[PATCH] lib/test_string.c: Add strncmp() tests

[Palmer Dabbelt]

The RISC-V strncmp() fails on some inputs, see the linked thread for
more details.  It turns out there were no strncmp() calls in the self
tests, this adds one.  It currently fails on RISC-V systems with Zbb
enabled with

    [    0.683479] String selftest failure 7.00001001

Link: https://lore.kernel.org/all/2801162.88bMQJbFj6@diego/
Reported-by: Heiko Stübner <heiko@xxxxxxxxx>
Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
---
This reports a checkpatch error for the __initconst, but I think it's
spurious as I've just pattern matched the above test.
---
 lib/test_string.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/lib/test_string.c b/lib/test_string.c
index c5cb92fb710e..8420379963ba 100644
--- a/lib/test_string.c
+++ b/lib/test_string.c
@@ -207,6 +207,40 @@ static __init int strspn_selftest(void)
 	return 0;
 }
 
+static __init int strncmp_selftest(void)
+{
+	static const struct strncmp_test {
+		const char *str_a;
+		const char *str_b;
+		unsigned long count;
+		unsigned long max_off;
+		size_t retval;
+	} tests[] __initconst = {
+		{ "/dev/vda", "/dev/",    5, 4, 0 },
+		{ "/dev/vda", "/dev/vdb", 5, 4, 0 },
+	};
+	size_t i;
+
+	for (i = 0; i < ARRAY_SIZE(tests); ++i) {
+		const struct strncmp_test *s = tests + i;
+		size_t off;
+
+		for (off = 0; off <= s->max_off; ++off) {
+			size_t res = strncmp(s->str_a + off,
+					     s->str_b + off,
+					     s->count - off);
+
+			if (res == 0 && s->retval != 0)
+				return 0x1000 + 0x100*off + 0x10*i + 0x0;
+			if (res > 0 && s->retval <= 0)
+				return 0x1000 + 0x100*off + 0x10*i + 0x1;
+			if (res < 0 && s->retval >= 0)
+				return 0x1000 + 0x100*off + 0x10*i + 0x2;
+		}
+	}
+	return 0;
+}
+
 static __exit void string_selftest_remove(void)
 {
 }
@@ -245,6 +279,11 @@ static __init int string_selftest_init(void)
 	if (subtest)
 		goto fail;
 
+	test = 7;
+	subtest = strncmp_selftest();
+	if (subtest)
+		goto fail;
+
 	pr_info("String selftests succeeded\n");
 	return 0;
 fail:
-- 
2.39.1