[PATCH] D30699: [ELF] - Stop producing broken output for R_386_GOT32X relocation.

[H.J. Lu via llvm-commits]

On Thu, Mar 30, 2017 at 1:49 PM, Rafael Espíndola
<rafael.espindola at gmail.com> wrote:
>> "name at GOT" is a memory operand.  It is usually computed as "G + A" to
>> get the GOT address.   If it is computed as G + A - GOT,
>>
>> op name at GOT, %reg
>>
>> will lead to random result since G + A - GOT is the GOT index, not address.
>> LEA is an exception, relocation in
>
> I can't reproduce this behavior with gnu tools.
>
> Given non-pic.s with just
>
> mov _start at GOT, %eax
>
> gas will produce a R_386_GOT32X even with --mrelax-relocations=no, so
> I used llvm-mc instead to get a R_386_GOT32.
>
> Then linking as
>
> ld.bfd -m elf_i386 start.o non-pic.o -o non-pic
>
> will produce
>
> 08048094 <_start>:
>  8048094:       a1 fc ff ff ff          mov    0xfffffffc,%eax
>
> So looks like it is computing G + A - GOT.
>

Relocate

mov _start at GOT, %eax

as G + A - GOT leads to random results.  Since compiler never generates
such sequence before, it never caused the real problem.  The updated
i386 psABI changes it to G + A so that it can be used to access GOT
without base register when PIC is off.  Ld in binutils 2.28 generates:

08048074 <_start>:
 8048074: c7 c0 74 80 04 08     mov    $0x8048074,%eax

as the GOT entry for _start contains the address of _start.

-- 
H.J.