-@node Project 2--User Programs, Project 3--Virtual Memory, Project 1--Threads, Top
+@node Project 2--User Programs
@chapter Project 2: User Programs
-Now that you've worked with Pintos and are familiar with its
+Now that you've worked with Pintos and are becoming familiar with its
infrastructure and thread package, it's time to start working on the
-parts of the system that will allow users to run programs on top of
-your operating system. The base code already supports loading and
-running a single user program at a time with little interactivity
-possible. You will allow multiple programs to be loaded in at once,
-and to interact with the OS via system calls.
+parts of the system that allow running user programs.
+The base code already supports loading and
+running user programs, but no I/O or interactivity
+is possible. In this project, you will enable programs to interact with
+the OS via system calls.
You will be working out of the @file{userprog} directory for this
-assignment. However, you will also be interacting with almost every
-other part of the code for this assignment. We will describe the
-relevant parts below. If you are confident in your HW1 code, you can
-build on top of it. However, if you wish you can start with a fresh
-copy of the code and re-implement @code{thread_join()}, which is the
-only part of project #1 required for this assignment.
-
-Up to now, all of the code you have written for Pintos has been part
+assignment, but you will also be interacting with almost every
+other part of Pintos. We will describe the
+relevant parts below.
+
+You can build project 2 on top of your project 1 submission or you can
+start fresh. No code from project 1 is required for this
+assignment. The ``alarm clock'' functionality may be useful in
+projects 3 and 4, but it is not strictly required.
+
+You might find it useful to go back and reread how to run the tests
+(@pxref{Testing}).
+
+@menu
+* Project 2 Background::
+* Project 2 Suggested Order of Implementation::
+* Project 2 Requirements::
+* Project 2 FAQ::
+* 80x86 Calling Convention::
+@end menu
+
+@node Project 2 Background
+@section Background
+
+Up to now, all of the code you have run under Pintos has been part
of the operating system kernel. This means, for example, that all the
test code from the last assignment ran as part of the kernel, with
full access to privileged parts of the system. Once we start running
user programs on top of the operating system, this is no longer true.
-This project deals with consequences of the change.
-
-We allow more than one user program to run at a time. Because user
-programs are written and compiled to work under the illusion that they
-have the entire machine, when you load into memory and run more than
-one process at a time, you must manage things correctly to maintain
-this illusion.
-
-Before we delve into the details of the new code that you'll be
-working with, you should probably undo the test cases from project 1.
-All you need to do is make sure the original
-@file{threads/pintostest.c} is in place. This will stop the tests
-from being run.
+This project deals with the consequences.
+
+We allow more than one process to run at a time. Each process has one
+thread (multithreaded processes are not supported). User programs are
+written under the illusion that they have the entire machine. This
+means that when you load and run multiple processes at a time, you must
+manage memory, scheduling, and other state correctly to maintain this
+illusion.
+
+In the previous project, we compiled our test code directly into your
+kernel, so we had to require certain specific function interfaces within
+the kernel. From now on, we will test your operating system by running
+user programs. This gives you much greater freedom. You must make sure
+that the user program interface meets the specifications described here,
+but given that constraint you are free to restructure or rewrite kernel
+code however you wish.
@menu
-* Project 2 Code to Hack::
+* Project 2 Source Files::
+* Using the File System::
* How User Programs Work::
-* Global Requirements::
-* Problem 2-1 Argument Passing::
-* Problem 2-2 System Calls::
-* User Programs FAQ::
-* 80x86 Calling Convention::
-* System Calls::
+* Virtual Memory Layout::
+* Accessing User Memory::
@end menu
-@node Project 2 Code to Hack, How User Programs Work, Project 2--User Programs, Project 2--User Programs
-@section Code to Hack
+@node Project 2 Source Files
+@subsection Source Files
The easiest way to get an overview of the programming you will be
doing is to simply go over each part you'll be working with. In
where the bulk of your work will be:
@table @file
-@item addrspace.c
-@itemx addrspace.h
-An address space keeps track of all the data necessary to execute a
-user program. Address space data is stored in @code{struct thread},
-but manipulated only by @file{addrspace.c}. Address spaces need to
-keep track of things like paging information for the process (so that
-it knows which memory the process is using). Address spaces also
-handle loading the program into memory and starting up the process's
-execution.
+@item process.c
+@itemx process.h
+Loads ELF binaries and starts processes.
+
+@item pagedir.c
+@itemx pagedir.h
+A simple manager for 80@var{x}86 hardware page tables.
+Although you probably won't want to modify this code for this project,
+you may want to call some of its functions.
+@xref{Page Tables}, for more information.
@item syscall.c
@itemx syscall.h
Whenever a user process wants to access some kernel functionality, it
-needs to do so via a system call. This is a skeleton system call
+invokes a system call. This is a skeleton system call
handler. Currently, it just prints a message and terminates the user
process. In part 2 of this project you will add code to do everything
else needed by system calls.
@itemx exception.h
When a user process performs a privileged or prohibited operation, it
traps into the kernel as an ``exception'' or ``fault.''@footnote{We
-will treat these terms as synonymous. There is no standard
-distinction between them, although the Intel processor manuals define
-them slightly differently on 80@var{x}86.} These files handle
+will treat these terms as synonyms. There is no standard
+distinction between them, although Intel processor manuals make
+a minor distinction between them on 80@var{x}86.} These files handle
exceptions. Currently all exceptions simply print a message and
-terminate the process. @strong{You should not need to modify this
-file for project 2.}
+terminate the process. Some, but not all, solutions to project 2
+require modifying @func{page_fault} in this file.
@item gdt.c
-@itemx gdt.c
+@itemx gdt.h
The 80@var{x}86 is a segmented architecture. The Global Descriptor
Table (GDT) is a table that describes the segments in use. These
-files set up the GDT. @strong{You should not need to modify these
-files for any of the projects.} However, you can read the code if
+files set up the GDT. You should not need to modify these
+files for any of the projects. You can read the code if
you're interested in how the GDT works.
@item tss.c
-@itemx tss.c
+@itemx tss.h
The Task-State Segment (TSS) is used for 80@var{x}86 architectural
task switching. Pintos uses the TSS only for switching stacks when a
-user process enters an interrupt handler, as does Linux. @strong{You
-should not need to modify these files for any of the projects.}
-However, you can read the code if you're interested in how the GDT
+user process enters an interrupt handler, as does Linux. You
+should not need to modify these files for any of the projects.
+You can read the code if you're interested in how the TSS
works.
@end table
-Elsewhere in the kernel, you will need to use some file system code.
-You will not actually write a file system until the end of the
-quarter, but since user programs need files to do anything
-interesting, we have provided a simple file system in the
-@file{filesys} directory. You will want to look over the
-@file{filesys.h} and @file{file.h} interfaces to understand how to use
-the file system. However, @strong{you should not modify the file
-system code for this project}. Proper use of the file system routines
-now will make life much easier for project 4, when you improve the
-file system implementation.
-
-Finally, in @file{lib/kernel}, you might want to use
-@file{bitmap.[ch]}. A bitmap is basically an array of bits, each of
-which can be true or false. Bitmaps are typically used to keep track
-of the usage of a large array of (identical) resources: if resource
-@var{n} is in use, then bit @var{n} of the bitmap is true. You might
-find it useful for tracking memory pages, for example.
-
-@node How User Programs Work, Global Requirements, Project 2 Code to Hack, Project 2--User Programs
-@section How User Programs Work
-
-Pintos can run normal C programs. In fact, it can run any program you
-want, provided it's compiled into the proper file format, and uses
-only the system calls you implement. (For example, @code{malloc()}
-makes use of functionality that isn't provided by any of the syscalls
-we require you to support.) The only other limitation is that Pintos
-can't run programs using floating point operations, since it doesn't
-include the necessary kernel functionality to save and restore the
-processor's floating-point unit when switching threads. You can look
-in @file{test} directory for some examples.
-
-Pintos loads ELF executables, where ELF is an executable format used
-by Linux, Solaris, and many other Unix and Unix-like systems.
-Therefore, you can use any compiler and linker that produce
-80@var{x}86 ELF executables to produce programs for Pintos. We
-recommend using the tools we provide in the @file{test} directory. By
-default, the @file{Makefile} in this directory will compile the test
-programs we provide. You can edit the @file{Makefile} to compile your
-own test programs as well.
