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SubjectRe: [PATCH] fbcon: Use kzalloc() in fbcon_prepare_logo()
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 positive 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"
> : "=&c" (d0), "=&D" (d1)
> : "a" (v), "1" (s), "0" (n)
> : "memory");
> return s;
> }
>
> . Plain memset() in arch/x86/include/asm/string_64.h is redirected to __msan_memset()
> but memsetXX() are not redirected to __msan_memsetXX(). That is, memory initialization
> 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 is 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.


--
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Straße, 33
80636 München

Geschäftsführer: Paul Manicle, Liana Sebastian
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg

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