linux-kernel - Re: [PATCH 2/3] x86/cpu: Elide KCSAN for cpu

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Date:   Tue, 3 May 2022 15:20:06 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, elver@...gle.com, jbaron@...mai.com,
        rostedt@...dmis.org, ardb@...nel.org,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH 2/3] x86/cpu: Elide KCSAN for cpu_has() and friends

On Mon, May 02, 2022 at 11:47:47AM -0700, Josh Poimboeuf wrote:
> On Mon, May 02, 2022 at 01:07:43PM +0200, Peter Zijlstra wrote:
> > vmlinux.o: warning: objtool: enter_from_user_mode+0x24: call to __kcsan_check_access() leaves .noinstr.text section
> > vmlinux.o: warning: objtool: syscall_enter_from_user_mode+0x28: call to __kcsan_check_access() leaves .noinstr.text section
> > vmlinux.o: warning: objtool: syscall_enter_from_user_mode_prepare+0x24: call to __kcsan_check_access() leaves .noinstr.text section
> > vmlinux.o: warning: objtool: irqentry_enter_from_user_mode+0x24: call to __kcsan_check_access() leaves .noinstr.text section
> > 
> > Reported-by: kernel test robot <lkp@...el.com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> 
> An explanation about *why* this fixes it would help, as I have no idea
> from looking at the patch.

How about something like:

| As x86 uses the <asm-generic/bitops/instrumented-*.h> headers, the
| regular forms of all bitops are instrumented with explicit calls to
| KASAN and KCSAN checks. As these are explicit calls, these are not
| suppressed by the noinstr function attribute.
|
| This can result in calls to those check functions in noinstr code, which
| objtool warns about:
|
| vmlinux.o: warning: objtool: enter_from_user_mode+0x24: call to __kcsan_check_access() leaves .noinstr.text section
| vmlinux.o: warning: objtool: syscall_enter_from_user_mode+0x28: call to __kcsan_check_access() leaves .noinstr.text section
| vmlinux.o: warning: objtool: syscall_enter_from_user_mode_prepare+0x24: call to __kcsan_check_access() leaves .noinstr.text section
| vmlinux.o: warning: objtool: irqentry_enter_from_user_mode+0x24: call to __kcsan_check_access() leaves .noinstr.text section
|
| Prevent this by using the arch_*() bitops, which are the underlying
| bitops without explciit instrumentation.

Thanks,
Mark.

> 
> > ---
> >  arch/x86/include/asm/cpufeature.h |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > --- a/arch/x86/include/asm/cpufeature.h
> > +++ b/arch/x86/include/asm/cpufeature.h
> > @@ -51,7 +51,7 @@ extern const char * const x86_power_flag
> >  extern const char * const x86_bug_flags[NBUGINTS*32];
> >  
> >  #define test_cpu_cap(c, bit)						\
> > -	 test_bit(bit, (unsigned long *)((c)->x86_capability))
> > +	 arch_test_bit(bit, (unsigned long *)((c)->x86_capability))
> >  
> >  /*
> >   * There are 32 bits/features in each mask word.  The high bits
> > 
> > 
> 
> -- 
> Josh
>