-
-@node Global Requirements, Problem 2-1 Argument Passing, How User Programs Work, Project 2--User Programs
-@section Global Requirements
-
-For testing and grading purposes, we have some simple requirements for
-your output. The kernel should print out the program's name and exit
-status whenever a process exits. Aside from this, it should print out
-no other messages. You may understand all those debug messages, but
-we won't, and it just clutters our ability to see the stuff we care
-about. Additionally, while it may be useful to hard-code which
-process will run at startup while debugging, before you submit your
-code you must make sure that it takes the start-up process name and
-arguments from the @samp{-ex} argument. The infrastructure for this
-is already there---you just need to make sure you enable it! For
-example, running @code{pintos -ex "testprogram 1 2 3 4"} will spawn
-@samp{testprogram 1 2 3 4} as the first process.
-
-@node Problem 2-1 Argument Passing, Problem 2-2 System Calls, Global Requirements, Project 2--User Programs
-@section Problem 2-1: Argument Passing
-
-Currently, @code{thread_execute()} does not support passing arguments
-to new processes. UNIX and other operating systems do allow passing
-command line arguments to a program, which accesses them via the argc,
-argv arguments to main. You must implement this functionality by
-extending @code{thread_execute()} so that instead of simply taking a
-program file name, it can take a program name with arguments as a
-single string. That is, @code{thread_execute("grep foo *.c")} should
-be a legal call. @xref{80x86 Calling Convention}, for information on
-exactly how this works.
-
-@strong{This functionality is extremely important.} Almost all our
-test cases rely on being able to pass arguments, so if you don't get
-this right, a lot of things will not appear to work correctly with our
-tests. If the tests fail, so do you. Fortunately, this part
-shouldn't be too hard.
-
-@node Problem 2-2 System Calls, User Programs FAQ, Problem 2-1 Argument Passing, Project 2--User Programs
-@section Problem 2-2: System Calls
-
-Implement the system call handler in @file{userprog/syscall.c} to
-properly deal with all the system calls described below. Currently,
-it ``handles'' system calls by terminating the process. You will need
-to decipher system call arguments and take the appropriate action for
-each.
-
-In addition, implement system calls and system call handling. You are
-required to support the following system calls, whose syscall numbers
-are defined in @file{lib/syscall-nr.h} and whose C functions called by
-user programs are prototyped in @file{lib/user/syscall.h}:
-
-@table @code
-@item SYS_halt
-@itemx void halt (void)
-Stops Pintos and prints out performance statistics. Note that this
-should be seldom used, since then you lose some information about
-possible deadlock situations, etc.
-
-@item SYS_exit
-@itemx void exit (int @var{status})
+@node Using the File System
+@subsection Using the File System
+
+You will need to interface to the file system code for this project,
+because
+user programs are loaded from the file system and many of the
+system calls you must implement deal with the file system. However,
+the focus of this project is not the file system, so we have
+provided a simple but complete file system in the @file{filesys}
+directory. You
+will want to look over the @file{filesys.h} and @file{file.h}
+interfaces to understand how to use the file system, and especially
+its many limitations.
+
+There is no need to modify the file system code for this project, and so
+we recommend that you do not. Working on the file system is likely to
+distract you from this project's focus.
+
+Proper use of the file system routines now
+will make life much easier for project 4, when you improve the file
+system implementation. Until then, you will have to tolerate the
+following limitations:
+
+@itemize @bullet
+@item
+No internal synchronization. Concurrent accesses will interfere with one
+another. You should use synchronization to ensure that only one process at a
+time is executing file system code.
+
+@item
+File size is fixed at creation time. The root directory is
+represented as a file, so the number of files that may be created is also
+limited.
+
+@item
+File data is allocated as a single extent, that is, data in a single
+file must occupy a contiguous range of sectors on disk. External
+fragmentation can therefore become a serious problem as a file system is
+used over time.
+
+@item
+No subdirectories.
+
+@item
+File names are limited to 14 characters.
+
+@item
+A system crash mid-operation may corrupt the disk in a way
+that cannot be repaired automatically. There is no file system repair
+tool anyway.
+@end itemize
+
+One important feature is included:
+
+@itemize @bullet
+@item
+Unix-like semantics for @func{filesys_remove} are implemented.
+That is, if a file is open when it is removed, its blocks
+are not deallocated and it may still be accessed by any
+threads that have it open, until the last one closes it. @xref{Removing
+an Open File}, for more information.
+@end itemize
+
+You need to be able to create simulated disks. The
+@command{pintos-mkdisk} program provides this functionality. From the
+@file{userprog/build} directory, execute @code{pintos-mkdisk fs.dsk@tie{}2}.
+This command creates a 2 MB simulated disk named @file{fs.dsk}. Then
+format the disk by passing @option{-f -q} on the kernel's command
+line: @code{pintos -f -q}. The @option{-f} option causes the disk to be
+formatted, and @option{-q} causes Pintos to exit as soon as the format
+is done.
+
+You'll need a way to copy files in and out of the simulated file system.
+The @code{pintos} @option{-p} (``put'') and @option{-g} (``get'')
+options do this. To copy @file{@var{file}} into the
+Pintos file system, use the command @file{pintos -p @var{file} -- -q}.
+(The @samp{--} is needed because @option{-p} is for the @command{pintos}
+script, not for the simulated kernel.) To copy it to the Pintos file
+system under the name @file{@var{newname}}, add @option{-a
+@var{newname}}: @file{pintos -p @var{file} -a @var{newname} -- -q}. The
+commands for copying files out of a VM are similar, but substitute
+@option{-g} for @option{-p}.
+
+Incidentally, these commands work by passing special commands
+@command{put} and @command{get} on the kernel's command line and copying
+to and from a special simulated ``scratch'' disk. If you're very
+curious, you can look at the @command{pintos} script as well as
+@file{filesys/fsutil.c} to learn the implementation details.
+
+Here's a summary of how to create and format a disk, copy the
+@command{echo} program into the new disk, and then run @command{echo},
+passing argument @code{x}. (Argument passing won't work until
+you implemented it.) It assumes
+that you've already built the
+examples in @file{examples} and that the current directory is
+@file{userprog/build}:
+
+@example
+pintos-mkdisk fs.dsk 2
+pintos -f -q
+pintos -p ../../examples/echo -a echo -- -q
+pintos -q run 'echo x'
+@end example
+
+The three final steps can actually be combined into a single command:
+
+@example
+pintos-mkdisk fs.dsk 2
+pintos -p ../../examples/echo -a echo -- -f -q run 'echo x'
+@end example
+
+If you don't want to keep the file system disk around for later use or
+inspection, you can even combine all four steps into a single command.
+The @code{--fs-disk=@var{n}} option creates a temporary disk
+approximately @var{n} megabytes in size just for the duration of the
+@command{pintos} run. The Pintos automatic test suite makes extensive
+use of this syntax:
+
+@example
+pintos --fs-disk=2 -p ../../examples/echo -a echo -- -f -q run 'echo x'
+@end example
+
+You can delete a file from the Pintos file system using the @code{rm
+@var{file}} kernel action, e.g.@: @code{pintos -q rm @var{file}}. Also,
+@command{ls} lists the files in the file system and @code{cat
+@var{file}} prints a file's contents to the display.
+
+@node How User Programs Work
+@subsection How User Programs Work
+
+Pintos can run normal C programs, as long as they fit into memory and use
+only the system calls you implement. Notably, @func{malloc} cannot be
+implemented because none of the system calls required for this project
+allow for memory allocation. Pintos also can't run programs that use
+floating point operations, since the kernel doesn't save and restore the
+processor's floating-point unit when switching threads.
+
+The @file{src/examples} directory contains a few sample user
+programs. The @file{Makefile} in this directory
+compiles the provided examples, and you can edit it
+compile your own programs as well. Some of the example programs will
+only work once projects 3 or 4 have been implemented.
+
+Pintos can load @dfn{ELF} executables with the loader provided for you
+in @file{userprog/process.c}. ELF is a file format used by Linux,
+Solaris, and many other operating systems for object files,
+shared libraries, and executables. You can actually use any compiler
+and linker that output 80@var{x}86 ELF executables to produce programs
+for Pintos. (We've provided compilers and linkers that should do just
+fine.)
+
+You should realize immediately that, until you copy a
+test program to the simulated disk, Pintos will be unable to do
+useful work. You won't be able to do
+interesting things until you copy a variety of programs to the disk.
+You might want to create a clean reference disk and copy that
+over whenever you trash your @file{fs.dsk} beyond a useful state,
+which may happen occasionally while debugging.
+
+@node Virtual Memory Layout
+@subsection Virtual Memory Layout
+
+Virtual memory in Pintos is divided into two regions: user virtual
+memory and kernel virtual memory. User virtual memory ranges from
+virtual address 0 up to @code{PHYS_BASE}, which is defined in
+@file{threads/vaddr.h} and defaults to @t{0xc0000000} (3 GB). Kernel
+virtual memory occupies the rest of the virtual address space, from
+@code{PHYS_BASE} up to 4 GB.
