Re: [PATCH v2 mips-next 2/4] MIPS: vmlinux.lds.S: add ".gnu.attributes" to DISCARDS - Fāng-ruì Sòng

From: "Fāng-ruì Sòng" <maskray@google.com>
To: Alexander Lobakin <alobakin@pm.me>
Cc: Kees Cook <keescook@chromium.org>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Jiaxun Yang <jiaxun.yang@flygoat.com>,
	Alex Smith <alex.smith@imgtec.com>,
	Ralf Baechle <ralf@linux-mips.org>,
	Markos Chandras <markos.chandras@imgtec.com>,
	linux-mips <linux-mips@vger.kernel.org>,
	"# 3.4.x" <stable@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 mips-next 2/4] MIPS: vmlinux.lds.S: add ".gnu.attributes" to DISCARDS
Date: Wed, 6 Jan 2021 14:43:08 -0800	[thread overview]
Message-ID: <CAFP8O3LY=puWC7-O+HakE0mBuZwyWi01RHt7+tBGBj5WQiT5qA@mail.gmail.com> (raw)
In-Reply-To: <202101061400.8F83981AE@keescook>

On Wed, Jan 6, 2021 at 2:07 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Wed, Jan 06, 2021 at 08:08:19PM +0000, Alexander Lobakin wrote:
> > Discard GNU attributes at link time as kernel doesn't use it at all.
> > Solves a dozen of the following ld warnings (one per every file):
> >
> > mips-alpine-linux-musl-ld: warning: orphan section `.gnu.attributes'
> > from `arch/mips/kernel/head.o' being placed in section
> > `.gnu.attributes'
> > mips-alpine-linux-musl-ld: warning: orphan section `.gnu.attributes'
> > from `init/main.o' being placed in section `.gnu.attributes'
> >
> > Misc: sort DISCARDS section entries alphabetically.
>
> Hmm, I wonder what is causing the appearance of .eh_frame? With help I
> tracked down all the causes of this on x86, arm, and arm64, so that's
> why it's not in the asm-generic DISCARDS section. I suspect this could
> be cleaned up for mips too?

On x86, 003602ad5516e59940de42e44c8d8033387bb363 "x86/*/Makefile: Use
-fno-asynchronous-unwind-tables to suppress .eh_frame sections"
noticed that some Makefiles
redefined KBUILD_CFLAGS and dropped -fno-asynchronous-unwind-tables.
Maybe mips has similar issues.

> Similarly for .gnu.attributes. What is generating that? (Or, more
> specifically, why is it both being generated AND discarded?)
>
> -Kees

gcc/config/mips/mips.c
https://github.com/gcc-mirror/gcc/blob/master/gcc/config/mips/mips.c#L9965
.gnu_attribute 4, 0 does not produce .gnu.attributes
(SHT_GNU_ATTRIBUTES) but there are likely code paths that
a non-zero value is used... So .gnu_attributes is likely needed to be excluded.

> >
> > Signed-off-by: Alexander Lobakin <alobakin@pm.me>
> > ---
> >  arch/mips/kernel/vmlinux.lds.S | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/mips/kernel/vmlinux.lds.S b/arch/mips/kernel/vmlinux.lds.S
> > index 83e27a181206..5d6563970ab2 100644
> > --- a/arch/mips/kernel/vmlinux.lds.S
> > +++ b/arch/mips/kernel/vmlinux.lds.S
> > @@ -221,9 +221,10 @@ SECTIONS
> >               /* ABI crap starts here */
> >               *(.MIPS.abiflags)
> >               *(.MIPS.options)
> > +             *(.eh_frame)
> > +             *(.gnu.attributes)
> >               *(.options)
> >               *(.pdr)
> >               *(.reginfo)
> > -             *(.eh_frame)
> >       }
> >  }
> > --
> > 2.30.0
> >
> >
>
> --
> Kees Cook

-- 
宋方睿