[uclibc-ng-devel] xtensa: fix strcmp

Message ID 1515486642-26344-1-git-send-email-jcmvbkbc@gmail.com
State New
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  • [uclibc-ng-devel] xtensa: fix strcmp
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Commit Message

Max Filippov Jan. 9, 2018, 8:30 a.m.
Loops with 'loop forever' annotation inside strcmp are actually meant to
loop forever. Falling through the end of the first loop may result in
equal strings being compared unequal, e.g.:

	#include <string.h>

	int main(void)
	{
		char a[4096] __attribute__((aligned(4)));
		char b[4096] __attribute__((aligned(4)));

		memset(a, ' ', 258 * 8);
		memset(b, ' ', 258 * 8);
		a[255 * 8] = 0;
		a[256 * 8] = 'a';
		b[255 * 8] = 0;
		b[256 * 8] = 'b';
		return !(strcmp(a, b) == 0);
	}

Falling through the end of the second loop may result in unequal strings
being compared as equal, e.g.:

	#include <string.h>

	int main(void)
	{
		char a[4096] __attribute__((aligned(4)));
		char b[4096] __attribute__((aligned(4)));

		memset(a, ' ', 514 * 4);
		memset(b, ' ', 514 * 4);
		a[514 * 4 + 0] = 'a';
		a[514 * 4 + 1] = 0;
		b[514 * 4 + 0] = 'b';
		b[514 * 4 + 1] = 0;
		return !(strcmp(a, b) != 0);
	}

Use 0 as a loop counter to make 2^32 - 1 iterations which is enough to
cover all addressable memory. While at it drop useless nop at the end of
the first loop and use a11 for all loop counters.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
---
 libc/string/xtensa/strcmp.S | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

Patch

diff --git a/libc/string/xtensa/strcmp.S b/libc/string/xtensa/strcmp.S
index 2dce590dbd42..a16da5da2e92 100644
--- a/libc/string/xtensa/strcmp.S
+++ b/libc/string/xtensa/strcmp.S
@@ -92,8 +92,8 @@  ENTRY (strcmp)
 	/* (2 mod 4) alignment for loop instruction */
 .Lunaligned:
 #if XCHAL_HAVE_LOOPS
-	_movi.n	a8, 0		/* set up for the maximum loop count */
-	loop	a8, .Lretdiff	/* loop forever (almost anyway) */
+	movi	a11, 0		/* set up for the maximum loop count */
+	loop	a11, .Lretdiff	/* loop forever (almost anyway) */
 #endif
 .Lnextbyte:
 	l8ui	a8, a2, 0
@@ -131,11 +131,10 @@  ENTRY (strcmp)
 #if XCHAL_HAVE_LOOPS
 .Laligned:
 	.begin	no-transform
+	movi	a11, 0
 	l32r	a4, .Lmask0	/* mask for byte 0 */
 	l32r	a7, .Lmask4
-	/* Loop forever.  (a4 is more than than the maximum number
-	   of iterations) */
-	loop	a4, .Laligned_done
+	loop	a11, .Laligned_done /* Loop forever. */
 
 	/* First unrolled loop body.  */
 	l32i	a8, a2, 0	/* get word from s1 */
@@ -156,8 +155,6 @@  ENTRY (strcmp)
 	addi	a2, a2, 8	/* advance s1 pointer */
 	addi	a3, a3, 8	/* advance s2 pointer */
 .Laligned_done:
-	or	a1, a1, a1	/* nop */
-
 .Lprobeq2:
 	/* Adjust pointers to account for the loop unrolling.  */
 	addi	a2, a2, 4
@@ -198,7 +195,7 @@  ENTRY (strcmp)
 #if XCHAL_HAVE_LOOPS
 
 	/* align (1 mod 4) */
-	loop	a4, .Leq	/* loop forever (a4 is bigger than max iters) */
+	loop	a11, .Leq	/* loop forever */
 	.end	no-transform
 
 	l32i	a8, a2, 0	/* get word from s1 */