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

[Rafael Espíndola via llvm-commits]

On 9 May 2016 at 10:20, H.J. Lu <hjl.tools at gmail.com> wrote:
> On Mon, May 9, 2016 at 7:18 AM, Rafael Espíndola
> <rafael.espindola at gmail.com> wrote:
>> On 9 May 2016 at 09:51, H.J. Lu <hjl.tools at gmail.com> wrote:
>>> On Mon, May 9, 2016 at 6:47 AM, Rafael Espíndola
>>> <rafael.espindola at gmail.com> wrote:
>>>>> 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.
>>>>
>>>> Given that this is being done before relocations, what is the
>>>> difference from just using _DYNAMIC?
>>>
>>> It is the link-time address of _DYNAMIC vs the run-time address of
>>>  _DYNAMIC.
>>
>>
>> Do you know how it is used?
>
>   /* Figure out the run-time load address of the dynamic linker itself.  */
>   bootstrap_map.l_addr = elf_machine_load_address ();
>
> /* Return the link-time address of _DYNAMIC.  Conveniently, this is the
>    first element of the GOT.  This must be inlined in a function which
>    uses global data.  */
> static inline ElfW(Addr) __attribute__ ((unused))
> elf_machine_dynamic (void)
> {
>   /* This produces an IP-relative reloc which is resolved at link time. */
>   extern const ElfW(Addr) _GLOBAL_OFFSET_TABLE_[] attribute_hidden;
>   return _GLOBAL_OFFSET_TABLE_[0];
> }
>
>
> /* Return the run-time load address of the shared object.  */
> static inline ElfW(Addr) __attribute__ ((unused))
> elf_machine_load_address (void)
> {
>   /* Compute the difference between the runtime address of _DYNAMIC as seen
>      by an IP-relative reference, and the link-time address found in the
>      special unrelocated first GOT entry.  */
>   extern ElfW(Dyn) _DYNAMIC[] attribute_hidden;
>   return (ElfW(Addr)) &_DYNAMIC - elf_machine_dynamic ();
> }

OK, seems similar to what netbsd does. I am still not sure why. glibc
just seems to add back elf_machine_dynamic to read the dynamic table,
but it could do that without subtracting and adding
elf_machine_dynamic :-)

In any case, I think this should be sufficient from lld's point of
view. I will update the patch to document and test it.


> Not going to work if a linker doesn't follow the psABI :-).

Sure, I would just be really surprised if this is all that is missing.

Cheers,
Rafael