linux-kernel - Re: [PATCH v3 9/9] arm64/build: Warn on orphan section placement

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Date:   Wed, 24 Jun 2020 08:36:20 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Will Deacon <will@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Peter Collingbourne <pcc@...gle.com>,
        James Morse <james.morse@....com>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>, x86@...nel.org,
        clang-built-linux@...glegroups.com, linux-arch@...r.kernel.org,
        linux-efi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 9/9] arm64/build: Warn on orphan section placement

On Wed, Jun 24, 2020 at 08:57:12AM +0100, Will Deacon wrote:
> On Tue, Jun 23, 2020 at 06:49:40PM -0700, Kees Cook wrote:
> > We don't want to depend on the linker's orphan section placement
> > heuristics as these can vary between linkers, and may change between
> > versions. All sections need to be explicitly named in the linker
> > script.
> > 
> > Avoid .eh_frame* by making sure both -fno-asychronous-unwind-tables and
> > -fno-unwind-tables are present in both CFLAGS and AFLAGS. Remove one
> > last instance of .eh_frame by removing the needless Call Frame Information
> > annotations from arch/arm64/kernel/smccc-call.S.
> > 
> > Add .plt, .data.rel.ro, .igot.*, and .iplt to discards as they are not
> > actually used. While .got.plt is also not used, it must be included
> > otherwise ld.bfd will fail to link with the error:
> > 
> >     aarch64-linux-gnu-ld: discarded output section: `.got.plt'
> > 
> > However, as it'd be better to validate that it stays effectively empty,
> > add an assert.
> > 
> > Explicitly include debug sections when they're present.
> > 
> > Fix a case of needless quotes in __section(), which Clang doesn't like.
> > 
> > Finally, enable orphan section warnings.
> > 
> > Thanks to Ard Biesheuvel for many hints on correct ways to handle
> > mysterious sections. :)
> 
> Sorry to be a pain, but this patch is doing 3 or 4 independent things at
> once. Please could you split it up a bit?
> e.g.
> 
>  - Removal of cfi directives from smccc macro
>  - Removal of quotes around section name for clang
>  - Avoid generating .eh_frame
>  - Ensure all sections are accounted for in linker script and warn on orphans
> 
> That way it's a bit easier to manage, we can revert/backport bits later if
> necessary and you get more patches in the kernel ;)

Yeah, this one patch did grow a bit. ;) I've split it up now.

> You can also add my Ack on all the patches:
> 
> Acked-by: Will Deacon <will@...nel.org>

Thanks!

-- 
Kees Cook