+
+User virtual memory is per-process.
+When the kernel switches from one process to another, it
+also switches user virtual address spaces by changing the processor's
+page directory base register (see @func{pagedir_activate} in
+@file{userprog/pagedir.c}). @struct{thread} contains a pointer to a
+process's page table.
+
+Kernel virtual memory is global. It is always mapped the same way,
+regardless of what user process or kernel thread is running. In
+Pintos, kernel virtual memory is mapped one-to-one to physical
+memory, starting at @code{PHYS_BASE}. That is, virtual address
+@code{PHYS_BASE} accesses physical
+address 0, virtual address @code{PHYS_BASE} + @t{0x1234} accesses
+physical address @t{0x1234}, and so on up to the size of the machine's
+physical memory.
+
+A user program can only access its own user virtual memory. An attempt to
+access kernel virtual memory causes a page fault, handled by
+@func{page_fault} in @file{userprog/exception.c}, and the process
+will be terminated. Kernel threads can access both kernel virtual
+memory and, if a user process is running, the user virtual memory of
+the running process. However, even in the kernel, an attempt to
+access memory at an unmapped user virtual address
+will cause a page fault.
+
+@menu
+* Typical Memory Layout::
+@end menu
+
+@node Typical Memory Layout
+@subsubsection Typical Memory Layout
+
+Conceptually, each process is
+free to lay out its own user virtual memory however it
+chooses. In practice, user virtual memory is laid out like this:
+
+@html
+<CENTER>
+@end html
+@example
+@group
+ PHYS_BASE +----------------------------------+
+ | user stack |
+ | | |
+ | | |
+ | V |
+ | grows downward |
+ | |
+ | |
+ | |
+ | |
+ | grows upward |
+ | ^ |
+ | | |
+ | | |
+ +----------------------------------+
+ | uninitialized data segment (BSS) |
+ +----------------------------------+
+ | initialized data segment |
+ +----------------------------------+
+ | code segment |
+ 0x08048000 +----------------------------------+
+ | |
+ | |
+ | |
+ | |
+ | |
+ 0 +----------------------------------+
+@end group
+@end example
+@html
+</CENTER>
+@end html
+
+In this project, the user stack is fixed in size, but in project 3 it
+will be allowed to grow. Traditionally, the size of the uninitialized
+data segment can be adjusted with a system call, but you will not have
+to implement this.
+
+The code segment in Pintos starts at user virtual address
+@t{0x08084000}, approximately 128 MB from the bottom of the address
+space. This value is specified in @bibref{SysV-i386} and has no deep
+significance.
+
+The linker sets the layout of a user program in memory, as directed by a
+``linker script'' that tells it the names and locations of the various
+program segments. You can learn more about linker scripts by reading
+the ``Scripts'' chapter in the linker manual, accessible via @samp{info
+ld}.
+
+To view the layout of a particular executable, run @command{objdump}
+(80@var{x}86) or @command{i386-elf-objdump} (SPARC) with the @option{-p}
+option.
+
+@node Accessing User Memory
+@subsection Accessing User Memory
+
+As part of a system
+call, the kernel must often access memory through pointers provided by a user
+program. The kernel must be very careful about doing so, because
+the user can pass a null pointer, a pointer to
+unmapped virtual memory, or a pointer to kernel virtual address space
+(above @code{PHYS_BASE}). All of these types of invalid pointers must
+be rejected without harm to the kernel or other running processes, by
+terminating the offending process and freeing its resources.
+
+There are at least two reasonable ways to do this correctly. The
+first method is to verify
+the validity of a user-provided pointer, then dereference it. If you
+choose this route, you'll want to look at the functions in
+@file{userprog/pagedir.c} and in @file{threads/vaddr.h}. This is the
+simplest way to handle user memory access.
+
+The second method is to check only that a user
+pointer points below @code{PHYS_BASE}, then dereference it.
+An invalid user pointer will cause a ``page fault'' that you can
+handle by modifying the code for @func{page_fault} in
+@file{userprog/exception.c}. This technique is normally faster
+because it takes advantage of the processor's MMU, so it tends to be
+used in real kernels (including Linux).
+
+In either case, you need to make sure not to ``leak'' resources. For
+example, suppose that your system call has acquired a lock or
+allocated memory with @func{malloc}. If you encounter an invalid user pointer
+afterward, you must still be sure to release the lock or free the page
+of memory. If you choose to verify user pointers before dereferencing
+them, this should be straightforward. It's more difficult to handle
+if an invalid pointer causes a page fault,
+because there's no way to return an error code from a memory access.
+Therefore, for those who want to try the latter technique, we'll
+provide a little bit of helpful code:
+
+@verbatim
+/* Reads a byte at user virtual address UADDR.
+ UADDR must be below PHYS_BASE.
+ Returns the byte value if successful, -1 if a segfault
+ occurred. */
+static int
+get_user (const uint8_t *uaddr)
+{
+ int result;
+ asm ("movl $1f, %0; movzbl %1, %0; 1:"
+ : "=&a" (result) : "m" (*uaddr));
+ return result;
+}
+
+/* Writes BYTE to user address UDST.
+ UDST must be below PHYS_BASE.
+ Returns true if successful, false if a segfault occurred. */
+static bool
+put_user (uint8_t *udst, uint8_t byte)
+{
+ int error_code;
+ asm ("movl $1f, %0; movb %b2, %1; 1:"
+ : "=&a" (error_code), "=m" (*udst) : "q" (byte));
+ return error_code != -1;
+}
+@end verbatim
+
+Each of these functions assumes that the user address has already been
+verified to be below @code{PHYS_BASE}. They also assume that you've
+modified @func{page_fault} so that a page fault in the kernel merely
+sets @code{eax} to @t{0xffffffff} and copies its former value
+into @code{eip}.
+
+@node Project 2 Suggested Order of Implementation
+@section Suggested Order of Implementation
+
+We suggest first implementing the following, which can happen in
+parallel:
+
+@itemize
+@item
+Argument passing (@pxref{Argument Passing}). Every user program will
+page fault immediately until argument passing is implemented.
+
+For now, you may simply wish to change
+@example
+*esp = PHYS_BASE;
+@end example
+@noindent to
+@example
+*esp = PHYS_BASE - 12;
+@end example
+in @func{setup_stack}. That will work for any test program that doesn't
+examine its arguments, although its name will be printed as
+@code{(null)}.
+
+Until you implement argument passing, you should only run programs
+without passing command-line arguments. Attempting to pass arguments to
+a program will include those arguments in the name of the program, which
+will probably fail.
+
+@item
+User memory access (@pxref{Accessing User Memory}). All system calls
+need to read user memory. Few system calls need to write to user
+memory.
+
+@item
+System call infrastructure (@pxref{System Calls}). Implement enough
+code to read the system call number from the user stack and dispatch to
+a handler based on it.
+
+@item
+The @code{exit} system call. Every user program that finishes in the
+normal way calls @code{exit}. Even a program that returns from
+@func{main} calls @code{exit} indirectly (see @func{_start} in
+@file{lib/user/entry.c}).
+
+@item
+The @code{write} system call for writing to fd 1, the system console.
+All of our test programs write to the console (the user process version
+of @func{printf} is implemented this way), so they will all malfunction
+until @code{write} is available.
+
+@item
+For now, change @func{process_wait} to an infinite loop (one that waits
+forever). The provided implementation returns immediately, so Pintos
+will power off before any processes actually get to run. You will
+eventually need to provide a correct implementation.
+@end itemize
+
+After the above are implemented, user processes should work minimally.
+At the very least, they can write to the console and exit correctly.
+You can then refine your implementation so that some of the tests start
+to pass.
+
+@node Project 2 Requirements
+@section Requirements
+
+@menu
+* Project 2 Design Document::
+* Process Termination Messages::
+* Argument Passing::
+* System Calls::
+* Denying Writes to Executables::
+@end menu
+
+@node Project 2 Design Document
+@subsection Design Document
+
+Before you turn in your project, you must copy @uref{userprog.tmpl, ,
+the project 2 design document template} into your source tree under the
+name @file{pintos/src/userprog/DESIGNDOC} and fill it in. We recommend
+that you read the design document template before you start working on
+the project. @xref{Project Documentation}, for a sample design document
+that goes along with a fictitious project.
+
+@node Process Termination Messages
+@subsection Process Termination Messages
+
+Whenever a user process terminates, because it called @code{exit}
+or for any other reason, print the process's name
+and exit code, formatted as if printed by @code{printf ("%s:
+exit(%d)\n", @dots{});}. The name printed should be the full name
+passed to @func{process_execute}, omitting command-line arguments.
