Support both -mstack-protector-guard={global,tls}

[Fangrui Song]

Thanks for the reply!

On 2020-11-24, Carlos O'Donell wrote:
>On 11/24/20 4:23 PM, Fangrui Song wrote:
>> Hi, I filed https://sourceware.org/bugzilla/show_bug.cgi?id=26817 about
>> supporting both -mstack-protector-guard={global,tls} (I believe musl
>> supports both). The feature request has a bit more context why
>> -mstack-protector-guard=global may be appealing in terms of performance
>> and security.
>
>In theory glibc supports both also... but only one at a time for a given
>architecture with the architecture making the choice of the implemented
>solution.
>
>I don't see a reason you couldn't support both, setting up a global and
>a TLS variable at the same time.
>We are already somewhat limited to having a single global process value.
>Some arches use a global so this limit is already applied.
>
>If glibc supported both, then the target application could choose to use
>TP+offset access or global access, whichever was faster?

Sounds good. The global __stack_chk_guard should probably be always available.
The TLS variable should be available as long as the target supports TLS (I
assume that some targets don't support TLS).