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. 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@xxxxxxxxxx> Cc: "Gustavo A. R. Silva" <gustavoars@xxxxxxxxxx> Cc: Nathan Chancellor <nathan@xxxxxxxxxx> Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> Cc: Nicolas Schier <nicolas@xxxxxxxxx> Cc: linux-kbuild@xxxxxxxxxxxxxxx Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> --- Makefile | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/Makefile b/Makefile index f836936fb4d8..07e5aec1daf5 100644 --- a/Makefile +++ b/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) -- 2.34.1
