On Wed, Jan 10, 2018 at 08:19:51AM +0100, Ingo Molnar wrote:
>
> * Willy Tarreau <[email protected]> wrote:
>
> > +#ifdef CONFIG_PAGE_TABLE_ISOLATION
> > + this_cpu_write(pti_disable,
> > + next_p->mm && next_p->mm->context.pti_disable);
> > +#endif
>
> Another pet peeve, please write:
>
> > + this_cpu_write(pti_disable, next_p->mm &&
> > next_p->mm->context.pti_disable);
>
> or consider introducing an 'mm_next' local variable, set to next_p->mm, and
> use
> that to shorten the sequence.
OK.
> More importantly, any strong reasons why the flag is logic-inverted? I.e. why
> not
> ::pti_enabled?
For me it's a matter of default case. Having a "pti_enabled" flag makes
one think the default is disabled and an action is required to turn it on.
With "pti_disabled", it becomes clearer that the default is enabled and an
action is required to turn it off. While it causes a double inversion for
the user due to the temporary choice of prctl name (we could have
ARCH_SET_PTI for example), I think it results on more readable code in
the sensitive parts like the asm one where these tests could possibly
end up inside #ifdefs. If we had "pit_enabled", something like this could
be confusing because it's not obvious whether this pti_enabled *enforces*
PTI or if its absence disables it :
#ifdef CONFIG_ALLOW_DISABLE_PTI
cmpb $0, PER_CPU_VAR(pti_enabled)
jz .Lend\@
#endif
But this is open to discussion of course.
Willy
