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From: "jimb@red-bean.com" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug dynamic-link/31285] New: Segfault when returning from main
 while a thread calls dlclose
Date: Wed, 24 Jan 2024 03:19:18 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
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X-Bugzilla-Product: glibc
X-Bugzilla-Component: dynamic-link
X-Bugzilla-Version: 2.38
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: jimb@red-bean.com
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X-Bugzilla-Priority: P2
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D31285

            Bug ID: 31285
           Summary: Segfault when returning from main while a thread calls
                    dlclose
           Product: glibc
           Version: 2.38
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: dynamic-link
          Assignee: unassigned at sourceware dot org
          Reporter: jimb@red-bean.com
  Target Milestone: ---

If the program's main thread returns while another thread calls dlclose on a
shared library that has a static variable with a destructor, dlclose may un=
map
the shared library's code while the main thread is still executing the
destructor, causing a crash.

This can manifest as intermittent crashes at exit, as in this issue:
https://github.com/gfx-rs/wgpu/issues/5084

The bug is that if the main thread calls `__run_exit_handlers` and finds the
shared library's destructor's entry in __exit_funcs` first, changes its fla=
vor
to `ef_free`, releases `__exit_funcs_lock`, and then calls the entry's
function, that function is arbitrary code which may take a long time to run.
While it does so, another thread may `dlclose` the shared library, skip past
the entry in `__exit_funcs` since it is marked as `ef_free`, and proceed to
unmap the shared library's code while the main thread is still working.

The attached test case reproduces the bug reliably. To reproduce, download =
the
attached test case as `dlclose-crash.tar.gz`, and then:

```
$ tar xf dlclose-crash.tar.gz=20
$ cd dlclose-crash/
$ make
g++ -g -export-dynamic main.cpp -o main
g++ -g  -shared -fPIC solib.cpp -o solib.so
$ ./main
Segmentation fault (core dumped)
$=20
```

--=20
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