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Many tools lie at your disposal for debugging Pintos. This appendix introduces you to a few of them.
printf()
Don't underestimate the value of printf(). The way
printf() is implemented in Pintos, you can call it from
practically anywhere in the kernel, whether it's in a kernel thread or
an interrupt handler, almost regardless of what locks are held.
printf() is useful for more than just examining data.
It can also help figure out when and where something goes wrong, even
when the kernel crashes or panics without a useful error message. The
strategy is to sprinkle calls to printf() with different strings
(e.g. "<1>", "<2>", ...) throughout the pieces of
code you suspect are failing. If you don't even see <1> printed,
then something bad happened before that point, if you see <1>
but not <2>, then something bad happened between those two
points, and so on. Based on what you learn, you can then insert more
printf() calls in the new, smaller region of code you suspect.
Eventually you can narrow the problem down to a single statement.
See section E.6 Triple Faults, for a related technique.
ASSERT Assertions are useful because they can catch problems early, before they'd otherwise be noticed. Ideally, each function should begin with a set of assertions that check its arguments for validity. (Initializers for functions' local variables are evaluated before assertions are checked, so be careful not to assume that an argument is valid in an initializer.) You can also sprinkle assertions throughout the body of functions in places where you suspect things are likely to go wrong. They are especially useful for checking loop invariants.
Pintos provides the ASSERT macro, defined in <debug.h>
,
for checking assertions.
These macros defined in <debug.h>
tell the compiler special
attributes of a function or function parameter. Their expansions are
GCC-specific.
printf()-like format string as the argument numbered
format (starting from 1) and that the corresponding value
arguments start at the argument numbered first. This lets the
compiler tell you if you pass the wrong argument types.
When the kernel panics, it prints a "backtrace," that is, a summary
of how your program got where it is, as a list of addresses inside the
functions that were running at the time of the panic. You can also
insert a call to debug_backtrace(), prototyped in
<debug.h>
, to print a backtrace at any point in your code.
debug_backtrace_all(), also declared in <debug.h>
,
prints backtraces of all threads.
The addresses in a backtrace are listed as raw hexadecimal numbers,
which are difficult to interpret. We provide a tool called
backtrace to translate these into function names and source
file line numbers.
Give it the name of your kernel.o
as the first argument and the
hexadecimal numbers composing the backtrace (including the 0x
prefixes) as the remaining arguments. It outputs the function name
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
call A), then it's a good sign that you're corrupting a kernel
thread's stack, because the backtrace is extracted from the stack.
Alternatively, it could be that the kernel.o
you passed to
backtrace is not the same kernel that produced
the backtrace.
Sometimes backtraces can be confusing without any corruption.
Compiler optimizations can cause surprising behavior. When a function
has called another function as its final action (a tail call), the
calling function may not appear in a backtrace at all. Similarly, when
function A calls another function B that never returns, the compiler may
optimize such that an unrelated function C appears in the backtrace
instead of A. Function C is simply the function that happens to be in
memory just after A. In the threads project, this is commonly seen in
backtraces for test failures; see pass() Fails, for more information.
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:
Call stack: 0xc0106eff 0xc01102fb 0xc010dc22 0xc010cf67 0xc0102319 0xc010325a 0x804812c 0x8048a96 0x8048ac8. |
You would then invoke the backtrace utility like shown below,
cutting and pasting the backtrace information into the command line.
This assumes that kernel.o
is in the current directory. You
would of course enter all of the following on a single shell command
line, even though that would overflow our margins here:
backtrace kernel.o 0xc0106eff 0xc01102fb 0xc010dc22 0xc010cf67 0xc0102319 0xc010325a 0x804812c 0x8048a96 0x8048ac8 |
The backtrace output would then look something like this:
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: intr_entry (threads/intr-stubs.S:38) 0x0804812c: (unknown) 0x08048a96: (unknown) 0x08048ac8: (unknown) |
(You will probably not see exactly the same addresses if you run the command above on your own kernel binary, because the source code you compiled and the compiler you used are probably different.)
