path: root/src/tools/gdb.md

# gdb(1)

# CLI

```markdown
  gdb [opts] [prg [-c coredump | -p pid]]
  gdb [opts] --args prg <prg-args>
    opts:
      -p <pid>        attach to pid
      -c <coredump>   use <coredump>
      -x <file>       execute script <file> before prompt
      -ex <cmd>       execute command <cmd> before prompt
      --tty <tty>     set I/O tty for debugee
      --batch         run in batch mode, exit after processing options (eg used
                      for scripting)
```

# Interactive usage

## Misc
```markdown
  tty <tty>
          Set <tty> as tty for debugee.
          Make sure nobody reads from target tty, easiest is to spawn a shell
          and run following in target tty:
          > while true; do sleep 1024; done

  sharedlibrary [<regex>]
          Load symbols of shared libs loaded by debugee. Optionally use <regex>
          to filter libs for symbol loading.

  display [/FMT] <expr>
          Print <expr> every time debugee stops. Eg print next instr, see
          examples below.

  undisplay [<num>]
          Delete display expressions either all or one referenced by <num>.

  info display
          List display expressions.
```

## Breakpoints
```markdown
  break [-qualified] <sym> thread <tnum>
          Set a breakpoint only for a specific thread.
          -qualified: Treat <sym> as fully qualified symbol (quiet handy to set
          breakpoints on C symbols in C++ contexts)

  break <sym> if <cond>
          Set conditional breakpoint (see examples below).

  delete [<num>]
          Delete breakpoint either all or one referenced by <num>.

  info break
          List breakpoints.

  cond <bp> <cond>
          Make existing breakpoint <bp> conditional with <cond>.

  cond <bp>
          Remove condition from breakpoint <bp>.

  tbreak
          Set temporary breakpoint, will be deleted when hit.
          Same syntax as `break`.

  rbreak <regex>
          Set breakpoints matching <regex>, where matching internally is done
          on: .*<regex>.*

  command [<bp_list>]
          Define commands to run after breakpoint hit. If <bp_list> is not
          specified attach command to last created breakpoint. Command block
          terminated with 'end' token.

          <bp_list>: Space separates list, eg 'command 2 5-8' to run command
          for breakpoints: 2,5,6,7,8.

  save break <file>
          Save breakpoints to <file>. Can be loaded with the `source` command.
```

## Watchpoints
```markdown
  watch [-location|-l] <expr> [thread <tnum>]
          Create a watchpoint for <expr>, will break if <expr> is written to.
          Watchpoints respect scope of variables, -l can be used to watch the
          memory location instead.

  rwatch ...
          Sets a read watchpoint, will break if <expr> is read from.

  awatch ...
          Sets an access watchpoint, will break if <expr> is written to or read
          from.
```

## Catchpoints
```markdown
  catch load [<regex>]
          Stop when shared libraries are loaded, optionally specify a <regex>
          to stop only on matches.
  catch unload [<regex>]
          Stop when shared libraries are unloaded, optionally specify a <regex>
          to stop only on matches.

  catch throw
          Stop when an exception is thrown.
  catch rethrow
          Stop when an exception is rethrown.
  catch catch
          Stop when an exception is caught.

  catch fork
          Stop at calls to fork (also stops at clones, as some systems
          implement fork via clone).

  catch syscall [<syscall> <syscall> ..]
          Stop at syscall. If no argument is given, stop at all syscalls.
          Optionally give a list of syscalls to stop at.
```

## Inspection
```markdown
  info functions [<regex>]
          List functions matching <regex>. List all functions if no <regex>
          provided.

  info variables [<regex>]
          List variables matching <regex>. List all variables if no <regex>
          provided.

  info register [<reg> <reg> ..]
          Dump content of all registers or only the specified <reg>ister.
```

## Signal handling
```markdown
  info handle [<signal>]
          Print how to handle <signal>. If no <signal> specified print for all
          signals.

  handle <signal> <action>
          Configure how gdb handles <signal> sent to debugee.
          <action>:
            stop/nostop       Catch signal in gdb and break.
            print/noprint     Print message when gdb catches signal.
            pass/nopass       Pass signal down to debugee.

  catch signal <signal>
          Create a catchpoint for <signal>.
```

