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Re: Undefined use of weak symbols in gnulib


From: H.J. Lu
Subject: Re: Undefined use of weak symbols in gnulib
Date: Tue, 27 Apr 2021 19:13:27 -0700

On Tue, Apr 27, 2021 at 7:10 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Tue, Apr 27, 2021 at 6:57 PM Bruno Haible <bruno@clisp.org> wrote:
> >
> > Hi Florian,
> >
> > > Here's a fairly representative test case, I think.
> > >
> > > #include <pthread.h>
> > > #include <stdio.h>
> > >
> > > extern __typeof (pthread_key_create) __pthread_key_create __attribute__ 
> > > ((weak));
> > > extern __typeof (pthread_once) pthread_once __attribute__ ((weak));
> > >
> > > void
> > > f1 (void)
> > > {
> > >   puts ("f1 called");
> > > }
> > >
> > > pthread_once_t once_var;
> > >
> > > void __attribute__ ((weak))
> > > f2 (void)
> > > {
> > >   if (__pthread_key_create != NULL)
> > >     pthread_once (&once_var, f1);
> > > }
> > >
> > > int
> > > main (void)
> > > {
> > >   f2 ();
> > > }
> > >
> > > Building it with “gcc -O2 -fpie -pie” and linking with binutils 2.30
> > > does not result in a crash with LD_PRELOAD=libpthread.so.0.
> >
> > Thank you for the test case. It helps the understanding.
> >
> > But I don't understand
> >   - why anyone would redeclare 'pthread_once', when it's a standard POSIX
> >     function,
> >   - why f2 is declared weak,
> >   - why the program skips its initializations in single-threaded mode,
> >   - why libpthread would be loaded through LD_PRELOAD or dlopen, given
> >     that the long-term statement has been that declaring a symbol weak
> >     has no effect on the dynamic linker [1][2][3][4]?
> >
> > How about the following test case instead?
> >
> > =====================================================================
> > #include <pthread.h>
> > #include <stdio.h>
> >
> > #pragma weak pthread_key_create
> > #pragma weak pthread_once
> >
> > void
> > do_init (void)
> > {
> >   puts ("initialization code");
> > }
> >
> > pthread_once_t once_var;
> >
> > void
> > init (void)
> > {
> >   if (pthread_key_create != NULL)
> >     {
> >       puts ("multi-threaded initialization");
> >       pthread_once (&once_var, do_init);
> >     }
> >   else
> >     do_init ();
> > }
> >
> > int
> > main (void)
> > {
> >   init ();
> > }
> > =====================================================================
> >
> > $ gcc -Wall -fpie -pie foo.c ; ./a.out
> > initialization code
> >
> > $ gcc -Wall -fpie -pie foo.c -Wl,--no-as-needed -lpthread ; ./a.out
> > multi-threaded initialization
> > initialization code
> >
> > What will change for this program with glibc 2.34?
> >
> > Bruno
> >
> > [1] https://sourceware.org/legacy-ml/libc-hacker/2000-06/msg00029.html
> > [2] https://www.akkadia.org/drepper/dsohowto.pdf page 6
> > [3] 
> > https://stackoverflow.com/questions/21092601/is-pthread-in-glibc-so-implemented-by-weak-symbol-to-provide-pthread-stub-functi/21103255
> > [4] 
> > https://stackoverflow.com/questions/20658809/dynamic-loading-and-weak-symbol-resolution
> >
>
> Does x86 show the same issue?  I fixed several undefined weak symbol
> bugs on x86:
>
> https://sourceware.org/bugzilla/show_bug.cgi?id=19636
> https://sourceware.org/bugzilla/show_bug.cgi?id=19704
> https://sourceware.org/bugzilla/show_bug.cgi?id=19719
>
> with a linker option:
>
>     'dynamic-undefined-weak'
>      'nodynamic-undefined-weak'
>           Make undefined weak symbols dynamic when building a dynamic
>           object, if they are referenced from a regular object file and
>           not forced local by symbol visibility or versioning.  Do not
>           make them dynamic if 'nodynamic-undefined-weak'.  If neither
>           option is given, a target may default to either option being
>           in force, or make some other selection of undefined weak
>           symbols dynamic.  Not all targets support these options.
>
> Alan extended the fix to PPC:
>
> commit 954b63d4c8645f86e40c7ef6c6d60acd2bf019de
> Author: Alan Modra <amodra@gmail.com>
> Date:   Wed Apr 19 01:26:57 2017 +0930
>
>     Implement -z dynamic-undefined-weak
>
>     -z nodynamic-undefined-weak is only implemented for x86.  (The sparc
>     backend has some support code but doesn't enable the option by
>     including ld/emulparams/dynamic_undefined_weak.sh, and since the
>     support looks like it may be broken I haven't enabled it.)  This patch
>     adds the complementary -z dynamic-undefined-weak, extends both options
>     to affect building of shared libraries as well as executables, and
>     adds support for the option on powerpc.
>

Another undefined weak symbol linker bug:

https://sourceware.org/bugzilla/show_bug.cgi?id=22269

-- 
H.J.



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