The first line in the backtrace refers to debug_panic(), the
function that implements kernel panics. Because backtraces commonly
result from kernel panics, debug_panic() will often be the first
function shown in a backtrace.
The second line shows file_seek() as 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 filesys/file.c
is the
assertion
ASSERT (file_ofs >= 0); |
(This line was also cited in the assertion failure message.)
Thus, file_seek() panicked because it passed a negative file offset
argument.
The third line indicates that seek() called file_seek(),
presumably without validating the offset argument. In this submission,
seek() implements the seek system call.
The fourth line shows that syscall_handler(), the system call
handler, invoked seek().
The fifth and sixth lines are the interrupt handler entry path.
The remaining lines are for addresses below 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 backtrace on the user program, like
so: (typing the command on a single line, of course):
backtrace tests/filesys/extended/grow-too-big 0xc0106eff 0xc01102fb 0xc010dc22 0xc010cf67 0xc0102319 0xc010325a 0x804812c 0x8048a96 0x8048ac8 |
The results look like this:
0xc0106eff: (unknown) 0xc01102fb: (unknown) 0xc010dc22: (unknown) 0xc010cf67: (unknown) 0xc0102319: (unknown) 0xc010325a: (unknown) 0x0804812c: test_main (...xtended/grow-too-big.c:20) 0x08048a96: main (tests/main.c:10) 0x08048ac8: _start (lib/user/entry.c:9) |
You can even specify both the kernel and the user program names on the command line, like so:
backtrace kernel.o tests/filesys/extended/grow-too-big 0xc0106eff 0xc01102fb 0xc010dc22 0xc010cf67 0xc0102319 0xc010325a 0x804812c 0x8048a96 0x8048ac8 |
The result is a combined backtrace:
In kernel.o: 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: intr_entry (threads/intr-stubs.S:38) In tests/filesys/extended/grow-too-big: 0x0804812c: test_main (...xtended/grow-too-big.c:20) 0x08048a96: main (tests/main.c:10) 0x08048ac8: _start (lib/user/entry.c:9) |
Here's an extra tip for anyone who read this far: backtrace
is smart enough to strip the Call stack: header and .
trailer from the command line if you include them. This can save you
a little bit of trouble in cutting and pasting. Thus, the following
command prints the same output as the first one we used:
backtrace kernel.o Call stack: 0xc0106eff 0xc01102fb 0xc010dc22 0xc010cf67 0xc0102319 0xc010325a 0x804812c 0x8048a96 0x8048ac8. |
You can run Pintos under the supervision of the GDB debugger.
First, start Pintos with the --gdb
option, e.g.
pintos --gdb -- run mytest. Second, open a second terminal on
the same machine and
use pintos-gdb to invoke GDB on
kernel.o
:(7)
pintos-gdb kernel.o |
target remote localhost:1234 |
Now GDB is connected to the simulator over a local
network connection. You can now issue any normal GDB
commands. If you issue the c
command, the simulated BIOS will take
control, load Pintos, and then Pintos will run in the usual way. You
can pause the process at any point with Ctrl+C.
You can read the GDB manual by typing info gdb at a
terminal command prompt. Here's a few commonly useful GDB commands:
0xprefix to specify an address in hex.)
Use break main to make GDB stop when Pintos starts running.
0xprefix to specify an address in hex.)
backtrace program described above.
0xprefix to specify an address in hex.)
We also provide a set of macros specialized for debugging Pintos,
written by Godmar Back gback@cs.vt.edu. You can type
help user-defined for basic help with the macros. Here is an
overview of their functionality, based on Godmar's documentation:
target remote localhost:1234.
struct list
that contains elements of the given type (without the word
struct) in which element is the struct list_elem member
that links the elements.
Example: dumplist all_list thread allelem prints all elements of
struct thread that are linked in struct list all_list using the
struct list_elem allelem which is part of struct thread.
struct thread of the thread whose backtrace it should show. For the
current thread, this is identical to the bt (backtrace) command.
It also works for any thread suspended in schedule(),
provided you know where its kernel stack page is located.
struct list in
which the threads are kept. Specify element as the
struct list_elem field used inside struct thread to link the threads
together.
