One day I noticed that my GDB autocomplete stopped working. I had a hunch my recent update had broken something in GDB, so I decided to investigate the internals of GDB autocomplete.
Introducing readline
Readline (https://en.wikipedia.org/wiki/GNU_Readline) is an open source library that provides utilities for interactive terminal programs. Readline provides functionality like history, autocomplete, line editing, vi/emacs modes, etc.
Readline in GDB
void _initialize_completer ();
void
_initialize_completer ()
{
/* Setup some readline completion globals. */
rl_completion_word_break_hook = gdb_completion_word_break_characters;
rl_attempted_completion_function = gdb_rl_attempted_completion_function;
set_rl_completer_word_break_characters (default_word_break_characters ());
/* Setup readline globals relating to filename completion. */
rl_filename_quote_characters = " \t\n\\\"'";
rl_filename_dequoting_function = gdb_completer_file_name_dequote;
rl_filename_quoting_function = gdb_completer_file_name_quote;
rl_directory_rewrite_hook = gdb_completer_directory_rewrite;
add_setshow_zuinteger_unlimited_cmd ("max-completions", no_class,
&max_completions, _("\
Set maximum number of completion candidates."), _("\
Show maximum number of completion candidates."), _("\
Use this to limit the number of candidates considered\n\
during completion. Specifying \"unlimited\" or -1\n\
disables limiting. Note that setting either no limit or\n\
a very large limit can make completion slow."),
NULL, NULL, &setlist, &showlist);
}The code shown above (gdb/completer.c) is where GDB initializes readline and sets up the completion function. All of the rl_* variables are global variables that exist inside the readline shared library. This means that if multiple different systems in the same program try to use readline at the same time, they will override each other.
Debugging GDB
How do you debug GDB? With another instance GDB of course!
Using gdb -ex 'set follow-fork-mode parent' -ex 'b _initialize_completer' --args /usr/bin/gdb to debug GDB with GDB. We can see that _initialize_completer is called successfully, so autocomplete is initialized properly. That means somewhere after autocomplete initialization something else is overriding GDB’s autocomplete.
Printing out the value of rl_attempt_completion_function shows that it has been overriden:
gef> p rl_attempted_completion_function
$1 = (rl_completion_func_t *) 0x7fffc8d8e947 <flex_complete>WTF is flex_complete and where is it from?
gef> xinfo flex_complete
-------------------------------------- xinfo: 0x7fffc8d8e947 --------------------------------------
[ Legend: Code | Heap | Stack | Writable | ReadOnly | None | RWX ]
Start End Size Offset Perm Path
0x00007fffc8d8e000 0x00007fffc8d90000 0x0000000000002000 0x0000000000003000 r-x /usr/lib/python3.13/lib-dynload/readline.cpython-313-x86_64-linux-gnu.so
Offset (from mapped): 0x7fffc8d8e000 + 0x947
Offset (from base): 0x7fffc8d8b000 + 0x3947
Offset (from segment): 0x7fffc8d8e020 (.text) + 0x927
Symbol: <flex_complete>
Inode: 24215410So turns out that it is an issue with python. Some random module is loading the python readline module and overriding the GDB autocomplete. It was at this point that I stumbled across a recent pwndbg github issue discussing the exact problem I had been facing.
https://github.com/pwndbg/pwndbg/issues/2232
Turns out that GDB already has a mechanism to disable python readline (here: https://github.com/bminor/binutils-gdb/blob/62e4d4d3ad68fe17113069b99d80a9ee9df87cb1/gdb/python/py-gdb-readline.c#L98-L111), but updating to python 3.13 broke this mechanism and messed up autocomplete.
To restore proper GDB autocomplete I have added a line to the top of my .gdbinit that sources a script to disable readline again:
import sys
import importlib.abc
# Fix gdb readline bug: https://github.com/pwndbg/pwndbg/issues/2232#issuecomment-25425645
class GdbRemoveReadlineFinder(importlib.abc.MetaPathFinder):
def find_spec(self, fullname, path=None, target=None):
if fullname == "readline":
raise ImportError("readline module disabled under GDB")
return None
sys.meta_path.insert(0, GdbRemoveReadlineFinder())