+Do not print these messages when a kernel thread that is not a user
+process terminates, or
+when the @code{halt} system call is invoked. The message is optional
+when a process fails to load.
+
+Aside from this, don't print any other
+messages that Pintos as provided doesn't already print. You may find
+extra messages useful during debugging, but they will confuse the
+grading scripts and thus lower your score.
+
+@node Argument Passing
+@subsection Argument Passing
+
+Currently, @func{process_execute} does not support passing arguments to
+new processes. Implement this functionality, by extending
+@func{process_execute} so that instead of simply taking a program file
+name as its argument, it divides it into words at spaces. The first
+word is the program name, the second word is the first argument, and so
+on. That is, @code{process_execute("grep foo bar")} should run
+@command{grep} passing two arguments @code{foo} and @code{bar}.
+
+Within a command line, multiple spaces are equivalent to a single
+space, so that @code{process_execute("grep @w{ }foo @w{ }@w{ }bar")}
+is equivalent to our original example. You can impose a reasonable
+limit on the length of the command line arguments. For example, you
+could limit the arguments to those that will fit in a single page (4
+kB). (There is an unrelated limit of 128 bytes on command-line
+arguments that the @command{pintos} utility can pass to the kernel.)
+
+You can parse argument strings any way you like. If you're lost,
+look at @func{strtok_r}, prototyped in @file{lib/string.h} and
+implemented with thorough comments in @file{lib/string.c}. You can
+find more about it by looking at the man page (run @code{man strtok_r}
+at the prompt).
+
+@xref{Program Startup Details}, for information on exactly how you
+need to set up the stack.
+
+@node System Calls
+@subsection System Calls
+
+Implement the system call handler in @file{userprog/syscall.c}. The
+skeleton implementation we provide ``handles'' system calls by
+terminating the process. It will need to retrieve the system call
+number, then any system call arguments, and carry out appropriate actions.
+
+Implement the following system calls. The prototypes listed are those
+seen by a user program that includes @file{lib/user/syscall.h}. (This
+header, and all others in @file{lib/user}, are for use by user
+programs only.) System call numbers for each system call are defined in
+@file{lib/syscall-nr.h}:
+
+@deftypefn {System Call} void halt (void)
+Terminates Pintos by calling @func{power_off} (declared in
+@file{threads/init.h}). This should be seldom used, because you lose
+some information about possible deadlock situations, etc.
+@end deftypefn
+
+@deftypefn {System Call} void exit (int @var{status})
Terminates the current user program, returning @var{status} to the
-kernel. A @var{status} of 0 indicates a successful exit. Other
-values may be used to indicate user-defined error conditions.
-
-@item SYS_exec
-@itemx pid_t exec (const char *@var{file})
-Run the executable in @var{file} and return the new process's program
-id (pid). If there is an error loading this program, returns pid -1,
-which otherwise should not be a valid id number.
-
-@item SYS_join
-@itemx int join (pid_t @var{pid})
-Joins the process @var{pid}, using the join rules from the last
-assignment, and returns the process's exit status. If the process was
-terminated by the kernel (i.e.@: killed due to an exception), the exit
-status should be -1. If the process was not a child process, the
-return value is undefined (but kernel operation must not be
-disrupted).
-
-@item SYS_create
-@itemx bool create (const char *@var{file})
-Create a new file called @var{file}. Returns -1 if failed, 0 if OK.
-
-@item SYS_remove
-@itemx bool remove (const char *@var{file})
-Delete the file called @var{file}. Returns -1 if failed, 0 if OK.
-
-@item SYS_open
-@itemx int open (const char *@var{file})
-Open the file called @var{file}. Returns a nonnegative integer handle
+kernel. If the process's parent @code{wait}s for it (see below), this
+is the status
+that will be returned. Conventionally, a @var{status} of 0 indicates
+success and nonzero values indicate errors.
+@end deftypefn
+
+@deftypefn {System Call} pid_t exec (const char *@var{cmd_line})
+Runs the executable whose name is given in @var{cmd_line}, passing any
+given arguments, and returns the new process's program id (pid). Must
+return pid -1, which otherwise should not be a valid pid, if
+the program cannot load or run for any reason.
+Thus, the parent process cannot return from the @code{exec} until it
+knows whether the child process successfully loaded its executable.
+You must use appropriate synchronization to ensure this.
+@end deftypefn
+
+@deftypefn {System Call} int wait (pid_t @var{pid})
+If process @var{pid} is still alive, waits until it dies.
+Then, returns the status that @var{pid} passed to @code{exit},
+or -1 if @var{pid}
+was terminated by the kernel (e.g.@: killed due to an exception). If
+@var{pid} does not refer to a child of the
+calling thread, or if @code{wait} has already been successfully
+called for the given @var{pid}, returns -1 immediately, without
+waiting.
+
+You must ensure that Pintos does not terminate until the initial
+process exits. The supplied Pintos code tries to do this by calling
+@func{process_wait} (in @file{userprog/process.c}) from @func{main}
+(in @file{threads/init.c}). We suggest that you implement
+@func{process_wait} according to the comment at the top of the
+function and then implement the @code{wait} system call in terms of
+@func{process_wait}.
+
+All of a process's resources, including its @struct{thread}, must be
+freed whether its parent ever waits for it or not, and regardless of
+whether the child exits before or after its parent.
+
+Children are not inherited: if @var{A} has child @var{B} and
+@var{B} has child @var{C}, then @code{wait(C)} always returns immediately
+when called from @var{A}, even if @var{B} is dead.
+
+Consider all the ways a wait can occur: nested waits (@var{A} waits
+for @var{B}, then @var{B} waits for @var{C}), multiple waits (@var{A}
+waits for @var{B}, then @var{A} waits for @var{C}), and so on.
+
+Implementing this system call requires considerably more work than any
+of the rest.
+@end deftypefn
+
+@deftypefn {System Call} bool create (const char *@var{file}, unsigned @var{initial_size})
+Creates a new file called @var{file} initially @var{initial_size} bytes
+in size. Returns true if successful, false otherwise.
+Creating a new file does not open it: opening the new file is a
+separate operation which would require a @code{open} system call.
+@end deftypefn
+
+@deftypefn {System Call} bool remove (const char *@var{file})
+Deletes the file called @var{file}. Returns true if successful, false
+otherwise.
+A file may be removed regardless of whether it is open or closed, and
+removing an open file does not close it. @xref{Removing an Open
+File}, for details.
+@end deftypefn
+
+@deftypefn {System Call} int open (const char *@var{file})
+Opens the file called @var{file}. Returns a nonnegative integer handle
called a ``file descriptor'' (fd), or -1 if the file could not be
-opened. File descriptors numbered 0 and 1 are reserved for the
-console. All open files associated with a process should be closed
-when the process exits or is terminated.
-
-@item SYS_filesize
-@itemx int filesize (int @var{fd})
-Returns the size, in bytes, of the file open as @var{fd}, or -1 if the
-file is invalid.
-
-@item SYS_read
-@itemx int read (int @var{fd}, void *@var{buffer}, unsigned @var{size})
-Read @var{size} bytes from the file open as @var{fd} into
-@var{buffer}. Returns the number of bytes actually read, or -1 if the
-file could not be read.
-
-@item SYS_write
-@itemx int write (int @var{fd}, const void *@var{buffer}, unsigned @var{size})
-Write @var{size} bytes from @var{buffer} to the open file @var{fd}.
-Returns the number of bytes actually written, or -1 if the file could
-not be written.
-
-@item SYS_close
-@itemx void close (int @var{fd})
-Close file descriptor @var{fd}.
-@end table
+opened.
+
+File descriptors numbered 0 and 1 are reserved for the console: fd 0
+(@code{STDIN_FILENO}) is standard input, fd 1 (@code{STDOUT_FILENO}) is
+standard output. The @code{open} system call will never return either
+of these file descriptors, which are valid as system call arguments only
+as explicitly described below.
+
+Each process has an independent set of file descriptors. File
+descriptors are not inherited by child processes.
+
+When a single file is opened more than once, whether by a single
+process or different processes, each @code{open} returns a new file
+descriptor. Different file descriptors for a single file are closed
+independently in separate calls to @code{close} and they do not share
+a file position.
+@end deftypefn
+
+@deftypefn {System Call} int filesize (int @var{fd})
+Returns the size, in bytes, of the file open as @var{fd}.
+@end deftypefn
+
+@deftypefn {System Call} int read (int @var{fd}, void *@var{buffer}, unsigned @var{size})
+Reads @var{size} bytes from the file open as @var{fd} into
+@var{buffer}. Returns the number of bytes actually read (0 at end of
+file), or -1 if the file could not be read (due to a condition other
+than end of file). Fd 0 reads from the keyboard using
+@func{input_getc}.
