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

[H.J. Lu via llvm-commits]

On Thu, Mar 30, 2017 at 3:13 PM, H.J. Lu <hjl.tools at gmail.com> wrote:
> On Thu, Mar 30, 2017 at 3:01 PM, Rafael Espíndola
> <rafael.espindola at gmail.com> wrote:
>> On 30 March 2017 at 17:59, H.J. Lu <hjl.tools at gmail.com> wrote:
>>> On Thu, Mar 30, 2017 at 2:52 PM, Rafael Espíndola
>>> <rafael.espindola at gmail.com> wrote:
>>>> On 30 March 2017 at 17:45, H.J. Lu <hjl.tools at gmail.com> wrote:
>>>>> On Thu, Mar 30, 2017 at 2:40 PM, Rafael Espíndola
>>>>> <rafael.espindola at gmail.com> wrote:
>>>>>>>> 08048074 <_start>:
>>>>>>>>  8048074: c7 c0 74 80 04 08     mov    $0x8048074,%eax
>>>>>>>>
>>>>>>>> as the GOT entry for _start contains the address of _start.
>>>>>>>
>>>>>>> Even with trunk I get
>>>>>>>
>>>>>>> 08048094 <_start>:
>>>>>>>  8048094:       a1 fc ff ff ff          mov    0xfffffffc,%eax
>>>>>>
>>>>>> And I still get -4 with the head of the binutils-2_28-branch branch.
>>>>>>
>>>>>
>>>>> The updated i386 psABI requires to encode "mov _start at GOT, %eax" with
>>>>> 0x8b opcode with R_386_GOT32X instead of 0xa1 opcode.
>>>>
>>>> What about files produced by old assemblers?
>>>
>>> For the older .o files, G + A - GOT is used.
>>
>> Which is incorrect?
>>
>> The current ABI also lists  R_386_GOT32 with the same expression as
>> R_386_GOT32X. When is a R_386_GOT32 required to be evaluated as G + A?
>>
>
> And it also says 0x8b opcode should be used.  For R_386_GOT32 without
> a base register on a1 opcode, it is undefined.
>

mov foo at GOT, %eax
mov %eax, foo at GOT

are special.   If

mov %eax, foo at GOT (0x89 opcode or 0xa2 opcode)

used, program behavior becomes random.

mov foo at GOT, %eax  (0xa0 opcode)

weren't used in the old programs since old linker will generate G + A - GOT,
which makes program behavior random.  The new assembler should always
generate 0x8b opcode for

mov foo at GOT, %eax

with R_386_GOT32X.

It is hard to define a reasonable computation for

mov foo at GOT, %eax  (0xa0 opcode)

with R_386_GOT32 without introducing more linker complexity.


-- 
H.J.