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Subject: Re: [PATCH v2 13/33] kmsan: Introduce memset_no_sanitize_memory()
From:   Ilya Leoshkevich <iii@linux.ibm.com>
To:     Alexander Potapenko <glider@google.com>
Cc:     Alexander Gordeev <agordeev@linux.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Marco Elver <elver@google.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Pekka Enberg <penberg@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-s390@vger.kernel.org,
        linux-trace-kernel@vger.kernel.org,
        Mark Rutland <mark.rutland@arm.com>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Sven Schnelle <svens@linux.ibm.com>
Date:   Wed, 13 Dec 2023 02:31:59 +0100
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On Fri, 2023-12-08 at 16:25 +0100, Alexander Potapenko wrote:
> > A problem with __memset() is that, at least for me, it always ends
> > up being a call. There is a use case where we need to write only 1
> > byte, so I thought that introducing a call there (when compiling
> > without KMSAN) would be unacceptable.
>=20
> Wonder what happens with that use case if we e.g. build with fortify-
> source.
> Calling memset() for a single byte might be indicating the code is
> not hot.

The original code has a simple assignment. Here is the relevant diff:

        if (s->flags & __OBJECT_POISON) {
-               memset(p, POISON_FREE, poison_size - 1);
-               p[poison_size - 1] =3D POISON_END;
+               memset_no_sanitize_memory(p, POISON_FREE, poison_size -
1);
+               memset_no_sanitize_memory(p + poison_size - 1,
POISON_END, 1);
        }

[...]


> As stated above, I don't think this is more or less working as
> intended.
> If we really want the ability to inline __memset(), we could
> transform
> it into memset() in non-sanitizer builds, but perhaps having a call
> is
> also acceptable?

Thanks for the detailed explanation and analysis. I will post
a version with a __memset() and let the slab maintainers decide if
the additional overhead is acceptable.