+@end deftypefn
+
+@deftypefn {System Call} int write (int @var{fd}, const void *@var{buffer}, unsigned @var{size})
+Writes @var{size} bytes from @var{buffer} to the open file @var{fd}.
+Returns the number of bytes actually written, which may be less than
+@var{size} if some bytes could not be written.
+
+Writing past end-of-file would normally extend the file, but file growth
+is not implemented by the basic file system. The expected behavior is
+to write as many bytes as possible up to end-of-file and return the
+actual number written, or 0 if no bytes could be written at all.
+
+Fd 1 writes to the console. Your code to write to the console should
+write all of @var{buffer} in one call to @func{putbuf}, at least as
+long as @var{size} is not bigger than a few hundred bytes. (It is
+reasonable to break up larger buffers.) Otherwise,
+lines of text output by different processes may end up interleaved on
+the console, confusing both human readers and our grading scripts.
+@end deftypefn
+
+@deftypefn {System Call} void seek (int @var{fd}, unsigned @var{position})
+Changes the next byte to be read or written in open file @var{fd} to
+@var{position}, expressed in bytes from the beginning of the file.
+(Thus, a @var{position} of 0 is the file's start.)
+
+A seek past the current end of a file is not an error. A later read
+obtains 0 bytes, indicating end of file. A later write extends the
+file, filling any unwritten gap with zeros. (However, in Pintos files
+have a fixed length until project 4 is complete, so writes past end of
+file will return an error.) These semantics are implemented in the
+file system and do not require any special effort in system call
+implementation.
+@end deftypefn
+
+@deftypefn {System Call} unsigned tell (int @var{fd})
+Returns the position of the next byte to be read or written in open
+file @var{fd}, expressed in bytes from the beginning of the file.
+@end deftypefn
+
+@deftypefn {System Call} void close (int @var{fd})
+Closes file descriptor @var{fd}.
+Exiting or terminating a process implicitly closes all its open file
+descriptors, as if by calling this function for each one.
+@end deftypefn
The file defines other syscalls. Ignore them for now. You will
implement some of them in project 3 and the rest in project 4, so be
sure to design your system with extensibility in mind.
-To implement syscalls, you will need to provide a way of copying data
-from the user's virtual address space into the kernel and vice versa.
+To implement syscalls, you need to provide ways to read and write data
+in user virtual address space.
+You need this ability before you can
+even obtain the system call number, because the system call number is
+on the user's stack in the user's virtual address space.
This can be a bit tricky: what if the user provides an invalid
-pointer, a pointer into kernel memory, or points to a block that is
+pointer, a pointer into kernel memory, or a block
partially in one of those regions? You should handle these cases by
-terminating the user process. You will need this code before you can
-even obtain the system call number, because the system call number is
-on the user's stack in the user's virtual address space. We recommend
+terminating the user process. We recommend
writing and testing this code before implementing any other system
-call functionality.
+call functionality. @xref{Accessing User Memory}, for more information.
-You must make sure that system calls are properly synchronized so that
+You must synchronize system calls so that
any number of user processes can make them at once. In particular, it
-is not safe to call into the filesystem code provided in the
-@file{filesys} directory from multiple threads at once. For now, we
-recommend adding a single lock that controls access to the filesystem
-code. You should acquire this lock before calling any functions in
-the @file{filesys} directory, and release it afterward. Because it
-calls into @file{filesys} functions, you will have to modify
-@file{addrspace_load()} in the same way. @strong{For now, we
-recommend against modifying code in the @file{filesys} directory.}
-
-We have provided you a function for each system call in
+is not safe to call into the file system code provided in the
+@file{filesys} directory from multiple threads at once. Your system
+call implementation must treat the file system code as a critical
+section. Don't forget
+that @func{process_execute} also accesses files. For now, we
+recommend against modifying code in the @file{filesys} directory.
+
+We have provided you a user-level function for each system call in
@file{lib/user/syscall.c}. These provide a way for user processes to
-invoke each system call from a C program. Each of them calls an
-assembly language routine in @file{lib/user/syscall-stub.S}, which in
-turn invokes the system call interrupt and returns.
+invoke each system call from a C program. Each uses a little inline
+assembly code to invoke the system call and (if appropriate) returns the
+system call's return value.
When you're done with this part, and forevermore, Pintos should be
bulletproof. Nothing that a user program can do should ever cause the
-OS to crash, halt, assert fail, or otherwise stop running. The sole
-exception is a call to the @code{halt} system call.
+OS to crash, panic, fail an assertion, or otherwise malfunction. It is
+important to emphasize this point: our tests will try to break your
+system calls in many, many ways. You need to think of all the corner
+cases and handle them. The sole way a user program should be able to
+cause the OS to halt is by invoking the @code{halt} system call.
+
+If a system call is passed an invalid argument, acceptable options
+include returning an error value (for those calls that return a
+value), returning an undefined value, or terminating the process.
+
+@xref{System Call Details}, for details on how system calls work.
+
+@node Denying Writes to Executables
+@subsection Denying Writes to Executables
+
+Add code to deny writes to files in use as executables. Many OSes do
+this because of the unpredictable results if a process tried to run code
+that was in the midst of being changed on disk. This is especially
+important once virtual memory is implemented in project 3, but it can't
+hurt even now.
+
+You can use @func{file_deny_write} to prevent writes to an open file.
+Calling @func{file_allow_write} on the file will re-enable them (unless
+the file is denied writes by another opener). Closing a file will also
+re-enable writes. Thus, to deny writes to a process's executable, you
+must keep it open as long as the process is still running.
+
+@node Project 2 FAQ
+@section FAQ
-@xref{System Calls}, for more information on how syscalls work.
+@table @asis
+@item How much code will I need to write?
-@node User Programs FAQ, 80x86 Calling Convention, Problem 2-2 System Calls, Project 2--User Programs
-@section FAQ
+Here's a summary of our reference solution, produced by the
+@command{diffstat} program. The final row gives total lines inserted
+and deleted; a changed line counts as both an insertion and a deletion.
-@enumerate 1
-@item General FAQs
+The reference solution represents just one possible solution. Many
+other solutions are also possible and many of those differ greatly from
+the reference solution. Some excellent solutions may not modify all the
+files modified by the reference solution, and some may modify files not
+modified by the reference solution.
-@enumerate 1
-@item
-@b{Do we need a working project 1 to implement project 2?}
+@verbatim
+ threads/thread.c | 13
+ threads/thread.h | 26 +
+ userprog/exception.c | 8
+ userprog/process.c | 247 ++++++++++++++--
+ userprog/syscall.c | 468 ++++++++++++++++++++++++++++++-
+ userprog/syscall.h | 1
+ 6 files changed, 725 insertions(+), 38 deletions(-)
+@end verbatim
-You may find the code for @code{thread_join()} to be useful in
-implementing the join syscall, but besides that, you can use
-the original code provided for project 1.
+@item The kernel always panics when I run @code{pintos -p @var{file} -- -q}.
-@item
-@b{Is there a way I can disassemble user programs?}
+Did you format the disk (with @samp{pintos -f})?
-@c FIXME
-The @command{objdump} utility can disassemble entire user programs or
-object files. Invoke it as @code{objdump -d @var{file}}. You can
-also use @code{gdb}'s @command{disassemble} command to disassemble
-individual functions in object files compiled with debug information.
+Is your file name too long? The file system limits file names to 14
+characters. A command like @samp{pintos -p ../../examples/echo -- -q}
+will exceed the limit. Use @samp{pintos -p ../../examples/echo -a echo
+-- -q} to put the file under the name @file{echo} instead.
-@item
-@b{Why can't I use many C include files in my Pintos programs?}
+Is the file system full?
-The C library we provide is very limited. It does not include many of
-the features that are expected of a real operating system's C library.
-The C library must be built specifically for the operating system (and
-architecture), since it must make system calls for I/O and memory
-allocation. (Not all functions do, of course, but usually the library
-is compiled as a unit.) If you wish to port libraries to Pintos, feel
-free.
+Does the file system already contain 16 files? The base Pintos file
+system has a 16-file limit.
-@item
-@b{How do I compile new user programs? How do I make 'echo' compile?}
+The file system may be so fragmented that there's not enough contiguous
+space for your file.
-You need to modify @file{tests/Makefile}.
+@item When I run @code{pintos -p ../file --}, @file{file} isn't copied.
-@item
-@b{Help, Solaris only allows 128 open files at once!}
+Files are written under the name you refer to them, by default, so in
+this case the file copied in would be named @file{../file}. You
+probably want to run @code{pintos -p ../file -a file --} instead.
