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From:   Alexander Potapenko <glider@google.com>
Date:   Mon, 11 Dec 2023 11:46:22 +0100
Message-ID: <CAG_fn=XyTZHU45EhinUSm-+Thux4VPCpT-jyf=cP7hNPcTbK8g@mail.gmail.com>
Subject: Re: [PATCH v2 30/33] s390/uaccess: Add KMSAN support to put_user()
 and get_user()
To:     Ilya Leoshkevich <iii@linux.ibm.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>
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On Tue, Nov 21, 2023 at 11:03=E2=80=AFPM Ilya Leoshkevich <iii@linux.ibm.co=
m> wrote:
>
> put_user() uses inline assembly with precise constraints, so Clang is
> in principle capable of instrumenting it automatically. Unfortunately,
> one of the constraints contains a dereferenced user pointer, and Clang
> does not currently distinguish user and kernel pointers. Therefore
> KMSAN attempts to access shadow for user pointers, which is not a right
> thing to do.
>
> An obvious fix to add __no_sanitize_memory to __put_user_fn() does not
> work, since it's __always_inline. And __always_inline cannot be removed
> due to the __put_user_bad() trick.
>
> A different obvious fix of using the "a" instead of the "+Q" constraint
> degrades the code quality, which is very important here, since it's a
> hot path.
>
> Instead, repurpose the __put_user_asm() macro to define
> __put_user_{char,short,int,long}_noinstr() functions and mark them with
> __no_sanitize_memory. For the non-KMSAN builds make them
> __always_inline in order to keep the generated code quality. Also
> define __put_user_{char,short,int,long}() functions, which call the
> aforementioned ones and which *are* instrumented, because they call
> KMSAN hooks, which may be implemented as macros.
>
> The same applies to get_user() as well.
>
> Acked-by: Heiko Carstens <hca@linux.ibm.com>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

I think this patch makes sense, but I don't feel myself qualified
enough to stamp it. Hope Heiko's ack is enough.