Re: CONFIG_DEBUG_INFO_BTF and CONFIG_GCC_PLUGIN_RANDSTRUCT

Kees Cook
2020-03-31T18:12:34Z

On Tue, Mar 31, 2020 at 12:41:04AM +0200, Daniel Borkmann wrote:
> On 3/30/20 7:20 PM, Kees Cook wrote:
> > 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.
>
> Is this expected to get fixed at some point wrt DWARF? Perhaps would make

No, gcc closed the issue as "won't fix".

> sense then to add a negative "depends" for both DWARF and BTF if the option
> GCC_PLUGIN_RANDSTRUCT is set given both would be incompatible/broken.

I hadn't just to keep wider randconfig build test coverage. That said, I
could make it be: depends COMPILE_TEST || !DWARF ...

I can certainly do that.

-Kees

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

--
Kees Cook