[PATCH] D20024: [ELF] - Protect first entries of got.plt with RelRo.

[H.J. Lu via llvm-commits]

On Fri, May 6, 2016 at 4:55 PM, H.J. Lu <hjl.tools at gmail.com> wrote:
> On Fri, May 6, 2016 at 4:09 PM, Joerg Sonnenberger via llvm-commits
> <llvm-commits at lists.llvm.org> wrote:
>> On Fri, May 06, 2016 at 03:37:28PM -0700, Rui Ueyama wrote:
>>> Does that mean the value the static linker writes to the first GOT slot
>>> will be ignored?
>>
>>         leaq    _GLOBAL_OFFSET_TABLE_(%rip),%rax
>>         leaq    _DYNAMIC(%rip),%rdi     # &_DYNAMIC
>>         movq    %rdi,%rbx
>>         subq    (%rax),%rbx             # relocbase
>>
>> ...and %rbx is then passed as argument to the self-relocation function.
>> There are some possible other ways to do it, but they are more involved
>> like parsing the aux vector. Given that this is very early startup code,
>> less magic is better here.
>
> Since the the link-time address of _DYNAMIC is stored in the first element
> of the GOT as specified by the x86-64 psABI, one can use
>
> movq _GLOBAL_OFFSET_TABLE_(%rip), %rax
>
> to get the link-time address of _DYNAMIC.
>

BTW, it can be written in C:

extern const long _GLOBAL_OFFSET_TABLE_[] __attribute__
((visibility("hidden")));
const long
foo ()
{
  return _GLOBAL_OFFSET_TABLE_[0];
}


-- 
H.J.