linux-kernel - Re: [PATCH] x86: suppress KMSAN reports in arch_within_stack

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Message-ID: <Y3tjJPtzyi9PD9VD@hirez.programming.kicks-ass.net>
Date:   Mon, 21 Nov 2022 12:38:12 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, Eric Biggers <ebiggers@...nel.org>
Subject: Re: [PATCH] x86: suppress KMSAN reports in arch_within_stack_frames()

On Mon, Nov 21, 2022 at 11:28:39AM +0100, Alexander Potapenko wrote:

> > > +__no_kmsan_checks
> > >  static inline int arch_within_stack_frames(const void * const stack,
> > >                                          const void * const stackend,
> > >                                          const void *obj, unsigned long len)
> >
> > Seems OK; but now I'm confused as to the exact distinction between
> > __no_sanitize_memory and __no_kmsan_checks.
> >
> > The comments there about seem to suggest __no_sanitize_memory ensures no
> > instrumentation at all, and __no_kmsan_checks some instrumentation but
> > doesn't actually check anything -- so what's left then?
> 
> __no_sanitize_memory prohibits all instrumentation whatsoever, whereas
> __no_kmsan_checks adds instrumentation that suppresses potential false
> positives around this function.
> 
> Quoting include/linux/compiler-clang.h:
> 
> /*
>  * The __no_kmsan_checks attribute ensures that a function does not produce
>  * false positive reports by:
>  *  - initializing all local variables and memory stores in this function;
>  *  - skipping all shadow checks;
>  *  - passing initialized arguments to this function's callees.
>  */
> 
> Does this answer your question?

So I read that comment; and it didn't click. So you're explicitly
initializing variables/arguments and explicitly not checking shadow
state vs, not doing explicit initialization and checking shadow state?

That is, it doesn't do the normal checks and adds explicit
initialization to avoid triggering discontent in surrounding functions?