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x86: Fix bug in strncmp-evex and strncmp-avx2 [BZ #28895]

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Logic can read before the start of `s1` / `s2` if both `s1` and `s2`
are near the start of a page. To avoid having the result contimated by
these comparisons the `strcmp` variants would mask off these
comparisons. This was missing in the `strncmp` variants causing
the bug. This commit adds the masking to `strncmp` so that out of
range comparisons don't affect the result.

test-strcmp, test-strncmp, test-wcscmp, and test-wcsncmp all pass as
well a full xcheck on x86_64 linux.
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
This commit is contained in:
Noah Goldstein 2022-02-15 20:27:21 -06:00
parent 0281c7a7ec
commit e108c02a5e
3 changed files with 25 additions and 0 deletions
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23
string/test-strncmp.c
View File

@ -423,6 +423,28 @@ check3 (void)
}
}
static void
check4 (void)
{
/* To trigger bug 28895; We need 1) both s1 and s2 to be within 32 bytes of
the end of the page. 2) For there to be no mismatch/null byte before the
first page cross. 3) For length (`n`) to be large enough for one string to
cross the page. And 4) for there to be either mismatch/null bytes before
the start of the strings. */
size_t size = 10;
size_t addr_mask = (getpagesize () - 1) ^ (sizeof (CHAR) - 1);
CHAR *s1 = (CHAR *)(buf1 + (addr_mask & 0xffa));
CHAR *s2 = (CHAR *)(buf2 + (addr_mask & 0xfed));
int exp_result;
STRCPY (s1, L ("tst-tlsmod%"));
STRCPY (s2, L ("tst-tls-manydynamic73mod"));
exp_result = SIMPLE_STRNCMP (s1, s2, size);
FOR_EACH_IMPL (impl, 0)
check_result (impl, s1, s2, size, exp_result);
}
static void
check_overflow (void)
{
@ -546,6 +568,7 @@ test_main (void)
check1 ();
check2 ();
check3 ();
check4 ();
printf ("%23s", "");
FOR_EACH_IMPL (impl, 0)

1
sysdeps/x86_64/multiarch/strcmp-avx2.S
View File

@ -661,6 +661,7 @@ L(ret8):
# ifdef USE_AS_STRNCMP
.p2align 4,, 10
L(return_page_cross_end_check):
andl %r10d, %ecx
tzcntl %ecx, %ecx
leal -VEC_SIZE(%rax, %rcx), %ecx
cmpl %ecx, %edx

1
sysdeps/x86_64/multiarch/strcmp-evex.S
View File

@ -689,6 +689,7 @@ L(ret8):
# ifdef USE_AS_STRNCMP
.p2align 4,, 10
L(return_page_cross_end_check):
andl %r10d, %ecx
tzcntl %ecx, %ecx
leal -VEC_SIZE(%rax, %rcx, SIZE_OF_CHAR), %ecx
# ifdef USE_AS_WCSCMP