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Date:   Wed, 17 May 2023 16:54:18 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Sam James <sam@gentoo.org>
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nicolas Schier <nicolas@fjasle.eu>,
        linux-kbuild@vger.kernel.org, Tom Rix <trix@redhat.com>,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
        linux-hardening@vger.kernel.org
Subject: Re: [PATCH] kbuild: Enable -fstrict-flex-arrays=3
Message-ID: <202305171653.8A5393C6@keescook>
References: <20230517232801.never.262-kees@kernel.org>
 <87lehmh647.fsf@gentoo.org>
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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