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

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Message-ID: <202107271434.039A9777@keescook>
Date:   Tue, 27 Jul 2021 14:47:22 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     linux-hardening@...r.kernel.org,
        "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 Tue, Jul 27, 2021 at 02:18:58PM -0700, Nathan Chancellor wrote:
> 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 :)

/me shakes fist at sky

Thank you! Fixed locally. :)

-Kees

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

-- 
Kees Cook