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

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Date:   Thu, 18 May 2023 00:47:41 +0100
From:   Sam James <sam@...too.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nicolas Schier <nicolas@...sle.eu>,
        linux-kbuild@...r.kernel.org, Tom Rix <trix@...hat.com>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] kbuild: Enable -fstrict-flex-arrays=3


Kees Cook <keescook@...omium.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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