* UNUSED NO_RETURN NO_INLINE PRINTF_FORMAT::
* Backtraces::
* i386-elf-gdb::
+* Debugging by Infinite Loop::
* Modifying Bochs::
* Debugging Tips::
@end menu
Give it the name of your @file{kernel.o} as the first argument and the
hexadecimal numbers composing the backtrace (including the @samp{0x}
prefixes) as the remaining arguments. It outputs the function name
-and source file line numbers that correspond to each address.
+and source file line numbers that correspond to each address.
If the translated form of a backtrace is garbled, or doesn't make
sense (e.g.@: function A is listed above function B, but B doesn't
@command{backtrace} does not correspond to the kernel that produced
the backtrace.
+@menu
+* Backtrace Example::
+@end menu
+
+@node Backtrace Example
+@subsection Example
+
+Here's an example. Suppose that Pintos printed out this following call
+stack, which is taken from an actual Pintos submission for the file
+system project:
+
+@example
+Call stack: 0xc0106eff 0xc01102fb 0xc010dc22 0xc010cf67 0xc0102319
+0xc010325a 0x804812c 0x8048a96 0x8048ac8.
+@end example
+
+You would then invoke the @command{backtrace} utility like shown below,
+cutting and pasting the backtrace information into the command line.
+This assumes that @file{kernel.o} is in the current directory. You
+would of course enter all of the following on a single shell command
+line:
+
+@example
+backtrace kernel.o 0xc0106eff 0xc01102fb 0xc010dc22 0xc010cf67 0xc0102319
+0xc010325a 0x804812c 0x8048a96 0x8048ac8
+@end example
+
+The backtrace output would then look something like this:
+
+@example
+0xc0106eff: debug_panic (../../lib/debug.c:86)
+0xc01102fb: file_seek (../../filesys/file.c:405)
+0xc010dc22: seek (../../userprog/syscall.c:744)
+0xc010cf67: syscall_handler (../../userprog/syscall.c:444)
+0xc0102319: intr_handler (../../threads/interrupt.c:334)
+0xc010325a: ?? (threads/intr-stubs.S:1554)
+0x804812c: ?? (??:0)
+0x8048a96: ?? (??:0)
+0x8048ac8: ?? (??:0)
+@end example
+
+(You will probably not get the same results if you run the command above
+on your own kernel binary, because the source code you compiled from is
+different from the source code that emitted the panic message.)
+
+The first line in the backtrace refers to @func{debug_panic}, the
+function that implements kernel panics. Because backtraces commonly
+result from kernel panics, @func{debug_panic} will often be the first
+function shown in a backtrace.
+
+The second line shows @func{file_seek} to be the function that panicked,
+in this case as the result of an assertion failure. In the source code
+tree used for this example, line 405 of @file{filesys/file.c} is the
+assertion
+
+@example
+ASSERT (file_ofs >= )0;
+@end example
+
+@noindent
+Thus, @func{file_seek} panicked because it passed a negative file offset
+argument.
+
+The third line indicates that @func{seek} called @func{file_seek},
+presumably without validating the offset argument. In this submission,
+@func{seek} implements the @code{seek} system call.
+
+The fourth line shows that @func{syscall_handler}, the system call
+handler, invoked @func{seek}.
+
+The fifth and sixth lines are the interrupt handler entry path.
+
+The remaining lines are for addresses below @code{PHYS_BASE}. This
+means that they refer to addresses in the user program, not in the
+kernel. If you know what user program was running when the kernel
+panicked, you can re-run @command{backtrace} on the user program, like
+so: (typing the command on a single line, of course):
+
+@example
+~/cs140/pintos/src/utils/backtrace grow-too-big 0xc0106eff 0xc01102fb
+0xc010dc22 0xc010cf67 0xc0102319 0xc010325a 0x804812c 0x8048a96
+0x8048ac8
+@end example
+
+The results look like this:
+
+@example
+0xc0106eff: ?? (??:0)
+0xc01102fb: ?? (??:0)
+0xc010dc22: ?? (??:0)
+0xc010cf67: ?? (??:0)
+0xc0102319: ?? (??:0)
+0xc010325a: ?? (??:0)
+0x804812c: test_main (/home/blp/cs140/pintos/grading/filesys/grow-too-big.c:20)
+0x8048a96: main (/home/blp/cs140/pintos/grading/filesys/fsmain.c:10)
+0x8048ac8: _start (../../src/lib/user/entry.c:9)
+@end example
+
@node i386-elf-gdb
@section @command{i386-elf-gdb}
(Use a @samp{0x} prefix to specify an address in hex.)
@end table
-If you notice unexplainable behavior while using @command{gdb}, there
+If you notice other strange behavior while using @command{gdb}, there
are three possibilities. The first is that there is a bug in your
modified Pintos. The second is that there is a bug in Bochs's
interface to @command{gdb} or in @command{gdb} itself. The third is
are quite likely, and you should seriously consider both. We hope
that the third is less likely, but it is also possible.
+@node Debugging by Infinite Loop
+@section Debugging by Infinite Loop
+
+If you get yourself into a situation where the machine reboots in a
+loop, you've probably hit a ``triple fault.'' In such a situation you
+might not be able to use @func{printf} for debugging, because the
+reboots might be happening even before everything needed for
+@func{printf} is initialized. In such a situation, you might want to
+try what I call ``debugging by infinite loop.''
+
+What you do is pick a place in the Pintos code, insert the statement
+@code{for (;;);} there, and recompile and run. There are two likely
+possibilities:
+
+@itemize @bullet
+@item
+The machine hangs without rebooting. If this happens, you know that
+the infinite loop is running. That means that whatever caused the
+problem must be @emph{after} the place you inserted the infinite loop.
+Now move the infinite loop later in the code sequence.
+
+@item
+The machine reboots in a loop. If this happens, you know that the
+machine didn't make it to the infinite loop. Thus, whatever caused the
+reboot must be @emph{before} the place you inserted the infinite loop.
+Now move the infinite loop earlier in the code sequence.
+@end itemize
+
+If you move around the infinite loop in a ``binary search'' fashion, you
+can use this technique to pin down the exact spot that everything goes
+wrong. It should only take a few minutes at most.
+
@node Modifying Bochs
@section Modifying Bochs
projects / pintos-anon / blobdiff