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

Kees Cook
2023-05-17T23:54:28Z

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