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SubjectRe: [PATCH v10 06/17] mm: slub: introduce metadata_access_enable()/metadata_access_disable()
On Sat, 31 Jan 2015 03:11:55 +0400 Andrey Ryabinin <ryabinin.a.a@gmail.com> wrote:

> >> > kasan_disable_local/kasan_enable_local are also undocumented doesn't
> >> > help.
> >> >
> >>
> >> Ok, How about this?
> >>
> >> /*
> >> * This hooks separate payload access from metadata access.
> >> * Useful for memory checkers that have to know when slub
> >> * accesses metadata.
> >> */
> >
> > "These hooks".
> >
> > I still don't understand :( Maybe I'm having a more-stupid-than-usual
> > day.
>
> I think it's me being stupid today ;) I'll try to explain better.
>
> > How can a function "separate access"? What does this mean? More
> > details, please. I think I've only once seen a comment which had too
> > much info!
> >
>
> slub could access memory marked by kasan as inaccessible (object's metadata).
> Kasan shouldn't print report in that case because this access is valid.
> Disabling instrumentation of slub.c code is not enough to achieve this
> because slub passes pointer to object's metadata into memchr_inv().
>
> We can't disable instrumentation for memchr_inv() because this is quite
> generic function.
>
> So metadata_access_enable/metadata_access_disable wrap some
> places in slub.c where access to object's metadata starts/end.
> And kasan_disable_local/kasan_enable_local just disable/enable
> error reporting in this places.

ooh, I see. Something like this?

/*
* slub is about to manipulate internal object metadata. This memory lies
* outside the range of the allocated object, so accessing it would normally
* be reported by kasan as a bounds error. metadata_access_enable() is used
* to tell kasan that these accesses are OK.
*/


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