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Date: Tue, 28 May 2024 17:45:12 +0200
From: Brian Johannesmeyer <bjohannesmeyer@gmail.com>
To: Alexander Potapenko <glider@google.com>
Cc: Marco Elver <elver@google.com>, Dmitry Vyukov <dvyukov@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	kasan-dev@googlegroups.com, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] kmsan: introduce test_unpoison_memory()
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References: <20240524232804.1984355-1-bjohannesmeyer@gmail.com>
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On Tue, May 28, 2024 at 12:20:15PM +0200, Alexander Potapenko wrote:
> You are right with your analysis.
> KMSAN stores a single origin for every aligned four-byte granule of
> memory, so we lose some information when more than one uninitialized
> value is combined in that granule.
> When writing an uninitialized value to memory, a viable strategy is to
> always update the origin. But if we partially initialize the granule
> with a store, it is better to preserve that granule's origin to
> prevent false negatives, so we need to check the resulting shadow slot
> before updating the origin.
> This is what the compiler instrumentation does, so
> kmsan_internal_set_shadow_origin() should behave in the same way.
> I found a similar bug in kmsan_internal_memmove_metadata() last year,
> but missed this one.

I appreciate the explanation. Makes sense.

> I am going to send a patch fixing this along with your test (with an
> updated description), if you don't object.

Yes, that's fine. Thank you.

-Brian