strcmp: fix overflow and possibly signedness error

Doing the strcmp return value as

	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@pengutronix.de>
Cc: Michael Buesch <mb@bu3sch.de>
Cc: Andreas Schwab <schwab@linux-m68k.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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