linux-hardening - Re: [PATCH 31/64] fortify: Explicitly disable Clang support

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Date:   Tue, 27 Jul 2021 14:18:58 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Kees Cook <keescook@...omium.org>, linux-hardening@...r.kernel.org
Cc:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Keith Packard <keithpac@...zon.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-staging@...ts.linux.dev, linux-block@...r.kernel.org,
        linux-kbuild@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH 31/64] fortify: Explicitly disable Clang support

On 7/27/2021 1:58 PM, Kees Cook 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@...omium.org>
> ---
>   security/Kconfig | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/security/Kconfig b/security/Kconfig
> index 0ced7fd33e4d..8f0e675e70a4 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 !CONFIG_CC_IS_CLANG

Should be !CC_IS_CLANG, Kconfig is hard :)

>   	help
>   	  Detect overflows of buffers in common string and memory functions
>   	  where the compiler can determine and validate the buffer sizes.
> 

Cheers,
Nathan