Clarify that multithreaded processes not supported.

[pintos-anon] / doc / userprog.texi

@node Project 2--User Programs, Project 3--Virtual Memory, Project 1--Threads, Top
@chapter Project 2: User Programs

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 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.

You can build project 2 on top of your project 1 submission or you can
start with a fresh copy.  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.

@menu
* Project 2 Background::        
* 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 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 Source Files::      
* Using the File System::       
* How User Programs Work::      
* Virtual Memory Layout::       
* Accessing User Memory::       
@end menu

@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
@file{userprog}, you'll find a small number of files, but here is
where the bulk of your work will be:

@table @file
@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 page directories and page tables.
Although you probably won't want to modify this code for this project,
you may want to call some of its functions.

@item syscall.c
@itemx syscall.h
Whenever a user process wants to access some kernel functionality, it
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.

@item exception.c
@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 Intel processor manuals define
them slightly differently on 80@var{x}86.}  These files handle
exceptions.  Currently all exceptions simply print a message and
terminate the process.  Some, but not all, solutions to project 2
require modifying @func{page_fault} in this file.

@item 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.}  You can read the code if
you're interested in how the GDT works.

@item 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.}
You can read the code if you're interested in how the TSS
works.
@end table

@node Using the File System
@subsection Using the File System

You will need to use some file system code for this project.  First,
user programs are loaded from the file system.  Second, many of the
system calls you must implement deal with the file system.  However,
the focus of this project is not on the file system code, so 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, and especially
its many limitations.  @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.  Until then, you will have to put up with the
following limitations:

@itemize @bullet
@item
No synchronization.  Concurrent accesses will interfere with one
another.  You should use a global lock 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 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} program 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've 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=2} option creates a temporary disk 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.  In fact, Pintos can run any program
you want, as long as it's compiled into the proper file format and uses
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.

Pintos loads @dfn{ELF} executables.  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 emulated 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/mmu.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 directory.

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_ADDR} accesses physical
address 0, virtual address @code{PHYS_ADDR} + @t{0x1234} access
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 a user virtual address that doesn't have a page
mapped into it will cause a page fault.

You must handle memory fragmentation gracefully, that is, a process that
needs @var{N} pages of user virtual memory must not require those pages
to be contiguous in kernel virtual memory.

@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/mmu.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.cc}.  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 a page of memory.  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
/* Tries to copy a byte from user address USRC to kernel address KDST.
   Returns true if successful, false if USRC is invalid. */
static inline bool get_user (uint8_t *kdst, const uint8_t *usrc) {
  int eax;
  asm ("movl $1f, %%eax; movb %2, %%al; movb %%al, %0; 1:"
       : "=m" (*kdst), "=&a" (eax) : "m" (*usrc));
  return eax != 0;
}

/* Tries to write BYTE to user address UDST.
   Returns true if successful, false if UDST is invalid. */
static inline bool put_user (uint8_t *udst, uint8_t byte) {
  int eax;
  asm ("movl $1f, %%eax; movb %b2, %0; 1:"
       : "=m" (*udst), "=&a" (eax) : "r" (byte));
  return eax != 0;
}
@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 causes
@code{eax} to be set to 0 and its former value copied into @code{eip}.

@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 foo 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).

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 appropriate actions.

Implement the following system calls.  The prototypes listed are those
seen by a user program that includes @file{lib/user/syscall.h}.  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}).  Note that this should be seldom used, since
then 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.  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.
@end deftypefn

@deftypefn {System Call} int wait (pid_t @var{pid})
Waits for process @var{pid} to die and returns the status it passed to
@code{exit}.  Returns -1 if @var{pid}
was terminated by the kernel (i.e.@: killed due to an exception).  If
@var{pid} is invalid or if it was not 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.
@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.

Consider implementing this function in terms of @func{filesys_create}.
@end deftypefn

@deftypefn {System Call} bool remove (const char *@var{file})
Deletes the file called @var{file}.  Returns true if successful, false
otherwise.

Consider implementing this function in terms of @func{filesys_remove}.
@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.  All open files associated with a process should be closed
when the process exits or is terminated.