Example: btthreadlist all_list allelem shows the backtraces of
all threads contained in struct list all_list, linked together by
allelem. This command is useful to determine where your threads
are stuck when a deadlock occurs. Please see the example scenario below.
btthreadlist all_list allelem.
Program received signal 0, Signal 0. 0xc0102320 in intr0e_stub () |
In that case, the bt command might not give a useful
backtrace. Use btpagefault instead.
You may also use btpagefault for page faults that occur in a user
process. In this case, you may wish to also load the user program's
symbol table using the loadusersymbols macro, as described above.
hook-stop will print a
message that says and explains further whether the page fault occurred
in the kernel or in user code.
If the exception occurred from user code, hook-stop will say:
pintos-debug: a page fault exception occurred in user mode pintos-debug: hit 'c' to continue, or 's' to step to intr_handler |
In Project 2, a page fault in a user process leads to the termination of
the process. You should expect those page faults to occur in the
robustness tests where we test that your kernel properly terminates
processes that try to access invalid addresses. To debug those, set a
break point in page_fault() in exception.c
, which you will
need to modify accordingly.
In Project 3, a page fault in a user process no longer automatically
leads to the termination of a process. Instead, it may require reading in
data for the page the process was trying to access, either
because it was swapped out or because this is the first time it's
accessed. In either case, you will reach page_fault() and need to
take the appropriate action there.
If the page fault did not occur in user mode while executing a user
process, then it occurred in kernel mode while executing kernel code.
In this case, hook-stop will print this message:
pintos-debug: a page fault occurred in kernel mode |
btpagefault command.
Before Project 3, a page fault exception in kernel code is always a bug
in your kernel, because your kernel should never crash. Starting with
Project 3, the situation will change if you use the get_user() and
put_user() strategy to verify user memory accesses
(see section 4.1.5 Accessing User Memory).
This section narrates a sample GDB session, provided by Godmar Back.
This example illustrates how one might debug a Project 1 solution in
which occasionally a thread that calls timer_sleep() is not woken
up. With this bug, tests such as mlfqs_load_1 get stuck.
This session was captured with a slightly older version of Bochs and the GDB macros for Pintos, so it looks slightly different than it would now. Program output is shown in normal type, user input in strong type.
First, I start Pintos:
$ pintos -v --gdb -- -q -mlfqs run mlfqs-load-1
Writing command line to /tmp/gDAlqTB5Uf.dsk...
bochs -q
========================================================================
Bochs x86 Emulator 2.2.5
Build from CVS snapshot on December 30, 2005
========================================================================
00000000000i[ ] reading configuration from bochsrc.txt
00000000000i[ ] Enabled gdbstub
00000000000i[ ] installing nogui module as the Bochs GUI
00000000000i[ ] using log file bochsout.txt
Waiting for gdb connection on localhost:1234
|
Then, I open a second window on the same machine and start GDB:
$ pintos-gdb kernel.o GNU gdb Red Hat Linux (6.3.0.0-1.84rh) Copyright 2004 Free Software Foundation, Inc. GDB is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. Type "show copying" to see the conditions. There is absolutely no warranty for GDB. Type "show warranty" for details. This GDB was configured as "i386-redhat-linux-gnu"... Using host libthread_db library "/lib/libthread_db.so.1". |
Then, I tell GDB to attach to the waiting Pintos emulator:
(gdb) debugpintos Remote debugging using localhost:1234 0x0000fff0 in ?? () Reply contains invalid hex digit 78 |
Now I tell Pintos to run by executing c (short for
continue) twice:
(gdb) c Continuing. Reply contains invalid hex digit 78 (gdb) c Continuing. |
Now Pintos will continue and output:
