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commit 5373c90f2ea3c3fa9931a684c9b81c648dfbe8d7
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Author: Noah Goldstein <goldstein.w.n@gmail.com>
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Date: Tue Feb 15 20:27:21 2022 -0600
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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`
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are near the start of a page. To avoid having the result contimated by
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these comparisons the `strcmp` variants would mask off these
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comparisons. This was missing in the `strncmp` variants causing
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the bug. This commit adds the masking to `strncmp` so that out of
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range comparisons don't affect the result.
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test-strcmp, test-strncmp, test-wcscmp, and test-wcsncmp all pass as
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well a full xcheck on x86_64 linux.
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Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
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(cherry picked from commit e108c02a5e23c8c88ce66d8705d4a24bb6b9a8bf)
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diff --git a/string/test-strncmp.c b/string/test-strncmp.c
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index 97e831d88fd24316..56e23670ae7f90e4 100644
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--- a/string/test-strncmp.c
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+++ b/string/test-strncmp.c
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@@ -438,13 +438,23 @@ check3 (void)
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static void
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check4 (void)
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{
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- const CHAR *s1 = L ("abc");
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- CHAR *s2 = STRDUP (s1);
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+ /* To trigger bug 28895; We need 1) both s1 and s2 to be within 32 bytes of
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+ the end of the page. 2) For there to be no mismatch/null byte before the
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+ first page cross. 3) For length (`n`) to be large enough for one string to
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+ cross the page. And 4) for there to be either mismatch/null bytes before
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+ the start of the strings. */
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+
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+ size_t size = 10;
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+ size_t addr_mask = (getpagesize () - 1) ^ (sizeof (CHAR) - 1);
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+ CHAR *s1 = (CHAR *)(buf1 + (addr_mask & 0xffa));
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+ CHAR *s2 = (CHAR *)(buf2 + (addr_mask & 0xfed));
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+ int exp_result;
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+ STRCPY (s1, L ("tst-tlsmod%"));
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+ STRCPY (s2, L ("tst-tls-manydynamic73mod"));
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+ exp_result = SIMPLE_STRNCMP (s1, s2, size);
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FOR_EACH_IMPL (impl, 0)
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- check_result (impl, s1, s2, SIZE_MAX, 0);
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-
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- free (s2);
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+ check_result (impl, s1, s2, size, exp_result);
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}
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int
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diff --git a/sysdeps/x86_64/multiarch/strcmp-avx2.S b/sysdeps/x86_64/multiarch/strcmp-avx2.S
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index cdded412a70bad10..f9bdc5ccd03aa1f9 100644
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--- a/sysdeps/x86_64/multiarch/strcmp-avx2.S
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+++ b/sysdeps/x86_64/multiarch/strcmp-avx2.S
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@@ -661,6 +661,7 @@ L(ret8):
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# ifdef USE_AS_STRNCMP
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.p2align 4,, 10
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L(return_page_cross_end_check):
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+ andl %r10d, %ecx
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tzcntl %ecx, %ecx
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leal -VEC_SIZE(%rax, %rcx), %ecx
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cmpl %ecx, %edx
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diff --git a/sysdeps/x86_64/multiarch/strcmp-evex.S b/sysdeps/x86_64/multiarch/strcmp-evex.S
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index ed56af8ecdad48b2..0dfa62bd149c02b4 100644
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--- a/sysdeps/x86_64/multiarch/strcmp-evex.S
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+++ b/sysdeps/x86_64/multiarch/strcmp-evex.S
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@@ -689,6 +689,7 @@ L(ret8):
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# ifdef USE_AS_STRNCMP
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.p2align 4,, 10
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L(return_page_cross_end_check):
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+ andl %r10d, %ecx
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tzcntl %ecx, %ecx
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leal -VEC_SIZE(%rax, %rcx, SIZE_OF_CHAR), %ecx
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# ifdef USE_AS_WCSCMP
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