On Fri, Dec 16, 2022 at 3:03 PM Tetsuo Handa
[off-list ref] wrote:

On 2022/12/15 18:36, Geert Uytterhoeven wrote:

quoted

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
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