Pintos booting with 4,096 kB RAM... Kernel command line: -q -mlfqs run mlfqs-load-1 374 pages available in kernel pool. 373 pages available in user pool. Calibrating timer... 102,400 loops/s. Boot complete. Executing 'mlfqs-load-1': (mlfqs-load-1) begin (mlfqs-load-1) spinning for up to 45 seconds, please wait... (mlfqs-load-1) load average rose to 0.5 after 42 seconds (mlfqs-load-1) sleeping for another 10 seconds, please wait... |
...until it gets stuck because of the bug I had introduced. I hit Ctrl+C in the debugger window:
Program received signal 0, Signal 0. 0xc010168c in next_thread_to_run () at ../../threads/thread.c:649 649 while (i <= PRI_MAX && list_empty (&ready_list[i])) (gdb) |
The thread that was running when I interrupted Pintos was the idle
thread. If I run backtrace, it shows this backtrace:
(gdb) bt
#0 0xc010168c in next_thread_to_run () at ../../threads/thread.c:649
#1 0xc0101778 in schedule () at ../../threads/thread.c:714
#2 0xc0100f8f in thread_block () at ../../threads/thread.c:324
#3 0xc0101419 in idle (aux=0x0) at ../../threads/thread.c:551
#4 0xc010145a in kernel_thread (function=0xc01013ff , aux=0x0)
at ../../threads/thread.c:575
#5 0x00000000 in ?? ()
|
Not terribly useful. What I really like to know is what's up with the
other thread (or threads). Since I keep all threads in a linked list
called all_list, linked together by a struct list_elem member
named allelem, I can use the btthreadlist macro from the
macro library I wrote. btthreadlist iterates through the list of
threads and prints the backtrace for each thread:
(gdb) btthreadlist all_list allelem
pintos-debug: dumping backtrace of thread 'main' @0xc002f000
#0 0xc0101820 in schedule () at ../../threads/thread.c:722
#1 0xc0100f8f in thread_block () at ../../threads/thread.c:324
#2 0xc0104755 in timer_sleep (ticks=1000) at ../../devices/timer.c:141
#3 0xc010bf7c in test_mlfqs_load_1 () at ../../tests/threads/mlfqs-load-1.c:49
#4 0xc010aabb in run_test (name=0xc0007d8c "mlfqs-load-1")
at ../../tests/threads/tests.c:50
#5 0xc0100647 in run_task (argv=0xc0110d28) at ../../threads/init.c:281
#6 0xc0100721 in run_actions (argv=0xc0110d28) at ../../threads/init.c:331
#7 0xc01000c7 in main () at ../../threads/init.c:140
pintos-debug: dumping backtrace of thread 'idle' @0xc0116000
#0 0xc010168c in next_thread_to_run () at ../../threads/thread.c:649
#1 0xc0101778 in schedule () at ../../threads/thread.c:714
#2 0xc0100f8f in thread_block () at ../../threads/thread.c:324
#3 0xc0101419 in idle (aux=0x0) at ../../threads/thread.c:551
#4 0xc010145a in kernel_thread (function=0xc01013ff , aux=0x0)
at ../../threads/thread.c:575
#5 0x00000000 in ?? ()
|
In this case, there are only two threads, the idle thread and the main
thread. The kernel stack pages (to which the struct thread points)
are at 0xc0116000 and 0xc002f000, respectively. The main thread
is stuck in timer_sleep(), called from test_mlfqs_load_1.
Knowing where threads are stuck can be tremendously useful, for instance when diagnosing deadlocks or unexplained hangs.
You can also use GDB to debug a user program running under Pintos.
To do that, use the loadusersymbols macro to load the program's
symbol table:
loadusersymbols program |
(gdb) loadusersymbols tests/userprog/exec-multiple
add symbol table from file "tests/userprog/exec-multiple" at
.text_addr = 0x80480a0
(gdb)
|
After this, you should be
able to debug the user program the same way you would the kernel, by
placing breakpoints, inspecting data, etc. Your actions apply to every
user program running in Pintos, not just to the one you want to debug,
so be careful in interpreting the results: GDB does not know
which process is currently active (because that is an abstraction
the Pintos kernel creates). Also, a name that appears in
both the kernel and the user program will actually refer to the kernel
name. (The latter problem can be avoided by giving the user executable
name on the GDB command line, instead of kernel.o
, and then using
loadusersymbols to load kernel.o
.)
loadusersymbols is implemented via GDB's add-symbol-file
command.
If the target remote command fails, then make sure that both
GDB and pintos are running on the same machine by
running hostname in each terminal. If the names printed
differ, then you need to open a new terminal for GDB on the
machine running pintos.
