linux-kernel - Re: [PATCH 7/9] arm64/build: Warn on orphan section placement

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Message-ID: <CAKwvOd=9JHM4yhfCWEa-8OhZ70v_pChjgKj=TSGZgxZrhS5a_g@mail.gmail.com>
Date:   Tue, 17 Mar 2020 16:10:53 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Will Deacon <will@...nel.org>, Borislav Petkov <bp@...e.de>,
        "H.J. Lu" <hjl.tools@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Collingbourne <pcc@...gle.com>,
        James Morse <james.morse@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Masahiro Yamada <masahiroy@...nel.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/9] arm64/build: Warn on orphan section placement

On Tue, Mar 17, 2020 at 4:01 PM Kees Cook <keescook@...omium.org> wrote:
>
> On Tue, Mar 17, 2020 at 09:56:14PM +0000, Will Deacon wrote:
> > On Thu, Feb 27, 2020 at 04:22:42PM -0800, 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.
> > >
> > > Explicitly include debug sections when they're present. Add .eh_frame*
> > > to discard as it seems that these are still generated even though
> > > -fno-asynchronous-unwind-tables is being specified. Add .plt and
> > > .data.rel.ro to discards as they are not actually used. Add .got.plt
> > > to the image as it does appear to be mapped near .data. Finally enable
> > > orphan section warnings.
> >
> > Hmm, I don't understand what .got.plt is doing here. Please can you
> > elaborate?
>
> I didn't track it down, but it seems to have been present (and merged
> into the kernel .data) for a while now. I can try to track this down if
> you want?

Yes, the presence of a procedure linkage table makes sense for symbol
interposition and lazy binding in userspace executables with runtime
shared object loading support, but not so much the kernel, I would
think.  (Though someone did just recently ask me if loadable kernel
modules could interpose weakly defined symbols in the kernel, and if
so what happens on unload.  I have no idea and suspect kernel modules
cannot do that, but I have looked into the kernel's runtime relocation
support.)
-- 
Thanks,
~Nick Desaulniers