pwndbg: Pwndbg Hangs with gdbserver on High Thread Count Programs

Description

Pwndbg gets stuck when used with gdbserver when debugging a program with 40+ threads, an example of which is Chromium running on Android.

Steps to reproduce

debug chromium apk on android

My setup

pwndbg commit hash: 13f467b0246367548a7fd9fd952391978ec1d47c

Gdb: 13.2 Python: 3.10.12 (main, Nov 20 2023, 15:14:05) [GCC 11.4.0] Pwndbg: 2023.07.17 build: 13f467b Capstone: 5.0.1280 Unicorn: 2.0.1

We have verified that this is not the issue with GDB.

The issue seems to be located in the update() function in the pwndbg/gdblib/stack.py file. This function takes a significantly long time to process when there is a high number of threads as it tries to find the stack boundary for each one.

When enabling the debug feature, we can see that Pwndbg gets hung up on the “stop” event for thread processing as shown here:

...
'obj' pwndbg.gdblib.hooks.reset_config (<gdb.NewObjFileEvent object at 0x7fde7a41bb10>,)
'obj' pwndbg.lib.cache.clear (<gdb.NewObjFileEvent object at 0x7fde7a41bb10>,)
0x0000007984eeab68 in __epoll_pwait () from /home/dev/android-solibs/system/lib64/libc.so
'stop' pwndbg.gdblib.events._start_stop (<gdb.StopEvent object at 0x7fde7a3fa2f0>,)
'stop' pwndbg.gdblib.regs.update_last (<gdb.StopEvent object at 0x7fde7a3fa2f0>,)
'stop' pwndbg.gdblib.stack.update (<gdb.StopEvent object at 0x7fde7a3fa2f0>,)

It appears that the current method of enumerating all thread information for context is inefficient for the case of debugging applications with a high number of threads. This should be more on-demand or limited to only showing information about the current thread.

Relevant Code:

@pwndbg.gdblib.events.stop
@pwndbg.lib.cache.cache_until("stop")
def update() -> None:
    """
    For each running thread, updates the known address range
    for its stack.
    """
    curr_thread = gdb.selected_thread()
    try:
    ...

Proposed Solution:

A potential solution to reduce processing time could be to only display thread information on-demand or solely display information about the current thread.