[v2] Fix x86_64 memchr for large input sizes

Message ID 1482782910-22756-1-git-send-email-adhemerval.zanella@linaro.org
State New
Headers show

Commit Message

Adhemerval Zanella Dec. 26, 2016, 8:08 p.m.
Changes from previous version

  - Simplify uintptr_t generation and check.

If noone opposes it I will commit it shortly.

--

Current optimized memchr for x86_64 does for input arguments pointers
module 64 in range of [49,63] if there is no searchr char in the rest
of 64-byte block a pointer addition which might overflow:

* sysdeps/x86_64/memchr.S

    77          .p2align 4
    78  L(unaligned_no_match):
    79          add     %rcx, %rdx

Add (uintptr_t)s % 16 to n in %rdx.

    80          sub     $16, %rdx
    81          jbe     L(return_null)

This patch fixes by adding a saturated math that sets a maximum pointer
value if it overflows (UINTPTR_MAX).

Checked on x86_64-linux-gnu and powerpc64-linux-gnu.

	[BZ# 19387]
	* sysdeps/x86_64/memchr.S (memchr): Avoid overflow in pointer
	addition.
	* string/test-memchr.c (do_test): Remove alignment limitation.
	(test_main): Add test that trigger BZ# 19387.
---
 string/test-memchr.c    | 9 ++++-----
 sysdeps/x86_64/memchr.S | 6 ++++++
 3 files changed, 18 insertions(+), 5 deletions(-)

-- 
2.7.4

Comments

Joseph Myers Dec. 30, 2016, 1:43 p.m. | #1
FWIW, I'm seeing string/test-memchr failing on i686; I suppose at least 
one of the i686 versions must have a similar issue.

-- 
Joseph S. Myers
jsm@polyomino.org.uk

Patch hide | download patch | download mbox

diff --git a/string/test-memchr.c b/string/test-memchr.c
index 0690cb4..10d696a 100644
--- a/string/test-memchr.c
+++ b/string/test-memchr.c
@@ -76,7 +76,6 @@  do_test (size_t align, size_t pos, size_t len, size_t n, int seek_char)
   size_t i;
   CHAR *result;
 
-  align &= 7;
   if ((align + len) * sizeof (CHAR) >= page_size)
     return;
 
@@ -194,12 +193,12 @@  test_main (void)
       do_test (i, 64, 256, SIZE_MAX, 0);
     }
 
-  for (i = 1; i < 16; ++i)
+  for (i = 1; i < 64; ++i)
     {
-      for (j = 1; j < 16; j++)
+      for (j = 1; j < 64; j++)
         {
-	  do_test (0, 16 - j, 16, SIZE_MAX, 23);
-	  do_test (i, 16 - j, 16, SIZE_MAX, 23);
+	  do_test (0, 64 - j, 64, SIZE_MAX, 23);
+	  do_test (i, 64 - j, 64, SIZE_MAX, 23);
         }
     }
 
diff --git a/sysdeps/x86_64/memchr.S b/sysdeps/x86_64/memchr.S
index 132eacb..66e986c 100644
--- a/sysdeps/x86_64/memchr.S
+++ b/sysdeps/x86_64/memchr.S
@@ -76,7 +76,13 @@  L(crosscache):
 
 	.p2align 4
 L(unaligned_no_match):
+        /* Calculate the last acceptable address and check for possible
+           addition overflow by using satured math:
+           rdx = rcx + rdx
+           rdx |= -(rdx < x)  */
 	add	%rcx, %rdx
+	sbb	%rax, %rax
+	or	%rax, %rdx
 	sub	$16, %rdx
 	jbe	L(return_null)
 	add	$16, %rdi