string.c: test *cmp for all possible 1-character strings

The switch to -funsigned-char made a pre-existing bug on m68k more
apparent. That is now fixed (by removing m68k's private strcmp(), see
commit 7c08461), but we still have quite a few architectures that
provide one or more of strcmp(), strncmp() and memcmp().

They probably all work fine for the cases where the input is all
ASCII, and/or where the caller only wants to know about equality or
not (i.e. only checks whether the return value is 0 or not).

Let's check that all these implementations also behave correctly for
bytes with the high bit set, and provide the correct ordering -
independent of us now building with -funsigned-char, the C standard
says that these *cmp functions should consider the buffers as
consisting of unsigned chars.

This is only intended to help find other latent bugs and can/should be
ripped out again before v6.2, or perhaps moved to test_string.c in
some form, but for now I think it's worth doing unconditionally.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>

lib/string.c

@@ -880,3 +880,30 @@ void *memchr_inv(const void *start, int c, size_t bytes)
 	return check_bytes8(start, value, bytes % 8);
 }
 EXPORT_SYMBOL(memchr_inv);
+
+static int sign(int x)
+{
+	return (x > 0) - (x < 0);
+}
+
+static int test_xxxcmp(void)
+{
+	char a[2], b[2];
+	int i, j;
+
+	a[1] = b[1] = 0;
+	for (i = 0; i < 256; ++i) {
+		a[0] = i;
+		for (j = 0; j < 256; ++j) {
+			b[0] = j;
+			WARN_ONCE(sign(strcmp(a, b)) != sign(i - j),
+				  "strcmp() broken for (%2ph, %2ph)\n", a, b);
+			WARN_ONCE(sign(memcmp(a, b, 2)) != sign(i - j),
+				  "memcmp() broken for (%2ph, %2ph)\n", a, b);
+			WARN_ONCE(sign(strncmp(a, b, 2)) != sign(i - j),
+				  "strncmp() broken for (%2ph, %2ph)\n", a, b);
+		}
+	}
+	return 0;
+}
+late_initcall(test_xxxcmp);