Re: [tip:x86/asm] x86/i386: Make sure stack-protector segment base is cache aligned

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 09/03/09 13:26, H. Peter Anvin wrote:
> With the new zero-based percpu segment, it seems we should be able to
> subsume the stack protector into the percpu segment and reference both
> via %gs -- we just have to reserve the first 24 bytes of the segment,
> and being able to reduce the number of segments we need in the kernel is
> good for multiple reasons.
>
> Tejun - am I missing something why that would be hard or impossible?
>   

Two problems:

    * gcc generates %gs: references for stack-protector, but we use %fs
      for percpu data (because restoring %fs is faster if it's a null
      selector; TLS uses %gs).  I guess we could use %fs if
      !CONFIG_CC_STACKPROTECTOR, or %gs if we are using it (though that
      has some fiddly ramifications for things like ptrace).
    * The i386 percpu %fs base is offset by -__per_cpu_start from the
      percpu variables, so we can directly refer to %fs:per_cpu__foo. 
      I'm not sure what it would take to unify i386 to use the same
      scheme as x86-64.

Neither looks insoluble.

    J