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

[H.J. Lu via llvm-commits]

On Thu, Mar 30, 2017 at 4:02 PM, Rui Ueyama <ruiu at google.com> wrote:
> But how do you know whether a R_386_GOT32 has a base register or not by
> looking at the location where a relocation is applied? IIUC, your old spec
> doesn't impose any restrictions on where you can use R_386_GOT32, so you can
> use that type of relocation anywhere. Even if it the location where a
> R_386_GOT32 looks like a 386 instruction, that could still be some other
> kind of data.

R_386_GOT32 is used on opcode with MODRM byte, except for opcode 0xa0,
which doesn't have MODRM byte.  If the byte before r_offset & 0xc7 == 0x5, it is
baseless and it can't be 0xa0 opcode.

H.J.
> 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.
>>
>>
>> --
>> H.J.
>
>



-- 
H.J.