On Mon, 30 Mar 2026 16:54:27 -0700 Kees Cook <[email protected]> wrote:
> 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. There are two differences: 1) I removed the check for p[0] being constant, just p[size - 1]. 2) The return value is 'not constant' (rather than SIZE_MAX) for non-constant strings. This includes any case where __builtin_strlen() returns a non-constant. I did wonder what happens if you have: static char foo[] = "foo"; then later: foo[1] = 0; The compiler could determine that both foo[0] and foo[3] are never changed, so (both versions of) __compiletime_strlen() would return the non-constant result of __builtin_strlen(). The current compilers don't report foo[3] as being constant unless the string itself in constant, but they might get 'better'. > 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. They needed changing to match the function returning 'not constant' rather than SIZE_MAX. Actually, as I noted in the other email (after writing, but not sending, this one) __compiletime_strlen() is fundamentally broken with the current compilers. Any uses should be replaced by __builtin_strlen(). David > > -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 > > >
