Re: [PATCH 3/3] jump_label,noinstr: Avoid instrumentation for JUMP_LABEL=n builds

[Peter Zijlstra]

On Mon, May 02, 2022 at 01:23:49PM +0200, Marco Elver wrote:
> On Mon, 2 May 2022 at 13:12, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > When building x86_64 with JUMP_LABEL=n it's possible for
> > instrumentation to sneak into noinstr:
> >
> > vmlinux.o: warning: objtool: exit_to_user_mode+0x14: call to static_key_count.constprop.0() leaves .noinstr.text section
> > vmlinux.o: warning: objtool: syscall_exit_to_user_mode+0x2d: call to static_key_count.constprop.0() leaves .noinstr.text section
> > vmlinux.o: warning: objtool: irqentry_exit_to_user_mode+0x1b: call to static_key_count.constprop.0() leaves .noinstr.text section
> >
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > ---
> >  include/linux/jump_label.h |    4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > --- a/include/linux/jump_label.h
> > +++ b/include/linux/jump_label.h
> > @@ -256,9 +256,9 @@ extern void static_key_disable_cpuslocke
> >  #include <linux/atomic.h>
> >  #include <linux/bug.h>
> >
> > -static inline int static_key_count(struct static_key *key)
> > +static __always_inline int static_key_count(struct static_key *key)
> >  {
> > -       return atomic_read(&key->enabled);
> > +       return READ_ONCE_NOCHECK(&key->enabled.count);
> >  }
> 
> Did arch_atomic_read() not work? Or should this also be unchecked in
> instrumented functions?

Urgh, also I think the above is broken for me forgetting to remove the
&.

I think arch_atomic_read() should work, lemme try.