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

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Message-ID: <CAKwvOdk1ir=D---9xVAxcErJWSGVxK1Mv6AC=TK3RVwNdcvFjw@mail.gmail.com>
Date:   Fri, 13 Nov 2020 11:34:11 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Florian Fainelli <f.fainelli@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Abbott Liu <liuwenliang@...wei.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Jian Cai <jiancai@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] kbuild: Always link with '-z norelro'

On Thu, Nov 12, 2020 at 10:06 PM Ard Biesheuvel <ardb@...nel.org> wrote:
>
> On Fri, 13 Nov 2020 at 01:53, Nathan Chancellor
> <natechancellor@...il.com> wrote:
> >
> > 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@...il.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@...gle.com>
> > > > Signed-off-by: Nathan Chancellor <natechancellor@...il.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.
> >
>
> Modules are partially linked objects, so there is no point in passing
> -z options for them.

Ok then.

Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>

More work to be done to boot this config, but at least we can link
without error.
-- 
Thanks,
~Nick Desaulniers