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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Wed, 25 Aug 2021 12:41:56 -0700
Message-ID: <CAKwvOd=A+ueGV2ihdy5GtgR2fQbcXjjAtVxv3=cPjffpebZB7A@mail.gmail.com>
Subject: Re: [PATCH for-next 16/25] fortify: Explicitly disable Clang support
To:     Kees Cook <keescook@chromium.org>
Cc:     linux-kernel@vger.kernel.org,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Daniel Micay <danielmicay@gmail.com>,
        Francis Laniel <laniel_francis@privacyrequired.com>,
        Bart Van Assche <bvanassche@acm.org>,
        David Gow <davidgow@google.com>, linux-mm@kvack.org,
        clang-built-linux@googlegroups.com, linux-hardening@vger.kernel.org
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On Sun, Aug 22, 2021 at 12:57 AM Kees Cook <keescook@chromium.org> wrote:
>
> Clang has never correctly compiled the FORTIFY_SOURCE defenses due to
> a couple bugs:
>
>         Eliding inlines with matching __builtin_* names
>         https://bugs.llvm.org/show_bug.cgi?id=50322
>
>         Incorrect __builtin_constant_p() of some globals
>         https://bugs.llvm.org/show_bug.cgi?id=41459
>
> In the process of making improvements to the FORTIFY_SOURCE defenses, the
> first (silent) bug (coincidentally) becomes worked around, but exposes
> the latter which breaks the build. As such, Clang must not be used with
> CONFIG_FORTIFY_SOURCE until at least latter bug is fixed (in Clang 13),
> and the fortify routines have been rearranged.
>
> Update the Kconfig to reflect the reality of the current situation.
>
> Signed-off-by: Kees Cook <keescook@chromium.org>

Acked-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
>  security/Kconfig | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/security/Kconfig b/security/Kconfig
> index 0ced7fd33e4d..fe6c0395fa02 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -191,6 +191,9 @@ config HARDENED_USERCOPY_PAGESPAN
>  config FORTIFY_SOURCE
>         bool "Harden common str/mem functions against buffer overflows"
>         depends on ARCH_HAS_FORTIFY_SOURCE
> +       # https://bugs.llvm.org/show_bug.cgi?id=50322
> +       # https://bugs.llvm.org/show_bug.cgi?id=41459
> +       depends on !CC_IS_CLANG
>         help
>           Detect overflows of buffers in common string and memory functions
>           where the compiler can determine and validate the buffer sizes.
> --
> 2.30.2
>
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-- 
Thanks,
~Nick Desaulniers