Re: [PATCH v2 03/14] x86,smap: Fix smap_{save,restore}() alternatives

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Apr 28, 2020 at 08:54:05PM -0400, Brian Gerst wrote:
> On Tue, Apr 28, 2020 at 3:21 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > As reported by objtool:
> >
> >   lib/ubsan.o: warning: objtool: .altinstr_replacement+0x0: alternative modifies stack
> >   lib/ubsan.o: warning: objtool: .altinstr_replacement+0x7: alternative modifies stack
> >
> > the smap_{save,restore}() alternatives violate (the newly enforced)
> > rule on stack invariance. That is, due to there only being a single
> > ORC table it must be valid to any alternative. These alternatives
> > violate this with the direct result that unwinds will not be correct
> > in between these calls.
> >
> > [ In specific, since we force SMAP on for objtool, running on !SMAP
> > hardware will observe a different stack-layout and the ORC unwinder
> > will stumble. ]
> >
> > So rewrite the functions to unconditionally save/restore the flags,
> > which gives an invariant stack layout irrespective of the SMAP state.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > ---
> >  arch/x86/include/asm/smap.h |   11 +++++++----
> >  1 file changed, 7 insertions(+), 4 deletions(-)
> >
> > --- a/arch/x86/include/asm/smap.h
> > +++ b/arch/x86/include/asm/smap.h
> > @@ -57,16 +57,19 @@ static __always_inline unsigned long sma
> >  {
> >         unsigned long flags;
> >
> > -       asm volatile (ALTERNATIVE("", "pushf; pop %0; " __ASM_CLAC,
> > -                                 X86_FEATURE_SMAP)
> > -                     : "=rm" (flags) : : "memory", "cc");
> > +       asm volatile ("# smap_save\n\t"
> > +                     "pushf; pop %0"
> > +                     : "=rm" (flags) : : "memory");
> > +
> > +       clac();
> >
> >         return flags;
> >  }
> >
> >  static __always_inline void smap_restore(unsigned long flags)
> >  {
> > -       asm volatile (ALTERNATIVE("", "push %0; popf", X86_FEATURE_SMAP)
> > +       asm volatile ("# smap_restore\n\t"
> > +                     "push %0; popf"
> >                       : : "g" (flags) : "memory", "cc");
> >  }
> 
> POPF is an expensive instruction that should be avoided if possible.
> A better solution would be to have the alternative jump over the
> push/pop when SMAP is disabled.

Yeah. I think I had that, but then confused myself again. I don't think
it matters much if you look at where it's used though.

Still, let me try the jmp thing again..