Hi, There is a case when GDB deadlocks or hang indefinetely when a breakpoint is hit and: 1) the breakpoint has attached commands and when one of the commands is `continue` 2) a stop event hook was registered before with a Python script and that hook executes a `gdb.execute` function This issue is painful for automating GDB and hits many GDB users that use plugins written in Python (see [0]). I would appreciate if it is fixed sooner than later, or, if you can provide any information that may be helpful for fixing or diagnosing this further. The bug can be reproduced by doing the following steps: ``` $ echo 'void foo(){}; int main() {foo();}' > example.c && gcc example.c $ cat gdbs.py import gdb def stop_handler (event): print("Python stop handler started") print("Thread is stopped: " + str(gdb.selected_thread().is_stopped())) print("Thread is running: " + str(gdb.selected_thread().is_running())) (gdb.execute("",to_string=True)) print("Thread is stopped: " + str(gdb.selected_thread().is_stopped())) print("Thread is running: " + str(gdb.selected_thread().is_running())) print("Python Stop handler finished\n") gdb.events.stop.connect (stop_handler) $ gdb --quiet --nx ./a.out Reading symbols from ./a.out... (No debugging symbols found in ./a.out) (gdb) starti Starting program: /home/dc/a.out Program stopped. 0x00007ffff7dd4090 in _start () from /lib64/ld-linux-x86-64.so.2 (gdb) break foo Breakpoint 1 at 0x5555555545fe (gdb) commands Type commands for breakpoint(s) 1, one per line. End with a line saying just "end". >continue >end (gdb) source gdbs.py (gdb) c Continuing. Breakpoint 1, 0x00005555555545fe in foo () Python stop handler started Thread is stopped: True Thread is running: False Thread is stopped: False Thread is running: True Python Stop handler finished ``` I was able to reproduce this bug with the following GDB versions: - 10.2-0ubuntu1~18.04~2 - 12.1 compiled from sources (../configure && make -j) [0] https://github.com/pwndbg/pwndbg/issues/425
This can be worked around in the following way. However, this is not a good solution since this means one can't script GDB using both its native CLI commands and Python at the same time. ``` import gdb def stop_handler (event): print("Python stop handler started") print("Thread is stopped: " + str(gdb.selected_thread().is_stopped())) print("Thread is running: " + str(gdb.selected_thread().is_running())) (gdb.execute("",to_string=True)) print("Thread is stopped: " + str(gdb.selected_thread().is_stopped())) print("Thread is running: " + str(gdb.selected_thread().is_running())) print("Python Stop handler finished\n") gdb.events.stop.connect (stop_handler) class Bp(gdb.Breakpoint): def stop(self): print("Breakpoint hit!") return True Bp("foo") ```
Created attachment 14819 [details] Possible patch for this problem. This patch seems to address the problem reported in this bug. I think there might be other bugs in this area though -- while I was testing this I hit some other strange behaviours that I still need to look into. Also I need to write some actual tests before this could be posted to the m/l.