Re: [PATCH v2] i386: Avoid avoid rely on linker optimization to avoid relocation

[Fangrui Song <maskray@google.com>]

On 2022-11-17, H.J. Lu wrote:
>On Thu, Nov 17, 2022 at 10:13 AM Adhemerval Zanella
><adhemerval.zanella@linaro.org> wrote:
>>
>> lld does not implement all the linker optimization to avoid the GOT
>> relocation as done by binutils (bfd/elf32-i386.c:elf_i386_convert_load_reloc).
>> The current 'movl main@GOT(%ebx), %eax' will then create a GOT
>> relocation when building with lld, which make static-pie status to
>> not being able to start the provided main function.
>>
>> The change uses a __wrap_main local symbol, which in turn calls main
>> (similar as used by aarch64 and s390x).
>>
>> Checked on i686-linux-gnu with binutils and lld.
>> ---
>>  sysdeps/i386/start.S | 13 +++++++++----
>>  1 file changed, 9 insertions(+), 4 deletions(-)
>>
>> diff --git a/sysdeps/i386/start.S b/sysdeps/i386/start.S
>> index 4ec04bdfd7..23e4f2b012 100644
>> --- a/sysdeps/i386/start.S
>> +++ b/sysdeps/i386/start.S
>> @@ -98,11 +98,10 @@ ENTRY (_start)
>>         pushl main@GOT(%ebx)
>>  # else
>>         /* Avoid relocation in static PIE since _start is called before
>> -          it is relocated.  Don't use "leal main@GOTOFF(%ebx), %eax"
>> -          since main may be in a shared object.  Linker will convert
>> -          "movl main@GOT(%ebx), %eax" to "leal main@GOTOFF(%ebx), %eax"
>> +          it is relocated.  This also avoid rely on linker optimization to
>> +          transform 'movl main@GOT(%ebx), %eax' to 'leal main@GOTOFF(%ebx)'
>>            if main is defined locally.  */
>> -       movl main@GOT(%ebx), %eax
>> +       leal __wrap_main@GOTOFF(%ebx), %eax
>>         pushl %eax
>>  # endif
>>
>> @@ -130,6 +129,12 @@ ENTRY (_start)
>>  1:     movl    (%esp), %ebx
>>         ret
>>  #endif
>> +
>> +#if defined PIC && !defined SHARED
>> +__wrap_main:
>> +       _CET_ENDBR
>> +       jmp     main@PLT
>> +#endif
>>  END (_start)
>>
>>  /* To fulfill the System V/i386 ABI we need this symbol.  Yuck, it's so
>> --
>> 2.34.1
>>
>
>LGTM.
>
>Thanks.

LGTM, too.

Reviewed-by: Fangrui Song <maskray@google.com>

BTW: do you know when start.o/start.os is assembled without -DPIC?

I find that in both --enable-static-pie and --disable-static-pie modes,
both start.o and start.os use -DPIC. start.os uses an additional -DSHARED.

Is the non-`#ifdef __PIC__` path dead?