-Solaris limits the number of file descriptors a process may keep open
-at any given time. The default limit is 128 open file descriptors.
+@item All my user programs die with page faults.
-To see the current limit for all new processes type @samp{limit} at
-the shell prompt and look at the line titled ``descriptors''. To
-increase this limit to the maximum allowed type @code{ulimit
-descriptors} in a @command{csh} derived shell or @code{unlimit
-descriptors} in a @command{sh} derived shell. This will increase the
-number of open file descriptors your Pintos process can use, but it
-will still be limited.
+This will happen if you haven't implemented argument passing
+(or haven't done so correctly). The basic C library for user programs tries
+to read @var{argc} and @var{argv} off the stack. If the stack
+isn't properly set up, this causes a page fault.
-Refer to the @command{limit(1)} man page for more information.
+@item All my user programs die with @code{system call!}
-@item
-@b{I can't seem to figure out how to read from and write to
-memory. What should I do?}
+You'll have to implement system calls before you see anything else.
+Every reasonable program tries to make at least one system call
+(@func{exit}) and most programs make more than that. Notably,
+@func{printf} invokes the @code{write} system call. The default system
+call handler just prints @samp{system call!} and terminates the program.
+Until then, you can use @func{hex_dump} to convince yourself that
+argument passing is implemented correctly (@pxref{Program Startup Details}).
-Here are some pointers:
+@item How can I disassemble user programs?
-FIXME
+The @command{objdump} (80@var{x}86) or @command{i386-elf-objdump}
+(SPARC) utility can disassemble entire user
+programs or object files. Invoke it as @code{objdump -d
+@var{file}}. You can use GDB's
+@code{disassemble} command to disassemble individual functions
+(@pxref{GDB}).
-@item
-@b{I'm also confused about reading from and writing to the stack. Can
-you help?}
+@item Why do many C include files not work in Pintos programs?
+@itemx Can I use lib@var{foo} in my Pintos programs?
-FIXME: relevant?
+The C library we provide is very limited. It does not include many of
+the features that are expected of a real operating system's C library.
+The C library must be built specifically for the operating system (and
+architecture), since it must make system calls for I/O and memory
+allocation. (Not all functions do, of course, but usually the library
+is compiled as a unit.)
-@itemize @bullet
-@item
-Only non-@samp{char} values will have issues when writing them to
-memory. If a digit is in a string, it is considered a character.
-However, the value of @code{argc} would be a non-char.
+The chances are good that the library you want uses parts of the C library
+that Pintos doesn't implement. It will probably take at least some
+porting effort to make it work under Pintos. Notably, the Pintos
+user program C library does not have a @func{malloc} implementation.
-@item
-You will need to write characters and non-characters into main memory.
+@item How do I compile new user programs?
-@item
-When you add items to the stack, you will be decrementing the stack
-pointer. You'll need to decrement the stack pointer before writing to
-the location.
+Modify @file{src/examples/Makefile}, then run @command{make}.
-@item
-Each character is 1 byte.
-@end itemize
-@end enumerate
+@item Can I run user programs under a debugger?
-@item Argument Passing FAQs
+Yes, with some limitations. @xref{Debugging User Programs}.
-@enumerate 1
-@item
-@b{What will be the format of command line arguments?}
+@item What's the difference between @code{tid_t} and @code{pid_t}?
-You should assume that command line arguments are delimited by white
-space.
+A @code{tid_t} identifies a kernel thread, which may have a user
+process running in it (if created with @func{process_execute}) or not
+(if created with @func{thread_create}). It is a data type used only
+in the kernel.
-@item
-@b{How do I parse all these argument strings?}
+A @code{pid_t} identifies a user process. It is used by user
+processes and the kernel in the @code{exec} and @code{wait} system
+calls.
-We recommend you look at @code{strtok_r()}, prototyped in
-@file{lib/string.h} and implemented with thorough comments in
-@file{lib/string.c}. You can find more about it by looking at the man
-page (run @code{man strtok_r} at the prompt).
+You can choose whatever suitable types you like for @code{tid_t} and
+@code{pid_t}. By default, they're both @code{int}. You can make them
+a one-to-one mapping, so that the same values in both identify the
+same process, or you can use a more complex mapping. It's up to you.
+@end table
-@item
-@b{Why is the top of the stack at @t{0xc0000000}? Isn't that off the
-top of user virtual memory? Shouldn't it be @t{0xbfffffff}?}
+@menu
+* Argument Passing FAQ::
+* System Calls FAQ::
+@end menu
-When the processor pushes data on the stack, it decrements the stack
-pointer first. Thus, the first (4-byte) value pushed on the stack
-will be at address @t{0xbffffffc}.
+@node Argument Passing FAQ
+@subsection Argument Passing FAQ
-Also, the stack should always be aligned to a 4-byte boundary, but
-@t{0xbfffffff} isn't.
-@end enumerate
+@table @asis
+@item Isn't the top of stack in kernel virtual memory?
-@item System Calls FAQs
+The top of stack is at @code{PHYS_BASE}, typically @t{0xc0000000}, which
+is also where kernel virtual memory starts.
+But before the processor pushes data on the stack, it decrements the stack
+pointer. Thus, the first (4-byte) value pushed on the stack
+will be at address @t{0xbffffffc}.
-@enumerate 1
-@item
-@b{What should I do with the parameter passed to @code{exit()}?}
+@item Is @code{PHYS_BASE} fixed?
-This value, the exit status of the process, must be returned to the
-thread's parent when @code{join()} is called.
+No. You should be able to support @code{PHYS_BASE} values that are
+any multiple of @t{0x10000000} from @t{0x80000000} to @t{0xf0000000},
+simply via recompilation.
+@end table
-@item
-@b{Can I just cast a pointer to a @code{struct file} object to get a
-unique file descriptor? Can I just cast a @code{struct thread *} to a
-@code{pid_t}? It's so much simpler that way!}
+@node System Calls FAQ
+@subsection System Calls FAQ
-This is a design decision you will have to make for yourself.
-However, note that most operating systems do distinguish between file
+@table @asis
+@item Can I just cast a @code{struct file *} to get a file descriptor?
+@itemx Can I just cast a @code{struct thread *} to a @code{pid_t}?
+
+You will have to make these design decisions yourself.
+Most operating systems do distinguish between file
descriptors (or pids) and the addresses of their kernel data
structures. You might want to give some thought as to why they do so
before committing yourself.
-@item
-@b{Can I set a maximum number of open files per process?}
-
-From a design standpoint, it would be better not to set an arbitrary
-maximum. That said, if your design calls for it, you may impose a
-limit of 128 open files per process (as the Solaris machines here do).
+@item Can I set a maximum number of open files per process?
-@item
-@b{What happens when two (or more) processes have a file open and one of
-them removes it?}
+It is better not to set an arbitrary limit. You may impose a limit of
+128 open files per process, if necessary.
-FIXME FIXME FIXME
+@item What happens when an open file is removed?
+@anchor{Removing an Open File}
-You should copy the standard Unix semantics for files. That is, when
-a file is removed an process which has a file descriptor for that file
-may continue to do operations on that descriptor. This means that
+You should implement the standard Unix semantics for files. That is, when
+a file is removed any process which has a file descriptor for that file
+may continue to use that descriptor. This means that
they can read and write from the file. The file will not have a name,
and no other processes will be able to open it, but it will continue
to exist until all file descriptors referring to the file are closed
or the machine shuts down.
+@item How can I run user programs that need more than 4 kB stack space?
+
+You may modify the stack setup code to allocate more than one page of
+stack space for each process. In the next project, you will implement a
+better solution.
+
+@item What should happen if an @code{exec} fails midway through loading?
+
+@code{exec} should return -1 if the child process fails to load for
+any reason. This includes the case where the load fails part of the
+way through the process (e.g.@: where it runs out of memory in the
+@code{multi-oom} test). Therefore, the parent process cannot return
+from the @code{exec} system call until it is established whether the
+load was successful or not. The child must communicate this
+information to its parent using appropriate synchronization, such as a
+semaphore (@pxref{Semaphores}), to ensure that the information is
+communicated without race conditions.
+@end table
+
+@node 80x86 Calling Convention
+@section 80@var{x}86 Calling Convention
+
+This section summarizes important points of the convention used for
+normal function calls on 32-bit 80@var{x}86 implementations of Unix.
+Some details are omitted for brevity. If you do want all the details,
+refer to @bibref{SysV-i386}.
+
+The calling convention works like this:
+
+@enumerate 1
@item
-@b{What happens if a system call is passed an invalid argument, such
-as Open being called with an invalid filename?}
+The caller pushes each of the function's arguments on the stack one by
+one, normally using the @code{PUSH} assembly language instruction.
