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Re: [x86-64 psABI] RFC: Extend x86-64 PLT entry to support MPX

On Tue, Jul 23, 2013 at 12:49:06PM -0700, H.J. Lu wrote:
> There are 2 psABI considerations:
>  1. Should PLT entries in all binaries, with and without MPX, be changed
>     to 32-byte or just the necessary ones?

Ugh, please don't.

>  2. Only branch to PLT entry with BND prefix needs 32-byte PLT entry. If
>     we use 32-byte PLT entry only when needed, it can be decided by:
>     a. A new MPX PLT relocation:
>        i. No new run-time relocation since MPX PLT relocation is
>       resolved to branch to PLT entry at link-time.
>        ii. Pro: No new section.
>        iii. Con:
>         Need a new relocation.
>         Can't mark executable nor shared library.

I think I prefer new relocation, @mpxplt or similar.  The linker could then
use the 32-byte PLT slot for both @plt and @mpxplt relocs if there is at
least one @mpxplt reloc for the symbol, otherwise it would use 16-byte PLT
slot.  And you can certainly mark executables or PIEs or shared libraries
this way, the linker could do that if it creates any 32-byte PLT slot.


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