linux-kernel - [PATCH] strcmp: fix overflow and possibly signedness error

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Date:	Wed, 18 Nov 2009 22:31:52 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	linux-kernel@...r.kernel.org
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Michael Buesch <mb@...sch.de>,
	Andreas Schwab <schwab@...ux-m68k.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH] strcmp: fix overflow and possibly signedness error

From: Linus Torvalds <torvalds@...ux-foundation.org>

	signed char __res = *cs - *ct;

is wrong for two reasons.  The subtraction can overflow because __res
doesn't use a type big enough.  Moreover the compared bytes should be
interpreted as unsigned char as specified by POSIX.

The same problem is fixed in strncmp.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc: Michael Buesch <mb@...sch.de>
Cc: Andreas Schwab <schwab@...ux-m68k.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
---
Hello Linus,

I resend with a (hopefully) nice commit log as this didn't appear in
your tree up to now.

Best regards
Uwe

 lib/string.c |   20 ++++++++++++++------
 1 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/lib/string.c b/lib/string.c
index b19b87a..e96421a 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -246,13 +246,17 @@ EXPORT_SYMBOL(strlcat);
 #undef strcmp
 int strcmp(const char *cs, const char *ct)
 {
-	signed char __res;
+	unsigned char c1, c2;
 
 	while (1) {
-		if ((__res = *cs - *ct++) != 0 || !*cs++)
+		c1 = *cs++;
+		c2 = *ct++;
+		if (c1 != c2)
+			return c1 < c2 ? -1 : 1;
+		if (!c1)
 			break;
 	}
-	return __res;
+	return 0;
 }
 EXPORT_SYMBOL(strcmp);
 #endif
@@ -266,14 +270,18 @@ EXPORT_SYMBOL(strcmp);
  */
 int strncmp(const char *cs, const char *ct, size_t count)
 {
-	signed char __res = 0;
+	unsigned char c1, c2;
 
 	while (count) {
-		if ((__res = *cs - *ct++) != 0 || !*cs++)
+		c1 = *cs++;
+		c2 = *ct++;
+		if (c1 != c2)
+			return c1 < c2 ? -1 : 1;
+		if (!c1)
 			break;
 		count--;
 	}
-	return __res;
+	return 0;
 }
 EXPORT_SYMBOL(strncmp);
 #endif
-- 
1.6.5.2

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