On Mon, Mar 30, 2026 at 02:20:02PM +0100, [email protected] wrote: > From: David Laight <[email protected]> > > 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.
This will need very very careful checking. It took a lot of tweaking to get __compiletime_strlen() to behave correctly across all supported versions of GCC and Clang. It has some very ugly glitches possible. I'm not saying your replacement is broken: I mean to say I'm going to need to spend a bunch of time proving to myself that this is a safe replacement. Outside of that, though, I don't think dropping the various SIZE_MAX tests is correct. Again, I'll need to spend some time double-checking. -Kees > > Signed-off-by: David Laight <[email protected]> > --- > 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 > -- Kees Cook
