[Bug libdw/30272] New: Unwinding multithreaded musl applications fails

["godlygeek at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30272

            Bug ID: 30272
           Summary: Unwinding multithreaded musl applications fails
           Product: elfutils
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libdw
          Assignee: unassigned at sourceware dot org
          Reporter: godlygeek at gmail dot com
                CC: elfutils-devel at sourceware dot org
  Target Milestone: ---

Unwinding multithreaded applications linked against musl libc on x86-64 seems
to fail, getting stuck on `__clone`:

TID 241:
...
#20 0x00007f6f2f74f08b start
#21 0x00007f6f2f75138e __clone
#22 0x00007f6f2f75138e __clone
#23 0x00007f6f2f75138e __clone
...
#253 0x00007f6f2f75138e __clone
#254 0x00007f6f2f75138e __clone
#255 0x00007f6f2f75138e __clone
eu-stack: tid 241: shown max number of frames (256, use -n 0 for unlimited)


GDB seems to detect the condition that libdw is getting stuck on, emitting a
warning message:

#44 0x00007f8f83e4d08b in start (p=0x7f8f836b8b00) at
src/thread/pthread_create.c:203
#45 0x00007f8f83e4f38e in __clone () at src/thread/x86_64/clone.s:22
Backtrace stopped: frame did not save the PC

I believe it's detecting that two frames in a row have the same DWARF CFA, if I
understand correctly.


Reproducer:

docker run -it --privileged python:3.10-alpine sh

And in the container:

apk add --update musl-dbg elfutils
python3.10 -c "import os, threading; threading.Thread(target=lambda:
os.system(f'eu-stack --pid={os.getpid()}')).start()"

That spawns a thread that forks a subprocess that runs `eu-stack` on its
parent, and reproduces the issue. If you remove the thread and just run:

python3.10 -c "import os; os.system(f'eu-stack --pid={os.getpid()}')"

then unwinding succeeds, ending at `_start`.

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