Re: [PATCH 2/4] arm64: prevent instrumentation of LL/SC atomics

[Mark Rutland]

On Tue, Oct 17, 2017 at 11:58:58AM +0100, Will Deacon wrote:
> On Tue, Oct 17, 2017 at 11:54:54AM +0100, Mark Rutland wrote:
> > On Tue, Oct 17, 2017 at 11:03:15AM +0100, Will Deacon wrote:
> > > On Mon, Oct 16, 2017 at 02:24:38PM +0100, Mark Rutland wrote:
> > > > While we build the LL/SC atomics as a C object file, this does not
> > > > follow the AAPCS. This does not interoperate with other C code, and can
> > > > only be called from special wrapper assembly.
> > > > 
> > > > Bulding a kernel with CONFIG_KCOV and CONFIG_ARM64_LSE_ATOMICS results
> > > > in the cmopiler inserting calls to __sanitizer_cov_trace_pc within the
> > > > LL/SC atomics. As __sanitizer_cov_trace_pc is built per the AAPCS, these
> > > > calls corrupt register values, resulting in failures at boot time.
> > > > 
> > > > Avoid this (and other similar issues) by opting out of all compiler
> > > > instrumentation. We can opt-in to specific instrumentation in future if
> > > > we want to.

> > > > diff --git a/arch/arm64/lib/Makefile b/arch/arm64/lib/Makefile
> > > > index a0abc142c92b..af77516f71b2 100644
> > > > --- a/arch/arm64/lib/Makefile
> > > > +++ b/arch/arm64/lib/Makefile
> > > > @@ -17,5 +17,6 @@ CFLAGS_atomic_ll_sc.o	:= -fcall-used-x0 -ffixed-x1 -ffixed-x2		\
> > > >  		   -fcall-saved-x10 -fcall-saved-x11 -fcall-saved-x12	\
> > > >  		   -fcall-saved-x13 -fcall-saved-x14 -fcall-saved-x15	\
> > > >  		   -fcall-saved-x18
> > > > +CC_INSTRUMENT_atomic_ll_sc.o := n
> > > 
> > > Does this mean we can lose the "notrace" definition of __LL_SC_INLINE
> > > when generating the out-of-line atomics?
> > 
> > Unfortunately not.
> > 
> > I'd missed -pg, since that isn't handled in scripts/Makefile.lib, and
> > doesn't seem to have a makefile-level disable.
> > 
> > I'll see if that can be remedied.
> 
> Thanks. It's a real shame to have a "just use this option to avoid
> instrumentation" if it doesn't actually catch everything. 

Agreed; it defeats the purpose of the exercise.

> We probably need to think about kprobes too, but not really sure what
> you can do there on a per-file basis.

Ugh; that's a much more painful one, yes. :(

Does that rely on any compiler options at all? I thought was all a
runtime thing.

Arguably it is somewhat separate for compiler instrumentation, and it
might make sense for that to be a separate option.

Thanks,
Mark.