+Arguments are pushed in right-to-left order.
-Pintos should not crash. You should have your system calls check for
-invalid arguments and return error codes.
+The stack grows downward: each push decrements the stack pointer, then
+stores into the location it now points to, like the C expression
+@samp{*--sp = @var{value}}.
@item
-@b{I've discovered that some of my user programs need more than one 4
-kB page of stack space. What should I do?}
+The caller pushes the address of its next instruction (the @dfn{return
+address}) on the stack and jumps to the first instruction of the callee.
+A single 80@var{x}86 instruction, @code{CALL}, does both.
-You may modify the stack setup code to allocate more than one page of
-stack space for each process.
+@item
+The callee executes. When it takes control, the stack pointer points to
+the return address, the first argument is just above it, the second
+argument is just above the first argument, and so on.
@item
-@b{What do I need to print on thread completion?}
+If the callee has a return value, it stores it into register @code{EAX}.
-You should print the complete thread name (as specified in the
-@code{SYS_exec} call) followed by the exit status code,
-e.g.@: @samp{example 1 2 3 4: 0}.
-@end enumerate
+@item
+The callee returns by popping the return address from the stack and
+jumping to the location it specifies, using the 80@var{x}86 @code{RET}
+instruction.
+
+@item
+The caller pops the arguments off the stack.
@end enumerate
-@node 80x86 Calling Convention, System Calls, User Programs FAQ, Project 2--User Programs
-@appendixsec 80@var{x}86 Calling Convention
-
-What follows is a quick and dirty discussion of the 80@var{x}86
-calling convention. Some of the basics should be familiar from CS
-107, and if you've already taken CS 143 or EE 182, then you should
-have seen even more of it. I've omitted some of the complexity, since
-this isn't a class in how function calls work, so don't expect this to
-be exactly correct in full, gory detail. If you do want all the
-details, you can refer to @cite{[SysV-i386]}.
-
-Whenever a function call happens, you need to put the arguments on the
-call stack for that function, before the code for that function
-executes, so that the callee has access to those values. The caller
-has to be responsible for this (be sure you understand why).
-Therefore, when you compile a program, the assembly code emitted will
-have in it, before every function call, a bunch of instructions that
-prepares for the call in whatever manner is conventional for the
-machine you're working on. This includes saving registers as needed,
-putting stuff on the stack, saving the location to return to somewhere
-(so that when the callee finishes, it knows where the caller code is),
-and some other bookkeeping stuff. Then you do the jump to the
-callee's code, and it goes along, assuming that the stack and
-registers are prepared in the appropriate manner. When the callee is
-done, it looks at the return location as saved earlier, and jumps back
-to that location. The caller may then have to do some cleanup:
-clearing arguments and the return value off the stack, restoring
-registers that were saved before the call, and so on.
-
-If you think about it, some of these things should remind you of
-context switching.
-
-As an aside, in general, function calls are not cheap. You have to do
-a bunch of memory writes to prepare the stack, you need to save and
-restore registers before and after a function call, you need to write
-the stack pointer, you have a couple of jumps which probably wrecks
-some of your caches. This is why inlining code can be much faster.
-
-@node Argument Passing to main
-@subsection Argument Passing to @code{main()}
-
-In @code{main()}'s case, there is no caller to prepare the stack
-before it runs. Therefore, the kernel needs to do it. Fortunately,
-since there's no caller, there are no registers to save, no return
-address to deal with, etc. The only difficult detail to take care of,
-after loading the code, is putting the arguments to @code{main()} on
-the stack.
-
-(The above is a small lie: most compilers will emit code where main
-isn't strictly speaking the first function. This isn't an important
-detail. If you want to look into it more, try disassembling a program
-and looking around a bit. However, you can just act as if
-@code{main()} is the very first function called.)
-
-Pintos is written for the 80@var{x}86 architecture. Therefore, we
-need to adhere to the 80@var{x}86 calling convention, which is
-detailed in the FAQ. Basically, you put all the arguments on the
-stack and move the stack pointer appropriately. The program will
-assume that this has been done when it begins running.
-
-So, what are the arguments to @code{main()}? Just two: an @samp{int}
-(@code{argc}) and a @samp{char **} (@code{argv}). @code{argv} is an
-array of strings, and @code{argc} is the number of strings in that
-array. However, the hard part isn't these two things. The hard part is
-getting all the individual strings in the right place. As we go
-through the procedure, let us consider the following example command:
-@samp{/bin/ls -l *.h *.c}.
-
-The first thing to do is to break the command line into individual
-strings: @samp{/bin/ls}, @samp{-l}, @samp{*.h}, and @samp{*.c}. These
-constitute the arguments of the command, including the program name
-itself (which belongs in @code{argv[0]}).
-
-These individual, null-terminated strings should be placed on the user
-stack. They may be placed in any order, as you'll see shortly,
-without affecting how main works, but for simplicity let's assume they
-are in reverse order (keeping in mind that the stack grows downward on
-an 80@var{x}86 machine). As we copy the strings onto the stack, we
-record their (virtual) stack addresses. These addresses will become
-important when we write the argument vector (two paragraphs down).
-
-After we push all of the strings onto the stack, we adjust the stack
-pointer so that it is word-aligned: that is, we move it down to the
-next 4-byte boundary. This is required because we will next be
-placing several words of data on the stack, and they must be aligned
-in order to be read correctly. In our example, as you'll see below,
-the strings start at address @t{0xffed}. One word below that would be
-at @t{0xffe9}, so we could in theory put the next word on the stack
-there. However, since the stack pointer should always be
-word-aligned, we instead leave the stack pointer at @t{0xffe8}.
-
-Once we align the stack pointer, we then push the elements of the
-argument vector (that is, the addresses of the strings @samp{/bin/ls},
-@samp{-l}, @samp{*.h}, and @samp{*.c}) onto the stack. This must be
-done in reverse order, such that @code{argv[0]} is at the lowest
-virtual address (again, because the stack is growing downward). This
-is because we are now writing the actual array of strings; if we write
-them in the wrong order, then the strings will be in the wrong order
-in the array. This is also why, strictly speaking, it doesn't matter
-what order the strings themselves are placed on the stack: as long as
-the pointers are in the right order, the strings themselves can really
-be anywhere. After we finish, we note the stack address of the first
-element of the argument vector, which is @code{argv} itself.
-
-Finally, we push @code{argv} (that is, the address of the first
-element of the @code{argv} array) onto the stack, along with the
-length of the argument vector (@code{argc}, 4 in this example). This
-must also be done in this order, since @code{argc} is the first
-argument to main and therefore is on first (smaller address) on the
-stack. We leave the stack pointer to point to the location where
-@code{argc} is, because it is at the top of the stack, the location
-directly below @code{argc}.
-
-All of which may sound very confusing, so here's a picture which will
-hopefully clarify what's going on. This represents the state of the
-stack and the relevant registers right before the beginning of the
-user program (assuming for this example a 16-bit virtual address space
-with addresses from @t{0x0000} to @t{0xffff}):
+Consider a function @func{f} that takes three @code{int} arguments.
+This diagram shows a sample stack frame as seen by the callee at the
+beginning of step 3 above, supposing that @func{f} is invoked as
+@code{f(1, 2, 3)}. The initial stack address is arbitrary:
+
+@html
+<CENTER>
+@end html
+@example
+ +----------------+
+ 0xbffffe7c | 3 |
+ 0xbffffe78 | 2 |
+ 0xbffffe74 | 1 |
+stack pointer --> 0xbffffe70 | return address |
+ +----------------+
+@end example
+@html
+</CENTER>
+@end html
+
+@menu
+* Program Startup Details::
+* System Call Details::
+@end menu
+
+@node Program Startup Details
+@subsection Program Startup Details
+
+The Pintos C library for user programs designates @func{_start}, in
+@file{lib/user/entry.c}, as the entry point for user programs. This
+function is a wrapper around @func{main} that calls @func{exit} if
+@func{main} returns:
+
+@example
+void
+_start (int argc, char *argv[])
+@{
+ exit (main (argc, argv));
+@}
+@end example
+
+The kernel must put the arguments for the initial function on the stack
+before it allows the user program to begin executing. The arguments are
+passed in the same way as the normal calling convention (@pxref{80x86
+Calling Convention}).
+
+Consider how to handle arguments for the following example command:
+@samp{/bin/ls -l foo bar}.
+First, break the command into words: @samp{/bin/ls},
+@samp{-l}, @samp{foo}, @samp{bar}. Place the words at the top of the
+stack. Order doesn't matter, because they will be referenced through
+pointers.
