linux-kernel - Re: [PATCH v4 16/35] mm: slub: Unpoison the memchr

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Date: Tue, 18 Jun 2024 16:38:31 +0200
From: Alexander Potapenko <glider@...gle.com>
To: Ilya Leoshkevich <iii@...ux.ibm.com>
Cc: Alexander Gordeev <agordeev@...ux.ibm.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Christoph Lameter <cl@...ux.com>, David Rientjes <rientjes@...gle.com>, Heiko Carstens <hca@...ux.ibm.com>, 
	Joonsoo Kim <iamjoonsoo.kim@....com>, Marco Elver <elver@...gle.com>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Pekka Enberg <penberg@...nel.org>, 
	Steven Rostedt <rostedt@...dmis.org>, Vasily Gorbik <gor@...ux.ibm.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Christian Borntraeger <borntraeger@...ux.ibm.com>, 
	Dmitry Vyukov <dvyukov@...gle.com>, Hyeonggon Yoo <42.hyeyoo@...il.com>, kasan-dev@...glegroups.com, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, linux-s390@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Sven Schnelle <svens@...ux.ibm.com>
Subject: Re: [PATCH v4 16/35] mm: slub: Unpoison the memchr_inv() return value

On Thu, Jun 13, 2024 at 5:39 PM Ilya Leoshkevich <iii@...ux.ibm.com> wrote:
>
> Even though the KMSAN warnings generated by memchr_inv() are suppressed
> by metadata_access_enable(), its return value may still be poisoned.
>
> The reason is that the last iteration of memchr_inv() returns
> `*start != value ? start : NULL`, where *start is poisoned. Because of
> this, somewhat counterintuitively, the shadow value computed by
> visitSelectInst() is equal to `(uintptr_t)start`.
>
> The intention behind guarding memchr_inv() behind
> metadata_access_enable() is to touch poisoned metadata without
> triggering KMSAN, so unpoison its return value.

What do you think about applying __no_kmsan_checks to these functions instead?