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bug #41576: weechat hangs/crashes if libpthread.so.0 is loaded after startup and GnuTLS is used

Submitter:  Johannes Nixdorf <mixi>
Submitted:  Thu 13 Feb 2014 10:56:47 AM UTC
Category:  compilation Severity:  3 - Normal
Item Group:  crash Status:  Fixed
Privacy:  Public Assigned to:  flashcode
Originator Name:  Open/Closed:  Closed
Release:  * 0.4.3 IRC nick:  mixi
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Thu 13 Feb 2014 03:49:35 PM UTC, comment #3: 

I can confirm this fix works fine here.

Johannes Nixdorf <mixi>
Thu 13 Feb 2014 03:11:11 PM UTC, comment #2: 

Committed (c324610226cef15ecfb1235113c8243b068084c8).

Johannes, could you please confirm if this fix is working fine for you? Thank you.

Sebastien Helleu <flashcode>
Group administrator
Thu 13 Feb 2014 02:11:45 PM UTC, comment #1: 

Yes, I can concur that this is indeed what's happening, and that the patch I've posted fixes the issue.

FlashCode asked for more explanation, so here goes:

The CMakeLists.txt has a section in it that only adds  “-lpthread” on OpenBSD platforms. For all other platforms, libpthread is not linked.

However, gnutls uses libpthread. The glibc developers were clever and set things up such that if you don't explicitly link against libpthread, and library code still uses the functions from it, you'll instead wind up using functions that all return 0. This is so that single-threaded programs aren't burdened with the overhead of mutexes and such, when they aren't needed.

So when weechat was loaded, it would also load gnutls. Gnutls would then make several calls to pthread_mutex_init. Since libpthread wasn't loaded, this function would be hitting the code inside libc, which would simply return 0. So the mutex would never be initialized and instead it would contain uninitialized junk from malloc(). This was fine, since all the other pthread functions that libc implements do the same – return 0 – so nothing bad happens since that data is never touched.

However, later in the weechat initialization, the python plugin is loaded. This plugin directly or indirectly links in the proper libpthread. This means that after this point, all function calls to pthread_mutex_lock and pthread_mutex_unlock are hitting the real pthread code, that actually works with the data and does a lot more than simply return 0. But because these mutexes were not initialized before with the zero-returning pthread_mutex_init, the lock and unlock functions are dealing with uninitialized random malloc() data. And so, in lots of cases, we crash, or abort().

My patch simply removes the “if OpenBSD” guard and links against libpthread always.

Jason A. Donenfeld <zx2c4>
Thu 13 Feb 2014 10:56:47 AM UTC, original submission:  

GnuTLS sets up mutexes with pthread_mutex_init before libpthread.so.0 is loaded. This works because glibc provides stubbed pthread functions, which are used when libpthread.so.0 is not loaded. Because some weechat plugins link to libpthread.so.0, it is loaded after the mutex is thought to be initialized with pthread_mutex_init (which is actually a stub, because the libpthread.so.0 wasn't loaded) and later calls to e.g. pthread_mutex_lock/pthread_mutex_unlock use code from libpthread.so.0, which assumes the mutex was properly initialized. This causes weechat to crash in some cases and hang in pthread_mutex_lock in other cases (as shown in the attached file).

This is not a problem when compiling weechat with autotools, because it links the weechat binary with libpthread.so.0. With cmake this only happens on openbsd.

Johannes Nixdorf <mixi>


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Attach Files:

Attached Files
file #30534:  0001-Link-against-pthread-on-all-platforms.patch added by None (913B - text/x-patch - This fixes the problem.)
file #30533:  weechat_bt_hang.txt added by mixi (3KiB - text/plain)


Depends on the following items: None found

Items that depend on this one: None found


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  • -email is unavailable- added by flashcode (Posted a comment)
  • -email is unavailable- added by zx2c4 (Posted a comment)
  • -email is unavailable- added by mixi (Submitted the item)

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    Follow 7 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2014-08-15 flashcode Open/ClosedOpen Closed
    2014-02-13 flashcode Categoryother compilation
        Item Groupother crash
        StatusNone Fixed
        Assigned toNone flashcode
    2014-02-13 None Attached File- Added 0001-Link-against-pthread-on-all-platforms.patch, #30534
    2014-02-13 mixi Attached File- Added weechat_bt_hang.txt, #30533

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