[PATCH] kbuild: Enable -fstrict-flex-arrays=3

[PATCH] kbuild: Enable -fstrict-flex-arrays=3

[Kees Cook]

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@kernel.org>
Cc: "Gustavo A. R. Silva" <gustavoars@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
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 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

Re: [PATCH] kbuild: Enable -fstrict-flex-arrays=3

[Gustavo A. R. Silva]

On Wed, May 17, 2023 at 04:28:04PM -0700, Kees Cook wrote:
> 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.

It's happening! :'-)

> 
> 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: "Gustavo A. R. Silva" <gustavoars@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
> Signed-off-by: Kees Cook <keescook@chromium.org>

Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Thanks!
--
Gustavo

> ---
>  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
> 

Re: [PATCH] kbuild: Enable -fstrict-flex-arrays=3

[Sam James]

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Kees Cook <keescook@chromium.org> writes:

> 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
>

Maybe link to https://people.kernel.org/kees/bounded-flexible-arrays-in-c as well 
just in case some confused soul ends up bisecting to this but doesn't
get the problem?

Not really required though, just a thought I had.

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Re: [PATCH] kbuild: Enable -fstrict-flex-arrays=3

[Kees Cook]

On Thu, May 18, 2023 at 12:47:41AM +0100, Sam James wrote:
> 
> Kees Cook <keescook@chromium.org> writes:
> 
> > 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
> >
> 
> Maybe link to https://people.kernel.org/kees/bounded-flexible-arrays-in-c as well 
> just in case some confused soul ends up bisecting to this but doesn't
> get the problem?
> 
> Not really required though, just a thought I had.

Ah yeah, good idea! :)

-- 
Kees Cook