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

[H.J. Lu via llvm-commits]

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.

-- 
H.J.