+
+Then, push the address of each string plus a null pointer sentinel, on
+the stack, in right-to-left order. These are the elements of
+@code{argv}. The null pointer sentinel ensures that @code{argv[argc]}
+is a null pointer, as required by the C standard. The order ensures
+that @code{argv[0]} is at the lowest virtual address. Word-aligned
+accesses are faster than unaligned accesses, so for best performance
+round the stack pointer down to a multiple of 4 before the first push.
+
+Then, push @code{argv} (the address of @code{argv[0]}) and @code{argc},
+in that order. Finally, push a fake ``return address'': although the
+entry function will never return, its stack frame must have the same
+structure as any other.
+
+The table below shows the state of the stack and the relevant registers
+right before the beginning of the user program, assuming
+@code{PHYS_BASE} is @t{0xc0000000}:
@html
<CENTER>
@end html
-@multitable {@t{0xffff}} {word-align} {@t{/bin/ls\0}}
-@item Address @tab Name @tab Data
-@item @t{0xfffc} @tab @code{*argv[3]} @tab @samp{*.c\0}
-@item @t{0xfff8} @tab @code{*argv[2]} @tab @samp{*.h\0}
-@item @t{0xfff5} @tab @code{*argv[1]} @tab @samp{-l\0}
-@item @t{0xffed} @tab @code{*argv[0]} @tab @samp{/bin/ls\0}
-@item @t{0xffec} @tab word-align @tab @samp{\0}
-@item @t{0xffe8} @tab @code{argv[3]} @tab @t{0xfffc}
-@item @t{0xffe4} @tab @code{argv[2]} @tab @t{0xfff8}
-@item @t{0xffe0} @tab @code{argv[1]} @tab @t{0xfff5}
-@item @t{0xffdc} @tab @code{argv[0]} @tab @t{0xffed}
-@item @t{0xffd8} @tab @code{argv} @tab @t{0xffdc}
-@item @t{0xffd4} @tab @code{argc} @tab 4
+@multitable {@t{0xbfffffff}} {return address} {@t{/bin/ls\0}} {@code{void (*) ()}}
+@item Address @tab Name @tab Data @tab Type
+@item @t{0xbffffffc} @tab @code{argv[3][@dots{}]} @tab @samp{bar\0} @tab @code{char[4]}
+@item @t{0xbffffff8} @tab @code{argv[2][@dots{}]} @tab @samp{foo\0} @tab @code{char[4]}
+@item @t{0xbffffff5} @tab @code{argv[1][@dots{}]} @tab @samp{-l\0} @tab @code{char[3]}
+@item @t{0xbfffffed} @tab @code{argv[0][@dots{}]} @tab @samp{/bin/ls\0} @tab @code{char[8]}
+@item @t{0xbfffffec} @tab word-align @tab 0 @tab @code{uint8_t}
+@item @t{0xbfffffe8} @tab @code{argv[4]} @tab @t{0} @tab @code{char *}
+@item @t{0xbfffffe4} @tab @code{argv[3]} @tab @t{0xbffffffc} @tab @code{char *}
+@item @t{0xbfffffe0} @tab @code{argv[2]} @tab @t{0xbffffff8} @tab @code{char *}
+@item @t{0xbfffffdc} @tab @code{argv[1]} @tab @t{0xbffffff5} @tab @code{char *}
+@item @t{0xbfffffd8} @tab @code{argv[0]} @tab @t{0xbfffffed} @tab @code{char *}
+@item @t{0xbfffffd4} @tab @code{argv} @tab @t{0xbfffffd8} @tab @code{char **}
+@item @t{0xbfffffd0} @tab @code{argc} @tab 4 @tab @code{int}
+@item @t{0xbfffffcc} @tab return address @tab 0 @tab @code{void (*) ()}
@end multitable
@html
</CENTER>
@end html
-In this example, the stack pointer would be initialized to @t{0xffd4}.
+In this example, the stack pointer would be initialized to
+@t{0xbfffffcc}.
+
+As shown above, your code should start the stack at the very top of
+the user virtual address space, in the page just below virtual address
+@code{PHYS_BASE} (defined in @file{threads/vaddr.h}).
+
+You may find the non-standard @func{hex_dump} function, declared in
+@file{<stdio.h>}, useful for debugging your argument passing code.
+Here's what it would show in the above example:
-Your code should start the stack at the very top of the user virtual
-address space, in the page just below virtual address @code{PHYS_BASE}
-(defined in @file{threads/mmu.h}).
+@verbatim
+bfffffc0 00 00 00 00 | ....|
+bfffffd0 04 00 00 00 d8 ff ff bf-ed ff ff bf f5 ff ff bf |................|
+bfffffe0 f8 ff ff bf fc ff ff bf-00 00 00 00 00 2f 62 69 |............./bi|
+bffffff0 6e 2f 6c 73 00 2d 6c 00-66 6f 6f 00 62 61 72 00 |n/ls.-l.foo.bar.|
+@end verbatim
-@node System Calls, , 80x86 Calling Convention, Project 2--User Programs
-@appendixsec System Calls
+@node System Call Details
+@subsection System Call Details
-We have already been dealing with one way that the operating system
+The first project already dealt with one way that the operating system
can regain control from a user program: interrupts from timers and I/O
devices. These are ``external'' interrupts, because they are caused
-by entities outside the CPU.
-
-The operating system is also called to deal with software exceptions,
-which are events generated in response to the code. These can be
-errors such as a page fault or division by zero. However, exceptions
-are also the means by which a user program can request services
-(``system calls'') from the operating system.
-
-Some exceptions are ``restartable'': the condition that caused the
-exception can be fixed and the instruction retried. For example, page
-faults call the operating system, but the user code should re-start on
-the load or store that caused the exception (not the next one) so that
-the memory access actually occurs. On the 80@var{x}86, restartable
-exceptions are called ``faults,'' whereas most non-restartable
-exceptions are classed as ``traps.'' Other architectures may define
-these terms differently.
+by entities outside the CPU (@pxref{External Interrupt Handling}).
+
+The operating system also deals with software exceptions, which are
+events that occur in program code (@pxref{Internal Interrupt
+Handling}). These can be errors such as a page fault or division by
+zero. Exceptions are also the means by which a user program
+can request services (``system calls'') from the operating system.
In the 80@var{x}86 architecture, the @samp{int} instruction is the
most commonly used means for invoking system calls. This instruction
-is handled in the same way that other software exceptions. In Pintos,
-user program invoke @samp{int $0x30} to make a system call. The
+is handled in the same way as other software exceptions. In Pintos,
+user programs invoke @samp{int $0x30} to make a system call. The
system call number and any additional arguments are expected to be
pushed on the stack in the normal fashion before invoking the
-interrupt.
-
-The normal calling convention pushes function arguments on the stack
-from right to left and the stack grows downward. Thus, when the
-system call handler @code{syscall_handler()} gets control, the system
-call number is in the 32-bit word at the caller's stack pointer, the
-first argument is in the 32-bit word at the next higher address, and
-so on. The caller's stack pointer is accessible to
-@code{syscall_handler()} as the @samp{esp} member of the @code{struct
-intr_frame} passed to it.
-
-Here's an example stack frame for calling a system call numbered 10
-with three arguments passed as 1, 2, and 3. The stack addresses are
-arbitrary:
-
-@html
-<CENTER>
-@end html
-@multitable {Address} {Value}
-@item Address @tab Value
-@item @t{0xfe7c} @tab 3
-@item @t{0xfe78} @tab 2
-@item @t{0xfe74} @tab 1
-@item @t{0xfe70} @tab 10
-@end multitable
-@html
-</CENTER>
-@end html
+interrupt (@pxref{80x86 Calling Convention}).
-In this example, the caller's stack pointer would be at @t{0xfe70}.
+Thus, when the system call handler @func{syscall_handler} gets control,
+the system call number is in the 32-bit word at the caller's stack
+pointer, the first argument is in the 32-bit word at the next higher
+address, and so on. The caller's stack pointer is accessible to
+@func{syscall_handler} as the @samp{esp} member of the
+@struct{intr_frame} passed to it. (@struct{intr_frame} is on the kernel
+stack.)
The 80@var{x}86 convention for function return values is to place them
-in the @samp{EAX} register. System calls that return a value can do
-so by modifying the @samp{eax} member of @code{struct intr_frame}.
+in the @code{EAX} register. System calls that return a value can do
+so by modifying the @samp{eax} member of @struct{intr_frame}.
+
+You should try to avoid writing large amounts of repetitive code for
+implementing system calls. Each system call argument, whether an
+integer or a pointer, takes up 4 bytes on the stack. You should be able
+to take advantage of this to avoid writing much near-identical code for
+retrieving each system call's arguments from the stack.
projects / pintos-anon / blobdiff