Re: Undefined use of weak symbols in gnulib

[Fangrui Song <i@maskray.me>]

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