Re: CONFIG_DEBUG_INFO_BTF and CONFIG_GCC_PLUGIN_RANDSTRUCT

Daniel Borkmann
2020-03-30T22:41:26Z

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
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.

Thanks,
Daniel

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