Re: [PATCH] kbuild: Always link with '-z norelro'

From: Nathan Chancellor <natechancellor@gmail.com>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>,
	Michal Marek <michal.lkml@markovi.net>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Russell King <linux@armlinux.org.uk>,
	Florian Fainelli <f.fainelli@gmail.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Abbott Liu <liuwenliang@huawei.com>,
	Linus Walleij <linus.walleij@linaro.org>,
	Jian Cai <jiancai@google.com>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] kbuild: Always link with '-z norelro'
Date: Thu, 12 Nov 2020 17:53:47 -0700	[thread overview]
Message-ID: <20201113005347.GA3625030@ubuntu-m3-large-x86> (raw)
In-Reply-To: <CAKwvOdkShrqgNDWO0bsPcPZLx-+u79mfmPrGy7CnSKZVdcYzSA@mail.gmail.com>

On Thu, Nov 12, 2020 at 04:44:46PM -0800, Nick Desaulniers wrote:
> On Thu, Nov 12, 2020 at 10:41 AM Nathan Chancellor
> <natechancellor@gmail.com> wrote:
> >
> > Commit 3bbd3db86470 ("arm64: relocatable: fix inconsistencies in linker
> > script and options") added '-z norelro' to the arm64 Makefile when
> > CONFIG_RELOCATABLE was set to help support ld.lld because ld.lld
> > defaults to '-z relro' but the kernel does not use program headers or
> > adhere to the section layout that is required for RELRO to work.
> >
> > Commit 3b92fa7485eb ("arm64: link with -z norelro regardless of
> > CONFIG_RELOCATABLE") unconditionally added it to LDFLAGS_vmlinux because
> > an error occurs with CONFIG_KASAN set even when CONFIG_RELOCATABLE is
> > unset.
> >
> > As it turns out, ARM experiences the same error after CONFIG_KASAN was
> > implemented, meaning that '-z norelro' needs to be added to that
> > Makefile as well (multi_v7_defconfig + CONFIG_KASAN=y + LD=ld.lld):
> >
> > $ make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- LLVM=1 zImage
> > ld.lld: error: section: .exit.data is not contiguous with other relro sections
> >
> > To avoid playing whack-a-mole with different architectures over time,
> > hoist '-z norelro' into the main Makefile. This does not affect ld.bfd
> > because '-z norelro' is the default for it.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1189
> > Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
> > Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> 
> Why not add it additionally to KBUILD_LDFLAGS_MODULE a la
> `--build-id=sha1` a few lines above? (or `LDFLAGS_MODULE`, but that
> looks unused?)  We probably don't want this for modules either.  In
> that case, you could add -z norelo to the two existing lines with
> `--build-id=sha1` above?

Yes, I can do that. I will send a v2 along tomorrow morning to let
others comment.

Cheers,
Nathan