## Multi-threading
```markdown
info thread
          List all threads.

thread apply <id> [<id>] <command>
          Run command on all threads listed by <id> (space separated list).
          When 'all' is specified as <id> the <command> is run on all threads.

thread name <name>
          The <name> for the current thread.
```

## Multi-process
```markdown
  set follow-fork-mode <child | parent>
          Specify which process to follow when debuggee makes a fork(2)
          syscall.

  set detach-on-fork <on | off>
          Turn on/off detaching from new child processes (on by default).
          Turning this off allows to debug multiple processes (inferiors) with
          one gdb session.

  info inferiors
          List all processes gdb debugs.

  inferior <id>
          Switch to inferior with <id>.
```

## Shell commands
```markdown
  shell <shell_cmd>
          Run the shell_cmd and print the output, can also contain a pipeline.

  pipe <gdb_cmd> | <shell_cmd>
          Evaluate the gdb_cmd and run the shell_cmd which receives the output
          of the gdb_cmd via stdin.
```

## Source file locations
```markdown
  dir <path>
          Add <path> to the beginning of the searh path for source files.

  show dir
          Show current search path.

  set substitute-path <from> <to>
          Add substitution rule checked during source file lookup.

  show substitute-path
          Show current substitution rules.
```

## Configuration

```markdown
  set disassembly-flavor <intel | att>
          Set the disassembly style "flavor".

  set pagination <on | off>
          Turn on/off gdb's pagination.

  set breakpoint pending <on | off | auto>
          on: always set pending breakpoints.
          off: error when trying to set pending breakpoints.
          auto: interatively query user to set breakpoint.

  set print pretty <on | off>
          Turn on/off pertty printing of structures.

  set style enabled <on | off>
          Turn on/off styling (eg colored output).

  set logging <on | off>
          Enable output logging to file (default gdb.txt).

  set logging file <fname>
          Change output log file to <fname>

  set logging redirect <on/off>
          on: only log to file.
          off: log to file and tty.

  set logging overwrite <on/off>
          on: Truncate log file on each run.
          off: Append to logfile (default).
```
> Logging options should be configured before logging is turned on.

# Text user interface (TUI)
```markdown
  C-x a     Toggle UI.
  C-l       Redraw UI (curses UI can be messed up after the debugee prints to
            stdout/stderr).
  C-x o     Change focus.
```

# User commands (macros)

Gdb allows to create & document user commands as follows:
```markdown
  define <cmd>
    # cmds
  end

  document <cmd>
    # docu
  end
```

To get all user commands or documentations one can use:
```markdown
  help user-defined
  help <cmd>
```

# Hooks

Gdb allows to create two types of command `hooks`
- `hook-` will be run before `<cmd>`
- `hookpost-` will be run after `<cmd>`
```markdown
  define hook-<cmd>
    # cmds
  end

  define hookpost-<cmd>
    # cmds
  end
```

# Examples

## Automatically print next instr
When ever the debugee stops automatically print the memory at the current
instruction pointer (`$rip` x86) and format as instruction `/i`.
```markdown
  # rip - x86
  display /i $rip

  # step instruction, after the step the next instruction is automatically printed
  si
```

## Conditional breakpoints
Create conditional breakpoints for a function `void foo(int i)` in the debugee.
```markdown
  # Create conditional breakpoint
  b foo if i == 42

  b foo     # would create bp 2
  # Make existing breakpoint conditional
  cond 2 i == 7
```

## Set breakpoint on all threads except one
Create conditional breakpoint using the `$_thread` [convenience
variable][gdb-convenience-vars].
```markdown
  # Create conditional breakpoint on all threads except thread 12.
  b foo if $_thread != 12
```

## Catch SIGSEGV and execute commands
This creates a `catchpoint` for the `SIGSEGV` signal and attached the `command`
to it.
```markdown
  catch signal SIGSEGV
  command
    bt
    c
  end
```