If you start GDB with pintos-gdb, it should load the Pintos
macros automatically. If you start GDB some other way, then you must
issue the command source pintosdir/src/misc/gdb-macros,
where pintosdir is the root of your Pintos directory, before you
can use them.
Yes, you can. DDD invokes GDB as a subprocess, so you'll need to tell
it to invokes pintos-gdb instead:
ddd --gdb --debugger pintos-gdb |
Yes, you can. Emacs has special support for running GDB as a
subprocess. Type M-x gdb and enter your pintos-gdb
command at the prompt. The Emacs manual has information on how to use
its debugging features in a section titled "Debuggers."
If you notice strange behavior while using GDB, there are three possibilities: a bug in your modified Pintos, a bug in Bochs's interface to GDB or in GDB itself, or a bug in the original Pintos code. The first and second are quite likely, and you should seriously consider both. We hope that the third is less likely, but it is also possible.
When a CPU exception handler, such as a page fault handler, cannot be invoked because it is missing or defective, the CPU will try to invoke the "double fault" handler. If the double fault handler is itself missing or defective, that's called a "triple fault." A triple fault causes an immediate CPU reset.
Thus, if you get yourself into a situation where the machine reboots in
a loop, that's probably a "triple fault." In a triple fault
situation, you might not be able to use printf() for debugging,
because the reboots might be happening even before everything needed for
printf() is initialized.
There are at least two ways to debug triple faults. First, you can run Pintos in Bochs under GDB (see section E.5 GDB). If Bochs has been built properly for Pintos, a triple fault under GDB will cause it to print the message "Triple fault: stopping for gdb" on the console and break into the debugger. (If Bochs is not running under GDB, a triple fault will still cause it to reboot.) You can then inspect where Pintos stopped, which is where the triple fault occurred.
Another option is what I call "debugging by infinite loop."
Pick a place in the Pintos code, insert the infinite loop
for (;;); there, and recompile and run. There are two likely
possibilities:
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.
An advanced debugging technique is to modify and recompile the simulator. This proves useful when the simulated hardware has more information than it makes available to the OS. For example, page faults have a long list of potential causes, but the hardware does not report to the OS exactly which one is the particular cause. Furthermore, a bug in the kernel's handling of page faults can easily lead to recursive faults, but a "triple fault" will cause the CPU to reset itself, which is hardly conducive to debugging.
In a case like this, you might appreciate being able to make Bochs
print out more debug information, such as the exact type of fault that
occurred. It's not very hard. You start by retrieving the source
code for Bochs 2.2.6 from http://bochs.sourceforge.net and
saving the file bochs-2.2.6.tar.gz
into a directory.
The script pintos/src/misc/bochs-2.2.6-build.sh
applies a number of patches contained in pintos/src/misc
to the Bochs tree, then builds Bochs and installs it in a directory
of your choice.
Run this script without arguments to learn usage instructions.
To use your bochs
binary with pintos,
put it in your PATH, and make sure that it is earlier than
unused/bochs
.
Of course, to get any good out of this you'll have to actually modify
Bochs. Instructions for doing this are firmly out of the scope of
this document. However, if you want to debug page faults as suggested
above, a good place to start adding printf()s is
BX_CPU_C::dtranslate_linear() in cpu/paging.cc
.
The page allocator in threads/palloc.c
and the block allocator in
threads/malloc.c
clear all the bytes in memory to
0xcc at time of free. Thus, if you see an attempt to
dereference a pointer like 0xcccccccc, or some other reference to
0xcc, there's a good chance you're trying to reuse a page that's
already been freed. Also, byte 0xcc is the CPU opcode for "invoke
interrupt 3," so if you see an error like Interrupt 0x03 (#BP
Breakpoint Exception), then Pintos tried to execute code in a freed page or
block.
An assertion failure on the expression sec_no < d->capacity
indicates that Pintos tried to access a file through an inode that has
been closed and freed. Freeing an inode clears its starting sector
number to 0xcccccccc, which is not a valid sector number for disks
smaller than about 1.6 TB.
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