[PATCH next 2/3] fortify: Optimise strnlen()

david.laight.linux
2026-03-30T13:20:12Z

From: David Laight <david.laight.linux@gmail.com>

If the string is constant there is no need to call __real_strlen()
even when maxlen is a variable - just return the smaller value.

If the size of the string variable is unknown fortify_panic() can't be
called, change the condition so that the compiler can optimise it away.

Change __compiletime_strlen(p) to return a 'non-constant' value
for non-constant strings (the same as __builtin_strlen()).
Simplify since it is only necessary to check that the size is constant
and that the last character is '\0'.
Explain why it is different from __builtin_strlen().
Update the kunit tests to match.

Signed-off-by: David Laight <david.laight.linux@gmail.com>
---
include/linux/fortify-string.h | 44 +++++++++++++++++-----------------
lib/tests/fortify_kunit.c | 8 +++----
2 files changed, 26 insertions(+), 26 deletions(-)

diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
index 214d237214d5..758afd7c5f8a 100644
--- a/include/linux/fortify-string.h
+++ b/include/linux/fortify-string.h
@@ -58,19 +58,22 @@ void __read_overflow2_field(size_t avail, size_t wanted) __compiletime_warning("
void __write_overflow(void) __compiletime_error("detected write beyond size of object (1st parameter)");
void __write_overflow_field(size_t avail, size_t wanted) __compiletime_warning("detected write beyond size of field (1st parameter); maybe use struct_group()?");

-#define __compiletime_strlen(p) \
-({ \
- char *__p = (char *)(p); \
- size_t __ret = SIZE_MAX; \
- const size_t __p_size = __member_size(p); \
- if (__p_size != SIZE_MAX && \
- __builtin_constant_p(*__p)) { \
- size_t __p_len = __p_size - 1; \
- if (__builtin_constant_p(__p[__p_len]) && \
- __p[__p_len] == '\0') \
- __ret = __builtin_strlen(__p); \
- } \
- __ret; \
+/*
+ * __builtin_strlen() generates a compile-time error for 'const char foo[4] = "abcd";'.
+ * But that is a valid source for both strnlen() and strscpy() with a constant
+ * length less than or equal to 4.
+ * __compiletime_strlen() returns a non-constant for such items.
+ * Beware of strings with embedded '\0', __builtin_strlen() can be much smaller
+ * than __member_size();
+ * The return value must only be used when it is a constant.
+ */
+extern size_t __fortify_undefined;
+#define __compiletime_strlen(p) \
+({ \
+ char *__p = (char *)(p); \
+ const size_t __p_size = __member_size(p); \
+ __p_size == SIZE_MAX || !statically_true(__p[__p_size - 1] == '\0') ? \
+ __fortify_undefined : __builtin_strlen(__p); \
})

#if defined(__SANITIZE_ADDRESS__)
@@ -215,16 +218,13 @@ __FORTIFY_INLINE __kernel_size_t strnlen(const char * const POS p, __kernel_size
const size_t p_len = __compiletime_strlen(p);
size_t ret;

- /* We can take compile-time actions when maxlen is const. */
- if (__builtin_constant_p(maxlen) && p_len != SIZE_MAX) {
- /* If p is const, we can use its compile-time-known len. */
- if (maxlen >= p_size)
- return p_len;
- }
+ /* If p is const, we can use its compile-time-known len. */
+ if (__builtin_constant_p(p_len))
+ return p_len < maxlen ? p_len : maxlen;

/* Do not check characters beyond the end of p. */
- ret = __real_strnlen(p, maxlen < p_size ? maxlen : p_size);
- if (p_size <= ret && maxlen != ret)
+ ret = __real_strnlen(p, p_size < maxlen ? p_size : maxlen);
+ if (ret == p_size && p_size < maxlen)
fortify_panic(FORTIFY_FUNC_strnlen, FORTIFY_READ, p_size, ret + 1, ret);
return ret;
}
@@ -289,7 +289,7 @@ __FORTIFY_INLINE ssize_t sized_strscpy(char * const POS p, const char * const PO
if (statically_true(p_size < SIZE_MAX)) {
len = __compiletime_strlen(q);

- if (len < SIZE_MAX && statically_true(len < size)) {
+ if (statically_true(len < size)) {
__underlying_memcpy(p, q, len + 1);
return len;
}
diff --git a/lib/tests/fortify_kunit.c b/lib/tests/fortify_kunit.c
index fc9c76f026d6..9b3b7201c02d 100644
--- a/lib/tests/fortify_kunit.c
+++ b/lib/tests/fortify_kunit.c
@@ -102,11 +102,11 @@ static void fortify_test_known_sizes(struct kunit *test)
KUNIT_EXPECT_EQ(test, __compiletime_strlen(unchanging_12), 12);

KUNIT_EXPECT_FALSE(test, __is_constexpr(__builtin_strlen(array_unknown)));
- KUNIT_EXPECT_EQ(test, __compiletime_strlen(array_unknown), SIZE_MAX);
+ KUNIT_EXPECT_FALSE(test, __is_constexpr(__compiletime_strlen(array_unknown)));

/* Externally defined and dynamically sized string pointer: */
KUNIT_EXPECT_FALSE(test, __is_constexpr(__builtin_strlen(test->name)));
- KUNIT_EXPECT_EQ(test, __compiletime_strlen(test->name), SIZE_MAX);
+ KUNIT_EXPECT_FALSE(test, __is_constexpr(__compiletime_strlen(test->name)));
}

/* This is volatile so the optimizer can't perform DCE below. */
@@ -128,12 +128,12 @@ static noinline size_t want_minus_one(int pick)
str = "1";
break;
}
- return __compiletime_strlen(str);
+ return __builtin_constant_p(__compiletime_strlen(str));
}

static void fortify_test_control_flow_split(struct kunit *test)
{
- KUNIT_EXPECT_EQ(test, want_minus_one(pick), SIZE_MAX);
+ KUNIT_EXPECT_FALSE(test, want_minus_one(pick));
}

#define KUNIT_EXPECT_BOS(test, p, expected, name) \
--
2.39.5