## Run `backtrace` on thread 1 (batch mode)
```markdown
  gdb --batch -ex 'thread 1' -ex 'bt' -p <pid>
```

## Script gdb for automating debugging sessions
To script gdb add commands into a file and pass it to gdb via `-x`.
For example create `run.gdb`:
```markdown
  set pagination off

  break mmap
  command
    info reg rdi rsi rdx
    bt
    c
  end

  #initial drop
  c
```
This script can be used as:
```markdown
  gdb --batch -x ./run.gdb -p <pid>
```

## Hook to automatically save breakpoints on `quit`
```markdown
define break-save
    save breakpoint $arg0.gdb.bp
end
define break-load
    source $arg0.gdb.bp
end

define hook-quit
    break-save quit
end
```

## Watchpoint on struct / class member
A symbolic watchpoint defined on a member variable for debugging is only valid
as long as the expression is in scope. Once out of scope the watchpoint gets
deleted.

When debugging some memory corruption we want to keep the watchpoint even the
expression goes out of scope to find the location that overrides the variable
and introduces the corruption.

```markdown
(gdb) l
1  struct S { int v; };
2
3  void set(struct S* s, int v) {
4      s->v = v;
5  }
6
7  int main() {
8      struct S s;
9      set(&s, 1);
10     set(&s, 2);
11     set(&s, 3);
...

(gdb) s
set (s=0x7fffffffe594, v=1) at test.c:4
4    s->v = v;

# Define a new watchpoint on the member of the struct. The expression however
# is only valid in the current functions scope.

(gdb) watch s->v
Hardware watchpoint 2: s->v

(gdb) c
Hardware watchpoint 2: s->v
Old value = 0
New value = 1
set (s=0x7fffffffe594, v=1) at test.c:5
5   }

# The watchpoint gets deleted as soon as we leave the function scope.

(gdb) c
Watchpoint 2 deleted because the program has left the block in
which its expression is valid.
main () at test.c:10
10      set(&s, 2);

# Define the watchpoint on the location of the object to watch.

(gdb) watch -l s->v

# This is equivalent to the following.

(gdb) p &s->v
$1 = (int *) 0x7fffffffe594

# Define a watchpoint to the address of the member variable of the s instance.
# This of course only makes sense as long as the s instance is not moved in memory.

(gdb) watch *0x7fffffffe594
Hardware watchpoint 3: *0x7fffffffe594

(gdb) c
Hardware watchpoint 3: *0x7fffffffe594
Old value = 1
New value = 2
set (s=0x7fffffffe594, v=2) at test.c:5
5   }

(gdb) c
Hardware watchpoint 3: *0x7fffffffe594
Old value = 2
New value = 3
set (s=0x7fffffffe594, v=3) at test.c:5
5   }
```

## Shell commands
```markdown
# Run shell commands.

(gdb) shell zcat /proc/config.gz | grep CONFIG_KVM=
CONFIG_KVM=m

# Pipe gdb command to shell command.

(gdb) pipe info proc mapping | grep libc
    0x7ffff7a1a000     0x7ffff7a42000    0x28000        0x0  r--p   /usr/lib/libc.so.6
    0x7ffff7a42000     0x7ffff7b9d000   0x15b000    0x28000  r-xp   /usr/lib/libc.so.6
    0x7ffff7b9d000     0x7ffff7bf2000    0x55000   0x183000  r--p   /usr/lib/libc.so.6
    0x7ffff7bf2000     0x7ffff7bf6000     0x4000   0x1d7000  r--p   /usr/lib/libc.so.6
    0x7ffff7bf6000     0x7ffff7bf8000     0x2000   0x1db000  rw-p   /usr/lib/libc.so.6
```

# Know Bugs

## Workaround `command + finish` bug
When using `finish` inside a `command` block, commands after `finish` are not
executed. To workaround that bug one can create a wrapper function which calls
`finish`.
```markdown
  define handler
    bt
    finish
    info reg rax
  end

  command
    handler
  end
```

[gdb-convenience-vars]: https://sourceware.org/gdb/onlinedocs/gdb/Convenience-Vars.html#Convenience-Vars