Re: [PATCH] x86/boot/compressed: Disable relocation relaxation for non-pie link

From: Nick Desaulniers <ndesaulniers@google.com>
To: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Fangrui Song <maskray@google.com>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	e5ten.arch@gmail.com,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Ard Biesheuvel <ardb@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"# 3.4.x" <stable@vger.kernel.org>
Subject: Re: [PATCH] x86/boot/compressed: Disable relocation relaxation for non-pie link
Date: Tue, 11 Aug 2020 16:51:23 -0700	[thread overview]
Message-ID: <CAKwvOdn5gCjcAVHZ3jHU+q=mD5rmFAHpEyHyLf7ixtdaQ3Z-PQ@mail.gmail.com> (raw)
In-Reply-To: <20200811234340.GA1318440@rani.riverdale.lan>

On Tue, Aug 11, 2020 at 4:43 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> On Tue, Aug 11, 2020 at 04:04:40PM -0700, Nick Desaulniers wrote:
> > On Tue, Aug 11, 2020 at 3:44 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
> > >
> > > On Tue, Aug 11, 2020 at 10:58:40AM -0700, Nick Desaulniers wrote:
> > > > > Cc: stable@vger.kernel.org # 4.19.x
> > > >
> > > > Thanks Arvind, good write up.  Just curious about this stable tag, how
> > > > come you picked 4.19?  I can see boot failures in our CI for x86+LLD
> > > > back to 4.9.  Can we amend that tag to use `# 4.9`? I'd be happy to
> > > > help submit backports should they fail to apply cleanly.
> > > > https://travis-ci.com/github/ClangBuiltLinux/continuous-integration/builds/179237488
> > > >
> > >
> > > 4.19 renamed LDFLAGS to KBUILD_LDFLAGS. For 4.4, 4.9 and 4.14 the patch
> > > needs to be modified, KBUILD_LDFLAGS -> LDFLAGS, so I figured we should
> > > submit backports separately. For 4.19 onwards, it should apply without
> > > changes I think.
> >
> > Cool, sounds good.  I'll keep an eye out for when stable goes to pick this up.
> >
> > tglx, Ingo, BP, can we pretty please get this in tip/urgent for
> > inclusion into 5.9?
> > --
> > Thanks,
> > ~Nick Desaulniers
>
> Another alternative is to just do this unconditionally instead of even
> checking for the -pie flag. None of the GOTPCRELs are in the
> decompressor, so they shouldn't be performance-sensitive at all.
>
> It still wouldn't apply cleanly to all the stable versions, but
> backporting would be even simpler.
>
> What do you think?
>
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 3962f592633d..10c2ba59d192 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -43,6 +43,7 @@ KBUILD_CFLAGS += -Wno-pointer-sign
>  KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
>  KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
>  KBUILD_CFLAGS += -D__DISABLE_EXPORTS
> +KBUILD_CFLAGS += $(call as-option,-Wa$(comma)-mrelax-relocations=no)

We'd still want it for KBUILD_AFLAGS, too, just to be safe. Maybe a
one line comment to the effect of `# remove me once we can link as
-pie` would help us rip off this band-aid in the future?  It's more
obvious that the added hunk can be reverted once -pie linkage is
achieved with the current patch; either are fine by me.  Thanks!

>
>  KBUILD_AFLAGS  := $(KBUILD_CFLAGS) -D__ASSEMBLY__
>  GCOV_PROFILE := n


-- 
Thanks,
~Nick Desaulniers