Re: [PATCH 0/3] Allow LLD 13.0.0 and improve compatibility with gold and clang

["Fāng-ruì Sòng via Libc-alpha" <libc-alpha@sourceware.org>]

On 2021-07-30, Fāng-ruì Sòng wrote:
>On Fri, Jul 30, 2021 at 12:57 AM Florian Weimer <fweimer@redhat.com> wrote:
>>
>> * Fangrui Song via Libc-alpha:
>>
>> > For malloc/tst-compathooks-on,
>> >
>> >     malloc/tst-compathooks-on: Symbol `__free_hook' has different size in shared object, consider re-linking
>> >
>> > the root cause is that lld's symbol versioning is different from GNU ld in an unusal case:
>> >
>> >     __asm__ (".symver " "__free_hook" "," "__free_hook" "@" "GLIBC_2.2.5");
>> >
>> > This leaves two symbols __free_hook and __free_hook@GLIBC_2.2.5.
>> > __free_hook is then attached a default version GLIBC_2.2.5.
>> > I think malloc/malloc-debug.c uses a fragile versioned symbol here.
>> > If the inline asm uses @@ the failure should go away.
>>
>> But we want to produce a compat symbol here.  With the current version
>> scripts, BFD ld will not export a symbol unless it is listed in the
>> version script.  That is, if I remove __free_hook from libc_malloc_debug
>> in malloc/Versions, I get an ABI check failure:
>>
>> --- ../sysdeps/unix/sysv/linux/x86_64/64/libc_malloc_debug.abilist      2021-07-27 16:14:51.516781791 +0200
>> +++ …/malloc/libc_malloc_debug.symlist  2021-07-30 09:55:09.818875449 +0200
>> @@ -3 +2,0 @@ GLIBC_2.16 aligned_alloc F
>> -GLIBC_2.2.5 __free_hook D 0x8
>>
>> If this works with a linker, it appears to ignore “local: *;” in version
>> nodes for versioned symbols.  That looks like a linker bug.
>>
>> Thanks,
>> Florian
>>
>
>I have a comment on https://sourceware.org/bugzilla/show_bug.cgi?id=23328#c6
>
>How does removing __free_hook from malloc/Versions break the ABI check test?
>
>% cat a.s
>.symver __free_hook, __free_hook@GLIBC_2.2.5
>.globl __free_hook
>__free_hook:
>  nop
>% cat a.ver
>GLIBC_2.2.5 {};  /* should not list the non-default version __free_hook */
>local { local: *; };
>% cc -c a.s
>% ld.bfd -shared --version-script=a.ver a.o -o a.so
>% readelf -W --dyn- a.so | grep free_hook
>     3: 0000000000001000     0 NOTYPE  GLOBAL DEFAULT    7
>__free_hook@GLIBC_2.2.5
>
>One non-default version symbol, as expected.

I'll implement the behavior in https://reviews.llvm.org/D107234 and
https://reviews.llvm.org/D107235

ld.lld linked libc.so has exactly the same set of non-SHN_ABS dynamic
symbols as ld.bfd linked libc.so.
(ld.bfd places version nodes into SHN_ABS symbols but they are unused by ld.so)