Re: [RFT PATCH] arm64: relocatable: build the kernel as a proper shared library

From: Will Deacon <will.deacon@arm.com>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Peter Smith <peter.smith@linaro.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFT PATCH] arm64: relocatable: build the kernel as a proper shared library
Date: Mon, 3 Dec 2018 18:57:45 +0000	[thread overview]
Message-ID: <20181203185745.GB29028@arm.com> (raw)
In-Reply-To: <CAKv+Gu9cZ6jMsoMG499g0416J6GW8KxJBmit9=F9ka4AqpZFZg@mail.gmail.com>

On Mon, Dec 03, 2018 at 06:11:47PM +0100, Ard Biesheuvel wrote:
> On Mon, 3 Dec 2018 at 18:08, Will Deacon <will.deacon@arm.com> wrote:
> >
> > Hi Ard,
> >
> > On Sat, Dec 01, 2018 at 12:53:24PM +0100, Ard Biesheuvel wrote:
> > > readelf complains about the section layout of vmlinux when building
> > > with CONFIG_RELOCATABLE=y (for KASLR):
> > >
> > >   readelf: Warning: [21]: Link field (0) should index a symtab section.
> > >   readelf: Warning: [21]: Info field (0) should index a relocatable section.
> > >
> > > Also, it seems that our use of '-pie -shared' is contradictory, and
> > > thus ambiguous. In general, the way KASLR is wired up at the moment
> > > is highly tailored to how ld.bfd happens to implement (and conflate)
> > > PIE executables and shared libraries, so given the current effort to
> > > support other toolchains, let's fix some of these issues as well.
> > >
> > > - Drop the -pie linker argument and just leave -shared. In ld.bfd,
> > >   the differences between them are unclear (except for the ELF type
> > >   of the produced image [0]) but lld chokes on seeing both at the
> > >   same time.
> > >
> > > - Rename the .rela output section to .rela.dyn, as is customary for
> > >   shared libraries and PIE executables.
> > >
> > > - Don't discard the .dynamic, .dynsym, .dynstr and .hash sections.
> > >   Instead, make sure that they are [mostly] empty by marking all
> > >   symbols as local, and emit them into the .init segment.
> >
> > These second two changes seem fairly arbitrary to me: is lld unhappy without
> > them? My concern is that we'll make some other change in the future and run
> > into more arbitrary breakage. Is there any build bots hooked up with lld so
> > help us catch regressions quickly? (i.e nightly builds of mainline and next)
> >
> 
> The .rela.dyn change is for readelf actually, not for lld. Discarding
> the .dyn* sections is indeed something lld chokes on.
> 
> It seems reasonable to make the vmlinux ELF image look as much as an
> ordinary shared library as we can, to avoid issues with tooling in
> general, not just lld. This is why I separated this patch from the one
> that caters for lld in particular.

Given that the LLD developers are looking at addressing the section
discards, can you repost without that part please?

Will

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