From 600b53381b0b04e0aa429dd1cae9eb08df58512c Mon Sep 17 00:00:00 2001
From: Egor Tensin <Egor.Tensin@gmail.com>
Date: Sun, 6 Nov 2022 23:11:06 +0100
Subject: notes: add gdb.md

---
 _notes/gdb.md      | 133 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 assets/css/gdb.css |   5 ++
 2 files changed, 138 insertions(+)
 create mode 100644 _notes/gdb.md
 create mode 100644 assets/css/gdb.css

diff --git a/_notes/gdb.md b/_notes/gdb.md
new file mode 100644
index 0000000..5e649ea
--- /dev/null
+++ b/_notes/gdb.md
@@ -0,0 +1,133 @@
+---
+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
diff --git a/assets/css/gdb.css b/assets/css/gdb.css
new file mode 100644
index 0000000..ac7ee1f
--- /dev/null
+++ b/assets/css/gdb.css
@@ -0,0 +1,5 @@
+td code {
+  /* Override Bootstrap styling in tables: */
+  padding: 0;
+  background-color: inherit;
+}
-- 
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