fortify: Short-circuit known-safe calls to strscpy()

Replacing compile-time safe calls of strcpy()-related functions with
strscpy() was always calling the full strscpy() logic when a builtin
would be better. For example:

	char buf[16];
	strcpy(buf, "yes");

would reduce to __builtin_memcpy(buf, "yes", 4), but not if it was:

	strscpy(buf, yes, sizeof(buf));

Fix this by checking if all sizes are known at compile-time.

Cc: linux-hardening@vger.kernel.org
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>

include/linux/fortify-string.h

@@ -314,6 +314,16 @@ __FORTIFY_INLINE ssize_t strscpy(char * const POS p, const char * const POS q, s
 	if (__compiletime_lessthan(p_size, size))
 		__write_overflow();
 
+	/* Short-circuit for compile-time known-safe lengths. */
+	if (__compiletime_lessthan(p_size, SIZE_MAX)) {
+		len = __compiletime_strlen(q);
+
+		if (len < SIZE_MAX && __compiletime_lessthan(len, size)) {
+			__underlying_memcpy(p, q, len + 1);
+			return len;
+		}
+	}
+
 	/*
 	 * This call protects from read overflow, because len will default to q
 	 * length if it smaller than size.

lib/strscpy_kunit.c

@@ -81,6 +81,8 @@ static void tc(struct kunit *test, char *src, int count, int expected,
 
 static void strscpy_test(struct kunit *test)
 {
+	char dest[8];
+
 	/*
 	 * tc() uses a destination buffer of size 6 and needs at
 	 * least 2 characters spare (one for null and one to check for
@@ -111,6 +113,17 @@ static void strscpy_test(struct kunit *test)
 	tc(test, "ab",   4, 2,	    2, 1, 1);
 	tc(test, "a",    4, 1,	    1, 1, 2);
 	tc(test, "",     4, 0,	    0, 1, 3);
+
+	/* Compile-time-known source strings. */
+	KUNIT_EXPECT_EQ(test, strscpy(dest, "", ARRAY_SIZE(dest)), 0);
+	KUNIT_EXPECT_EQ(test, strscpy(dest, "", 3), 0);
+	KUNIT_EXPECT_EQ(test, strscpy(dest, "", 1), 0);
+	KUNIT_EXPECT_EQ(test, strscpy(dest, "", 0), -E2BIG);
+	KUNIT_EXPECT_EQ(test, strscpy(dest, "Fixed", ARRAY_SIZE(dest)), 5);
+	KUNIT_EXPECT_EQ(test, strscpy(dest, "Fixed", 3), -E2BIG);
+	KUNIT_EXPECT_EQ(test, strscpy(dest, "Fixed", 1), -E2BIG);
+	KUNIT_EXPECT_EQ(test, strscpy(dest, "Fixed", 0), -E2BIG);
+	KUNIT_EXPECT_EQ(test, strscpy(dest, "This is too long", ARRAY_SIZE(dest)), -E2BIG);
 }
 
 static struct kunit_case strscpy_test_cases[] = {