Re: [PATCH] Rewritten v9/64-bit sparc strcmp.

[David Miller <davem at davemloft dot net>]

From: Aurelien Jarno <aurelien@aurel32.net>
Date: Tue, 29 Apr 2014 11:53:39 +0200

> This method doesn't work when comparing a 0x00 char in string 1 and 0x01
> char in string 2. In that case the mask for this byte is 0x01 and the
> corresponding xor is also  0x01. The result of the comparison therefore
> depends on the garbage after the end of the string.
> 
> On Debian [1] this causes for example debian-installer to fail to build
> [2], and it might be the source of the random segfaults which we are
> trying to debug for a few years.
> 
> [1] http://bugs.debian.org/746310
> [2] http://bugs.debian.org/731806

Right you are, if there are any zeros after the first zero we will
potentially miscompare.

The only solution I can see at the moment is to clear all except the
topmost bit in the mask.

Can you please test this patch?

Meanwhile I'll try to adjust string/test-strcmp.c so that it
explicitly tests this situation.

Thanks!

diff --git a/sysdeps/sparc/sparc64/strcmp.S b/sysdeps/sparc/sparc64/strcmp.S
index 8925396..312924a 100644
--- a/sysdeps/sparc/sparc64/strcmp.S
+++ b/sysdeps/sparc/sparc64/strcmp.S
@@ -121,6 +121,37 @@ ENTRY(strcmp)
 	movleu	%xcc, -1, %o0
 	srlx	rTMP1, 7, rTMP1
 
+	/* In order not to be influenced by bytes after the zero byte, we
+	 * have to retain only the highest bit in the mask for the comparison
+	 * with rSTRXOR to work properly.
+	 */
+	mov	0, rTMP2
+	andcc	rTMP1, 0x0100, %g0
+	
+	movne	%xcc, 8, rTMP2
+	sllx	rTMP1, 63 - 16, %o1
+
+	movrlz	%o1, 16, rTMP2
+	sllx	rTMP1, 63 - 24, %o1
+
+	movrlz	%o1, 24, rTMP2
+	sllx	rTMP1, 63 - 32, %o1
+
+	movrlz	%o1, 32, rTMP2
+	sllx	rTMP1, 63 - 40, %o1
+
+	movrlz	%o1, 40, rTMP2
+	sllx	rTMP1, 63 - 48, %o1
+
+	movrlz	%o1, 48, rTMP2
+	sllx	rTMP1, 63 - 56, %o1
+
+	movrlz	%o1, 56, rTMP2
+
+	srlx	rTMP1, rTMP2, rTMP1
+
+	sllx	rTMP1, rTMP2, rTMP1
+
 	cmp	rTMP1, rSTRXOR
 	retl
 	 movgu	%xcc, 0, %o0