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

Kees Cook
2021-07-27T21:47:31Z

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@chromium.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