[PATCH v2 2/2] lib/test_string.c: Add strncmp() tests

["Björn Töpel" <bjorn@kernel.org>]

From: Palmer Dabbelt <palmer@rivosinc.com>

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.

Link: https://lore.kernel.org/all/2801162.88bMQJbFj6@diego/
Reported-by: Heiko Stübner <heiko@sntech.de>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
---

v1->v2: Added two more tests (pos/neg). (Andy)
        Minor code style issues. (Andy)
        Fixed checkpatch errors.

---
lib/test_string.c | 43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/lib/test_string.c b/lib/test_string.c
index 550229084c41..b95037eb138b 100644
--- a/lib/test_string.c
+++ b/lib/test_string.c
@@ -209,6 +209,44 @@ static __init int strspn_selftest(void)
 	return 0;
 }
 
+struct strncmp_test {
+	const char *str_a;
+	const char *str_b;
+	unsigned long count;
+	unsigned long max_off;
+	int retval;
+};
+
+static __init int strncmp_selftest(void)
+{
+	size_t i;
+	static const struct strncmp_test tests[] __initconst = {
+		{ "/dev/vda", "/dev/", 5, 4, 0 },
+		{ "/dev/vda", "/dev/vdb", 5, 4, 0 },
+		{ "00000000---11111", "00000000---11112", 12, 4, 0 },
+		{ "ABC", "AB", 3, 0, 67 },
+		{ "ABA", "ABZ", 3, 0, -25 },
+		{ "ABC", "ABC", 3, 0, 0 },
+	};
+
+	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++) {
+			int 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)
 {
 }
@@ -247,6 +285,11 @@ 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:
-- 
2.37.2