Re: CONFIG_DEBUG_INFO_BTF and CONFIG_GCC_PLUGIN_RANDSTRUCT

Slava Bacherikov
2020-03-31T21:24:52Z

31.03.2020 23:23, Andrii Nakryiko пишет:
> On Tue, Mar 31, 2020 at 12:58 PM Kees Cook <keescook@chromium.org> wrote:
>>
>> On Tue, Mar 31, 2020 at 12:50:07PM -0700, Andrii Nakryiko wrote:
>>> On Tue, Mar 31, 2020 at 11:12 AM Kees Cook <keescook@chromium.org> wrote:
>>>>
>>>> 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.
>>>
>>> I've asked Slava in [0] to disable all three known configs that break
>>> DWARF and subsequently BTF, I hope it's ok to just do it in one patch.
>>> Currently all these appear to result in invalid BTF due to various
>>> DWARF modifications:
>>>
>>> - DEBUG_INFO_REDUCED (see [1])
>>> - DEBUG_INFO_SPLIT (see [0]
>>> - GCC_PLUGIN_RANDSTRUCT (this discussion).
>>>
>>> [0] https://lore.kernel.org/bpf/CAEf4BzadnfAwfa1D0jZb=01Ou783GpK_U7PAYeEJca-L9kdnVA@mail.gmail.com/
>>> [1] https://lore.kernel.org/bpf/CAEf4BzZri8KpwLcoPgjiVx_=QmJ2W9UzBkDqSO2rUWMzWogkKg@mail.gmail.com/
>>
>> Sure! That'd by fine by me. I'd just like it to be a "|| COMPILE_TEST"
>> for GCC_PLUGIN_RANDSTRUCT. Feel free to CC me for an Ack. :)
>>
>
> +cc Slava
>
> I'm unsure what COMPILE_TEST dependency (or is it anti-dependency?)
> has to do with BTF generation and reading description in Kconfig
> didn't clarify it for me. Can you please elaborate just a bit? Thanks!
>
>> -Kees

Hi,

Regarding COMPILE_TEST, DEBUG_INFO has dependency on:

DEBUG_KERNEL && !COMPILE_TEST

And DEBUG_INFO_BTF depends on DEBUG_INFO, so enabling COMPILE_TEST
would block DEBUG_INFO and so DEBUG_INFO_BTF as well. Unless I don't
understand something and there is some other reason to add it.

--
Slava

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