[Bug ld/26762] New: -Wl, --as-needed removes strong symbols in favour of weak symbols

[slyfox at inbox dot ru]

https://sourceware.org/bugzilla/show_bug.cgi?id=26762

            Bug ID: 26762
           Summary: -Wl,--as-needed removes strong symbols in favour of
                    weak symbols
           Product: binutils
           Version: 2.34
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: ld
          Assignee: unassigned at sourceware dot org
          Reporter: slyfox at inbox dot ru
                CC: ccoutant at gmail dot com
  Target Milestone: ---

Original bug was a failed attempt to build llvm-11 with LDFLAGS="-flto
-Wl,--as-needed": https://bugs.gentoo.org/749162.

There std::call_once is implemented via weak pthread_once() symbol:
https://gcc.gnu.org/PR55394. Presence of -Wl,--as-needed removes libpthread.so
and breaks the program at runtime as pthread_once() has to have a real
implementation. gcc plans to fix std::call_once by avoiding pthread_once() for
unrelated reason (solving it will take a while).

But the problem seems to be more general when local weak symbols are used to
probe presence of a real implementation dynamically.

What do you think of changing 'ld' to prefer strong symbols from shared
libraries and keeping DT_NEEDED entries even if local object files define weak
symbols?

Toy example:

$ cat a.c
// gcc a.c -o a -Wl,--as-needed -pthread && ./a
#include <stdio.h>
#include <pthread.h>

static pthread_once_t once = PTHREAD_ONCE_INIT;

static __typeof(pthread_once) __gthrw_pthread_once __attribute__
((__weakref__("pthread_once"), __copy__ (pthread_once)));

static void ir(void)
{
    printf("foo\n");
}

int main(void)
{
    if (__gthrw_pthread_once)
        __gthrw_pthread_once(&once, &ir);
}
$ gcc a.c -o a -Wl,--as-needed -pthread -fuse-ld=bfd && ./a
$ gcc a.c -o a -Wl,--as-needed -pthread -fuse-ld=gold && ./a
$ gcc a.c -o a -Wl,--as-needed -pthread -fuse-ld=lld && ./a

$ gcc a.c -o a -Wl,--no-as-needed -pthread -fuse-ld=bfd && ./a
foo

Ideally all 4 tests should call pthread_once from libpthread.so.0 instead of
calling local weak reference.

Thanks!

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