Re: [PATCH v3 3/9] efi/libstub: Remove .note.gnu.property

From: Will Deacon <will@kernel.org>
To: Dave Martin <Dave.Martin@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	linux-efi <linux-efi@vger.kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-arch <linux-arch@vger.kernel.org>,
	Fangrui Song <maskray@google.com>,
	Masahiro Yamada <masahiroy@kernel.org>, X86 ML <x86@kernel.org>,
	Russell King <linux@armlinux.org.uk>,
	Ard Biesheuvel <ardb@kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@suse.de>,
	Kees Cook <keescook@chromium.org>, Arnd Bergmann <arnd@arndb.de>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Peter Collingbourne <pcc@google.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	James Morse <james.morse@arm.com>
Subject: Re: [PATCH v3 3/9] efi/libstub: Remove .note.gnu.property
Date: Wed, 24 Jun 2020 16:26:46 +0100	[thread overview]
Message-ID: <20200624152646.GA6768@willie-the-truck> (raw)
In-Reply-To: <20200624134854.GF25945@arm.com>

On Wed, Jun 24, 2020 at 02:48:55PM +0100, Dave Martin wrote:
> On Wed, Jun 24, 2020 at 12:26:47PM +0100, Will Deacon wrote:
> > On Wed, Jun 24, 2020 at 12:46:32PM +0200, Ard Biesheuvel wrote:
> > > On Wed, 24 Jun 2020 at 12:44, Will Deacon <will@kernel.org> wrote:
> > > > For the kernel Image, how do we remove these sections? The objcopy flags
> > > > in arch/arm64/boot/Makefile look both insufficient and out of date. My
> > > > vmlinux ends up with both a ".notes" and a ".init.note.gnu.property"
> > > > segment.
> > > 
> > > The latter is the fault of the libstub make rules, that prepend .init
> > > to all section names.
> > 
> > Hmm. I tried adding -mbranch-protection=none to arm64 cflags for the stub,
> > but I still see this note in vmlinux. It looks like it comes in via the
> > stub copy of lib-ctype.o, but I don't know why that would force the
> > note. The cflags look ok to me [1] and I confirmed that the note is
> > being generated by the compiler.
> > 
> > > I'm not sure if there is a point to having PAC and/or BTI in the EFI
> > > stub, given that it runs under the control of the firmware, with its
> > > memory mappings and PAC configuration etc.
> > 
> > Agreed, I just can't figure out how to get rid of the note.
> 
> Because this section is generated by the linker itself I think you might
> have to send it to /DISCARD/ in the link, or strip it explicitly after
> linking.

Right, but why is the linker generating that section in the first place? I'm
compiling with -mbranch-protection=none and all the other objects linked
into the stub do not have the section.

I wonder if it's because lib/ctype.c doesn't have any executable code...

Will

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