Re: Undefined use of weak symbols in gnulib

[Fangrui Song]

On 2021-04-27, H.J. Lu via Binutils wrote:
> 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

I don't consider the first two bugs.
Whether a dynamic relocation is emitted depends on
(1) whether .dynsym exists (2) architecture (3) relocation type (4)
-no-pie/-pie/-shared (5) -z {,no}dynamic-undefined-weak.

It is unlikely a user can summarize rules which can be relied upon.

We can step back and look at these from a different perspective:
figure out what should be defined, then everything else has no hard rule
and we can choose whatever to simplify rules.

* absolute relocation resolves to 0. There may or may not be dynamic 
relocations.
* PC-relative relocation doesn't make sense.



I changed LLD to use a simple rule:

* -no-pie and -pie: no dynamic relocation
* -shared: dynamic relocation

https://maskray.me/blog/2021-04-25-weak-symbol

> > 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