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From:   Alexander Potapenko <glider@google.com>
Date:   Fri, 16 Dec 2022 16:52:14 +0100
Message-ID: <CAG_fn=VJrJDNSea6DksLt5uBe_sDu0+8Ofg+ifscOyDdMKj3XQ@mail.gmail.com>
Subject: Re: [PATCH] fbcon: Use kzalloc() in fbcon_prepare_logo()
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     Geert Uytterhoeven <geert@linux-m68k.org>,
        Marco Elver <elver@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Daniel Vetter <daniel@ffwll.ch>, Helge Deller <deller@gmx.de>,
        Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
        DRI <dri-devel@lists.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Fri, Dec 16, 2022 at 3:03 PM Tetsuo Handa
<penguin-kernel@i-love.sakura.ne.jp> wrote:
>
> On 2022/12/15 18:36, Geert Uytterhoeven wrote:
> > The next line is:
> >
> >         scr_memsetw(save, erase, array3_size(logo_lines, new_cols, 2));
> >
> > So how can this turn out to be uninitialized later below?
> >
> >         scr_memcpyw(q, save, array3_size(logo_lines, new_cols, 2));
> >
> > What am I missing?
>
> Good catch. It turned out that this was a KMSAN problem (i.e. a false pos=
itive report).
>
> On x86_64, scr_memsetw() is implemented as
>
>         static inline void scr_memsetw(u16 *s, u16 c, unsigned int count)
>         {
>                 memset16(s, c, count / 2);
>         }
>
> and memset16() is implemented as
>
>         static inline void *memset16(uint16_t *s, uint16_t v, size_t n)
>         {
>                 long d0, d1;
>                 asm volatile("rep\n\t"
>                              "stosw"
>                              : "=3D&c" (d0), "=3D&D" (d1)
>                              : "a" (v), "1" (s), "0" (n)
>                              : "memory");
>                 return s;
>         }
>
> . Plain memset() in arch/x86/include/asm/string_64.h is redirected to __m=
san_memset()
> but memsetXX() are not redirected to __msan_memsetXX(). That is, memory i=
nitialization
> via memsetXX() results in KMSAN's shadow memory being not updated.
>
> KMSAN folks, how should we fix this problem?
> Redirect assembly-implemented memset16(size) to memset(size*2) if KMSAN i=
s enabled?
>

I think the easiest way to fix it would be disable memsetXX asm
implementations by something like:

---------------------------------------------------------------------------=
----------------------
diff --git a/arch/x86/include/asm/string_64.h b/arch/x86/include/asm/string=
_64.h
index 888731ccf1f67..5fb330150a7d1 100644
--- a/arch/x86/include/asm/string_64.h
+++ b/arch/x86/include/asm/string_64.h
@@ -33,6 +33,7 @@ void *memset(void *s, int c, size_t n);
 #endif
 void *__memset(void *s, int c, size_t n);

+#if !defined(__SANITIZE_MEMORY__)
 #define __HAVE_ARCH_MEMSET16
 static inline void *memset16(uint16_t *s, uint16_t v, size_t n)
 {
@@ -68,6 +69,7 @@ static inline void *memset64(uint64_t *s, uint64_t
v, size_t n)
                     : "memory");
        return s;
 }
+#endif

 #define __HAVE_ARCH_MEMMOVE
 #if defined(__SANITIZE_MEMORY__) && defined(__NO_FORTIFY)
---------------------------------------------------------------------------=
----------------------

This way we'll just pick the existing C implementations instead of
reinventing them.


--=20
Alexander Potapenko
Software Engineer

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