kbuild: Enable -fstrict-flex-arrays=3

The -fstrict-flex-arrays=3 option is now available with the release
of GCC 13[1] and Clang 16[2]. This feature instructs the compiler to
treat only C99 flexible arrays as dynamically sized for the purposes of
object size calculations. In other words, the ancient practice of using
1-element arrays, or the GNU extension of using 0-sized arrays, as a
dynamically sized array is disabled. This allows CONFIG_UBSAN_BOUNDS,
CONFIG_FORTIFY_SOURCE, and other object-size aware features to behave
unambiguously in the face of trailing arrays: only C99 flexible arrays
are considered to be dynamically sized. For yet more detail, see:
https://people.kernel.org/kees/bounded-flexible-arrays-in-c

Enabling this will help track down any outstanding cases of fake
flexible arrays that need attention in kernel code.

[1] https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html#index-fstrict-flex-arrays
[2] https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-fstrict-flex-arrays

Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nicolas Schier <nicolas@fjasle.eu>
Cc: linux-kbuild@vger.kernel.org
Co-developed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Signed-off-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>

Makefile

@@ -1026,6 +1026,12 @@ KBUILD_CFLAGS += -Wno-pointer-sign
 # globally built with -Wcast-function-type.
 KBUILD_CFLAGS += $(call cc-option, -Wcast-function-type)
 
+# To gain proper coverage for CONFIG_UBSAN_BOUNDS and CONFIG_FORTIFY_SOURCE,
+# the kernel uses only C99 flexible arrays for dynamically sized trailing
+# arrays. Enforce this for everything that may examine structure sizes and
+# perform bounds checking.
+KBUILD_CFLAGS += $(call cc-option, -fstrict-flex-arrays=3)
+
 # disable stringop warnings in gcc 8+
 KBUILD_CFLAGS += $(call cc-disable-warning, stringop-truncation)