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---
title: GDB
subtitle: cheat sheet
links:
- {rel: stylesheet, href: 'assets/css/gdb.css'}
---
Core dumps
----------
* Where are my core dumps?
cat /proc/sys/kernel/core_pattern
* Put core dumps in a directory:
mkdir /coredumps
chmod 0777 /coredumps
echo '/coredumps/core.%e.%p' | tee /proc/sys/kernel/core_pattern
* Still no dumps :-(
ulimit -c unlimited
* If dumps are piped to systemd-coredump, you can examine them using
`coredumpctl`.
<div markdown="1" class="table-responsive">
| List dumps | `coredumpctl`
| Debug the last dump | `coredumpctl gdb`
| Extract the last dump | `coredumpctl dump -o core`
{: .table .table-bordered .table-condensed }
</div>
.gdbinit
--------
# Without these, gdb is hardly usable:
set pagination off
set confirm off
set print pretty on
# Save history:
set history save on
set history filename ~/.gdb-history
set history size 10000
Basics
------
<div markdown="1" class="table-responsive">
| Run | `r`
| Continue | `c`
| Create breakpoint | `b FUNC`
| List breakpoints | `i b`
| Disable breakpoint | `dis N`
| Enable breakpoint | `en N`
| Delete breakpoint | `d N`
| Call stack | `bt`
| Call stack: all threads | `thread apply all bt`
| Go to frame | `f N`
| Disassemble | `disas FUNC`
| Step over line | `n`
| Step over instruction | `si`
| Step out of frame | `fin`
{: .table .table-bordered .table-condensed }
</div>
Hint: put this in your ~/.gdbinit and use `bta` as a shortcut:
define bta
thread apply all backtrace
end
Data inspection
---------------
<div markdown="1" class="table-responsive">
| Disassemble 5 instructions | `x/5i 0xdeadbeef`
| Print a 64-bit address | `x/1xg 0xdeadbeef`
| Print a 32-bit address | `x/1xw 0xdeadbeef`
| Print anything | `p sa->__sigaction_handler.sa_handler`
| Describe a type | `ptype struct sigaction`
| Describe a type with offsets | `ptype /o struct sigaction`
| Disassemble all code sections | `objdump -d /proc/self/exe`
| Disassemble a single section | `objdump -d -j .init /proc/self/exe`
| Display the section contents | `objdump -s -j .data /proc/self/exe`
{: .table .table-bordered .table-condensed }
</div>
Hint: put this in your ~/.gdbinit:
define xxd
dump binary memory /tmp/dump.bin $arg0 ((char *)$arg0)+$arg1
shell xxd -groupsize 1 /tmp/dump.bin
shell rm -f /tmp/dump.bin
end
You can then use `xxd ADDR LEN` to display, in my opinion, the best formatting
for memory dumps:
(gdb) xxd main 24
00000000: f3 0f 1e fa 41 57 41 89 ff bf 05 00 00 00 41 56 ....AWA.......AV
00000010: 49 89 f6 41 55 41 54 55 I..AUATU
Debuginfod
----------
If your distribution provides a Debuginfod server, use it!
For example, see [Arch], [Debian], [Fedora].
In ~/.gdbinit, add
set debuginfod enabled on
[Arch]: https://wiki.archlinux.org/title/Debuginfod
[Debian]: https://wiki.debian.org/Debuginfod
[Fedora]: https://fedoraproject.org/wiki/Debuginfod
Intel syntax
------------
This is just me being a baby duck.
In ~/.gdbinit:
set disassembly-flavor intel
With `objdump`:
objdump -Mintel -d /proc/self/exe
|
