Re: [PATCH 0/3] Improve lld support and current status

[Fangrui Song <maskray@google.com>]

On 2021-10-27, Tulio Magno Quites Machado Filho wrote:
>Fangrui Song via Libc-alpha <libc-alpha@sourceware.org> writes:
>
>> On 2021-10-26, Adhemerval Zanella wrote:
>>>The powerpc64le fails at libc.so start:
>>>
>>>  (gdb) bt
>>>  #0  0x00007ffff7f2a980 in __gep_setup___vmx__sigjmp_save ()
>>>    from
>>>  /home/azanella/glibc/build/powerpc64le-linux-gnu-power9-lld/libc.so.6
>>>  #1  0x00007ffff7d7ac5c in __libc_start_call_main ()
>>>     from
>>>  /home/azanella/glibc/build/powerpc64le-linux-gnu-power9-lld/libc.so.6
>
>This is a local call from libc.so to libc.so.
>It should be using the local entry point for __vmx__sigjmp_save.
>
>>>  #2  0x00007ffff7d7ae90 in __libc_start_main_impl ()
>>>     from
>>>  /home/azanella/glibc/build/powerpc64le-linux-gnu-power9-lld/libc.so.6
>>>  #3  0x0000000000000000 in ?? ()
>>>  (gdb) disas
>>>  Dump of assembler code for function __gep_setup___vmx__sigjmp_save:
>>>  => 0x00007ffff7f2a980 <+0>:     .long 0x613ffe6
>>>     0x00007ffff7f2a984 <+4>:     li      r12,-1280
>
>This is a pla, but this GDB isn't able to disassemble it.  This instruction
>shouldn't be used unless when configuring glibc using --with-cpu=power10.
>
>>>And powerpc64-linux-gnu fails on dynamic loaders start:
>>>
>>>  (gdb) bt
>>>  #0  0x00007ffff7ffa9a8 in _start ()
>>>  (gdb) disas
>>>  Dump of assembler code for function _start:
>>>  => 0x00007ffff7ffa9a8 <+0>:     .long 0x0
>>>     0x00007ffff7ffa9ac <+4>:     .long 0x0
>>>     0x00007ffff7ffa9b0 <+8>:     .long 0x0
>>>     0x00007ffff7ffa9b4 <+12>:    .long 0x0
>
>I've been told that LLD doesn't support the powerpc ELFv1 yet.

You are right:) The incomplete PowerPC64 ELF v1 support was removed by
https://reviews.llvm.org/D46316 (2018-05).  Is there remaining ELF v1
usage for powerpc64le in glibc?

(I only know some 64-bit ELF v2 and some 32-bit, but know really little about 64-bit ELF v1.)

Legacy ELF v1 is also the issue that powerpc64 big-endian Linux kernel
cannot be linked with LLD (https://github.com/ClangBuiltLinux/linux/issues/602).

There is little benefit retroactively adding ELF v1 support to LLD, so
migration to ELF v2 will be nice :)

>> CCed the powerpc maintainer.
>
>Thanks!
>
>-- 
>Tulio Magno