[RFC PATCH 12/17] gcc-plugins: objtool: Add plugin to detect switch table on arm64

[Nick Desaulniers]

On Wed, Jan 27, 2021 at 3:27 PM Josh Poimboeuf <jpoimboe at redhat.com> wrote:
>
> On Wed, Jan 27, 2021 at 02:15:57PM -0800, Nick Desaulniers wrote:
> > > From: Raphael Gault <raphael.gault at arm.com>
> > >
> > > This plugins comes into play before the final 2 RTL passes of GCC and
> > > detects switch-tables that are to be outputed in the ELF and writes
> > > information in an ".discard.switch_table_info" section which will be
> > > used by objtool.
> > >
> > > Signed-off-by: Raphael Gault <raphael.gault at arm.com>
> > > [J.T.: Change section name to store switch table information,
> > >        Make plugin Kconfig be selected rather than opt-in by user,
> > >        Add a relocation in the switch_table_info that points to
> > >        the jump operation itself]
> > > Signed-off-by: Julien Thierry <jthierry at redhat.com>
> >
> > Rather than tightly couple this feature to a particular toolchain via
> > plugin, it might be nice to consider what features could be spec'ed out
> > for toolchains to implement (perhaps via a -f flag).
>
> The problem is being able to detect switch statement jump table vectors.
>
> For a given indirect branch (due to a switch statement), what are all
> the corresponding jump targets?
>
> We would need the compiler to annotate that information somehow.

Makes sense, the compiler should have this information.  How is this
problem solved on x86?

>
> > Distributions (like Android, CrOS) wont be able to use such a feature as
> > is.
>
> Would a Clang plugin be out of the question?

Generally, we frown on out of tree kernel modules for a couple reasons.

Maintaining ABI compatibility when the core kernel changes is
generally not instantaneous; someone has to notice the ABI has changed
which will be more delayed than if the module was in tree.  Worse is
when semantics subtly change.  While we must not break userspace, we
provide no such guarantees within the kernel proper.

Also, it's less likely that out of tree kernel modules have been
reviewed by kernel developers.  They may not have the same quality,
use the recommended interfaces, follow coding conventions, etc..

Oh, did I say "out of tree kernel modules?"  I meant "compiler
plugins."  But it's two different sides of the same coin to me.

FWIW, I think the approach taken by -mstack-protector-guard-reg= is a
useful case study.  It was prototyped as a GCC extension, then added
to GCC proper, then added to LLVM (currently only x86, but most of the
machinery is in place in the compiler to get it running on arm64).  My
recommendation is to skip the plugin part and work on a standard
interface for compilers to implement, with input from compiler
developers.
-- 
Thanks,
~Nick Desaulniers