Re: [PATCH -rcu/kcsan 23/23] objtool, kcsan: Remove memory barrier instrumentation from noinstr

Peter Zijlstra
2021-11-11T11:35:22Z

On Thu, Nov 11, 2021 at 11:11:00AM +0100, Marco Elver wrote:
> On Tue, 5 Oct 2021 at 17:13, Marco Elver <elver@google.com> wrote:

> > So this is where I'd like to hear if the approach of:
> >
> > | #if !defined(CONFIG_ARCH_WANTS_NO_INSTR) || defined(CONFIG_STACK_VALIDATION)
> > | ...
> > | #else
> > | #define kcsan_noinstr noinstr
> > | static __always_inline bool within_noinstr(unsigned long ip)
> > | {
> > | return (unsigned long)__noinstr_text_start <= ip &&
> > | ip < (unsigned long)__noinstr_text_end;

Provided these turn into compile time constants this stands a fair
chance of working I suppose. Once this needs data loads things get a
*lot* more tricky.

> > | }
> > | #endif
> >
> > and then (using the !STACK_VALIDATION definitions)
> >
> > | kcsan_noinstr void instrumentation_may_appear_in_noinstr(void)
> > | {
> > | if (within_noinstr(_RET_IP_))
> > | return;
> >
> > works for the non-x86 arches that select ARCH_WANTS_NO_INSTR.
> >
> > If it doesn't I can easily just remove kcsan_noinstr/within_noinstr, and
> > add a "depends on !ARCH_WANTS_NO_INSTR || STACK_VALIDATION" to the
> > KCSAN_WEAK_MEMORY option.
> >
> > Looking at a previous discussion [1], however, I was under the
> > impression that this would work.
> >
> > [1] https://lkml.kernel.org/r/CANpmjNMAZiW-Er=2QDgGP+_3hg1LOvPYcbfGSPMv=aR6MVTB-g@mail.gmail.com
>
> I'll send v2 of this series after 5.16-rc1. So far I think we haven't
> been able to say the above doesn't work, which means I'll assume it
> works on non-x86 architectures with ARCH_WANTS_NO_INSTR until we get
> evidence of the opposite.

Fair enough.