Re: CONFIG_DEBUG_INFO_BTF and CONFIG_GCC_PLUGIN_RANDSTRUCT

Kees Cook
2020-03-30T17:20:56Z

On Mon, Mar 30, 2020 at 06:17:32PM +0200, Jann Horn wrote:
> On Mon, Mar 30, 2020 at 5:59 PM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> > On Mon, Mar 30, 2020 at 8:14 AM Jann Horn <jannh@google.com> wrote:
> > >
> > > I noticed that CONFIG_DEBUG_INFO_BTF seems to partly defeat the point
> > > of CONFIG_GCC_PLUGIN_RANDSTRUCT.
> >
> > Is it a theoretical stmt or you have data?
> > I think it's the other way around.
> > gcc-plugin breaks dwarf and breaks btf.
> > But I only looked at gcc patches without applying them.
>
> Ah, interesting - I haven't actually tested it, I just assumed
> (perhaps incorrectly) that the GCC plugin would deal with DWARF info
> properly.

Yeah, GCC appears to create DWARF before the plugin does the
randomization[1], so it's not an exposure, but yes, struct randomization
is pretty completely incompatible with a bunch of things in the kernel
(by design). I'm happy to add negative "depends" in the Kconfig if it
helps clarify anything.

-Kees

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84052

--
Kees Cook