look for pthread_atfork() also in lib c_nonshared

[DerDakon]

Newer versions of glibc do not have pthread_nonshared anymore, instead the symbol is in c_nonshared. This fixes a crash on start on those platforms (namely with syslog-ng).

The fix was inspired by #15 (comment).

[hydrapolic]

We added this to Gentoo Linux and it fixed the segfault on ppc64 - https://bugs.gentoo.org/675338.

[hbakken]

The fix from the pull request seems to fix this, and syslog-ng is (understandably) not very keen on pulling an ivykis fix which isn't upstream. Is there any chance of getting this pulled soon?

[DerDakon]

ping?

[buytenh]

@hydrapolic @hbakken @DerDakon Just for my information, do you also get
this segmentation fault if you build and install ivykis from an ivykis
source tarball and then build syslog-ng with --with-ivykis=system ?

[hbakken]

@hydrapolic @hbakken @DerDakon Just for my information, do you also get
this segmentation fault if you build and install ivykis from an ivykis
source tarball and then build syslog-ng with --with-ivykis=system ?

Yes, unless the above-mentioned patch is applied to ivykis, it still crashes.

[buytenh]

@hbakken OK, thank you for testing. Could you please upload a build
log for that somewhere?

(My only aarch64 environment is emulated via QEMU and everything
there is painfully slow.)

[hbakken]

Uh, sorry, I haven't tested in a while, and I need to build a new os base for my embedded system to trigger the bug, which takes a bit, so that's going to be a bit of a hassle.
The fix seems rather straight-forward to me, though. We search for a function in the wrong library, fails to find it, thus crashing.

[hbakken]

...so it seems I need to clarify my previous comment: That was the case back on April 1st. If any of the commits on ivykis since then has fixed this, I haven't tested it. I did, however, try to build syslog-ng against ivykis as of ~1 Apr, and it failed.

[MrAnno]

I have a reproduction of this issue on a public arm64 server (without syslog-ng).
@buytenh I've sent you SSH access via email.

Basically, the following example program and glibc 2.28 are needed:

#define _POSIX_C_SOURCE 200809L
#include <iv.h>

int main()
{
  iv_init();
  iv_main();
  iv_deinit();
}

$ gcc -std=c99 -Wall -Wextra -Werror -g -pthread ivykis-reprod.c -o ivykis-reprod -livykis

#0  0x0000000000000000 in ?? ()
#1  0x0000ffffb7fb3654 in pthr_atfork (child=<optimized out>, parent=<optimized out>, prepare=<optimized out>) at pthr.h:76
#2  iv_signal_init () at iv_signal.c:114

$ nm -g /usr/lib/aarch64-linux-gnu/libc_nonshared.a | grep atfork
pthread_atfork.oS:
0000000000000000 T __pthread_atfork
0000000000000000 W pthread_atfork
                 U __register_atfork

[buytenh]

I think I understand this a bit better now (see the update on issue #15).

It seems that we don't even need to pull in lib*_nonshared.a by
hand (which is what AC_CHECK_LIB() does, it adds the library you're
checking for to LIBS if it is found), as this patch, which only
defines HAVE_* but doesn't pull in any libraries, also seems to do
the trick for me:

diff --git a/configure.ac b/configure.ac
index e9b10c0..4f560fc 100644
--- a/configure.ac
+++ b/configure.ac
@@ -87,7 +87,8 @@ esac
 # libpthread_nonshared.a without pulling in libpthread.so.0, explicitly
 # link in libpthread_nonshared.a if it is available.
 #
-AC_CHECK_LIB([pthread_nonshared], [pthread_atfork])
+AC_CHECK_LIB([c_nonshared], [pthread_atfork], [AC_DEFINE(HAVE_LIBC_NONSHARED, 1, Blah)])
+AC_CHECK_LIB([pthread_nonshared], [pthread_atfork], [AC_DEFINE(HAVE_LIBPTHREAD_NONSHARED, 1, Blah)])

 # Checks for header files.
 AC_CHECK_HEADERS([process.h])
diff --git a/src/pthr.h b/src/pthr.h
index a41eaf3..a1807d3 100644
--- a/src/pthr.h
+++ b/src/pthr.h
@@ -42,7 +42,7 @@ static inline int pthreads_available(void)
  * symbol because that causes it to be undefined even if you link
  * libpthread_nonshared.a in explicitly.
  */
-#ifndef HAVE_LIBPTHREAD_NONSHARED
+#if !defined(HAVE_LIBC_NONSHARED) && !defined(HAVE_LIBPTHREAD_NONSHARED)
 #pragma weak pthread_atfork
 #endif

That is, merely the not marking of pthread_atfork as weak if it is
provided by one of two archive libraries seems to be sufficient to fix
this issue, as per #15 (comment) .

[buytenh]

Does https://github.com/buytenh/ivykis/commits/libc-nonshared work for you?

[buytenh]

I've had positive feedback for this fix, so I've merged it as commit
a5e9cad ("Avoid marking pthread_atfork() as weak if provided by
libc_nonshared.a."), and I have released ivykis v0.42.4 with this fix
included.

Thanks again to all involved!

[hydrapolic]

Thank you