File descriptors numbered 0 and 1 are reserved for the console: fd 0
is standard input (@code{stdin}), fd 1 is standard output
(@code{stdout}).  These special file descriptors are valid as system
call arguments only as explicitly described below.

Consider implementing this function in terms of @func{filesys_open}.
@end deftypefn

@deftypefn {System Call} int filesize (int @var{fd})
Returns the size, in bytes, of the file open as @var{fd}.

Consider implementing this function in terms of @func{file_length}.
@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{kbd_getc}.

Consider implementing this function in terms of @func{file_read}.
@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, or -1 if the file 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 -1 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.  Otherwise,
lines of text output by different processes may end up interleaved on
the console, confusing both human readers and our grading scripts.

Consider implementing this function in terms of @func{file_write}.
@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.

Consider implementing this function in terms of @func{file_seek}.
@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.

Consider implementing this function in terms of @func{file_tell}.
@end deftypefn

@deftypefn {System Call} void close (int @var{fd})
Closes file descriptor @var{fd}.

Consider implementing this function in terms of @func{file_close}.
@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 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 a block
partially in one of those regions?  You should handle these cases by
terminating the user process.  We recommend
writing and testing this code before implementing any other system
call functionality.

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 file system 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 file system
code.  You should acquire this lock before calling any functions in
the @file{filesys} directory, and release it afterward.  Don't forget
that @func{process_execute} also accesses files.  @strong{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 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, 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.

@node Project 2 FAQ
@section FAQ

@table @asis
@item How much code will I need to write?

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.

@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

@item The kernel always panics when I run @code{pintos -p @var{file} -- -q}.

Did you format the disk (with @samp{pintos -f})?

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.

Is the file system full?

Does the file system already contain 16 files?  The base Pintos file
system has a 16-file limit.

The file system may be so fragmented that there's not enough contiguous
space for your file.

@item When I run @code{pintos -p ../file --}, @file{file} isn't copied.

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.

@item All my user programs die with page faults.

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.

@item All my user programs die with @code{system call!}

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}).

@item How can I can disassemble user programs?

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 @code{gdb}'s
@command{disassemble} command to disassemble individual functions
(@pxref{gdb}).

@item Why do many C include files not work in Pintos programs?

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.)

@item Can I use lib@var{foo} in my Pintos programs?

The chances are good that lib@var{foo} 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 How do I compile new user programs?

Modify @file{src/examples/Makefile}, then run @command{make}.

@item Can I run user programs under a debugger?

Yes, with some limitations.  @xref{Debugging User Programs}.

@item What's the difference between @code{tid_t} and @code{pid_t}?

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.

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.

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.

@item Keyboard input doesn't work with @command{pintos} option @option{-v}.

Serial input isn't implemented.  Don't use @option{-v} if you
want to use the shell or otherwise need keyboard input.
@end table

@menu
* Argument Passing FAQ::        
* System Calls FAQ::            
@end menu

@node Argument Passing FAQ
@subsection Argument Passing FAQ

@table @asis
@item Isn't the top of stack off the top of user virtual memory?

The top of stack is at @code{PHYS_BASE}, typically @t{0xc0000000}, which
is also where kernel virtual memory starts.
But 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}.

@item Is @code{PHYS_BASE} fixed?

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

@node System Calls FAQ
@subsection System Calls FAQ

@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 Can I set a maximum number of open files per process?

It is better not to set an arbitrary limit.  You may impose a limit of
128 open files per process, if necessary.

@item What happens when an open file is removed?
@anchor{Removing an Open File}

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.
@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,
you can refer to @bibref{SysV-i386}.

The basic calling convention works like this:

@enumerate 1
@item
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.

@item
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.

@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
If the callee has a return value, it stores it into register @code{EAX}.

@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

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 stack addresses are 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 is responsible for setting up the arguments for the initial
function on the stack, in accordance with the calling convention
explained in the preceding section, before it allows the user program to
begin executing.

Consider the following example command: @samp{/bin/ls -l foo bar}.
First, the kernel must break the command into words, as @samp{/bin/ls},
@samp{-l}, @samp{foo}, and @samp{bar}, and place them 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 order ensure 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 show 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{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{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/mmu.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:

@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 Call Details
@subsection System Call Details

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 (@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 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.

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 @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.