[llvm-dev] Need help with code generation

Tue Mar 22 14:33:30 PDT 2016

On Tue, Mar 22, 2016 at 1:29 PM, Rui Ueyama <ruiu at google.com> wrote:

> On Tue, Mar 22, 2016 at 9:19 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>>
>>
>> On Tue, Mar 22, 2016 at 1:15 PM, Rui Ueyama <ruiu at google.com> wrote:
>>
>>> On Tue, Mar 22, 2016 at 9:00 PM, David Blaikie <dblaikie at gmail.com>
>>> wrote:
>>>
>>>>
>>>>
>>>> On Tue, Mar 22, 2016 at 12:36 PM, Rui Ueyama <ruiu at google.com> wrote:
>>>>
>>>>> On Tue, Mar 22, 2016 at 7:36 PM, Rui Ueyama <ruiu at google.com> wrote:
>>>>>
>>>>>> I have a question. If there is a ELF verifier function that walks
>>>>>> every part of an ELF file to verify that the file is sane, and if you can
>>>>>> call that before calling LLD's function, are you guys happy with that?
>>>>>>
>>>>>
>>>>> I'd like to get you guys opinion on this question.
>>>>>
>>>>
>>>> I'd still find it problematic that lld itself would consider
>>>> crash-on-invalid "not a bug" to the point of not reviewing/approving
>>>> patches to fix such issues. That's what I'm concerned about in this thread.
>>>>
>>>
>>> That's one way to see that. The other view is it as a whole has a
>>> boolean option *IsInputTrustworthy* and works accordingly. What matters
>>> most is what we provide to the users as a guarantee.  You have an opinion
>>> that that should be implemented within LLD, but that would now an internal
>>> design choice. Hypothetically we had such pass to verify inputs, and if you
>>> send a patch to "fix" crash bug of LLD, we wouldn't probably reject that
>>> but instead argue that that needs to be addressed in the verifier pass
>>> instead. This is about "how" something should be implemented and usual
>>> design choice discussion, no?
>>>
>>
>> OK, sorry - some confusion. I assume that you wouldn't run this verifier
>> pass by default in lld-the-command-line-tool, right? (I would guess it
>> wouldn't meet your performance criteria)
>>
>
> Correct.
>
>
>> So from that perspective, lld-the-command-line-tool would still be
>> crashing-by-design on certain inputs in its default/normal/user-facing mode
>> & that would be seem problematic to me.
>>
>
> I disagree. You are in almost all case handling valid object files created
> by compilers, and if omitting some error checks for really pathetic case
> would make code simpler and improve performance, I think having that option
> is worth it (and I believe that's the case, at least those who are actually
> writing code seems to take that stance.) Also, if you give broken object
> files, you wouldn't get an output anyways. The only difference that a user
> can observe is whether it dies with an error message or not. We could even
> catch an segfault and run the verifier on the input again to print out an
> error after something goes wrong.
>

Sure, I understand that we disagree here, I was merely answering your
question "I have a question. If there is a ELF verifier function that walks
every part of an ELF file to verify that the file is sane, and if you can
call that before calling LLD's function, are you guys happy with that?" to
help you understand that point of disagreement/my position (& probably the
position of other people on this thread)

(UB is more than just segfaulting, though. So there are more possible
failures (including silently producing output & exiting with 0) than
segfault.)

- David

>
> If you're proposing having this verifier run by default - sure, then I
>> can't construct an input that crashes the linker, it'll fail with an error
>> message an exit. Yes, that would be fine by me - the way the feature is
>> implemented is not something I mean to imply constraints on.
>>
>> - David
>>
>>
>>>
>>>
>>>>>
>>>>>> On Tue, Mar 22, 2016 at 6:39 PM, Hal Finkel via llvm-dev <
>>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>>
>>>>>>>
>>>>>>> ------------------------------
>>>>>>>
>>>>>>> *From: *"David Blaikie via llvm-dev" <llvm-dev at lists.llvm.org>
>>>>>>> *To: *"Rafael Espíndola" <rafael.espindola at gmail.com>
>>>>>>> *Cc: *"llvm-dev" <llvm-dev at lists.llvm.org>, "Bruce Hoult" <
>>>>>>> bruce at hoult.org>
>>>>>>> *Sent: *Tuesday, March 22, 2016 10:18:03 AM
>>>>>>> *Subject: *Re: [llvm-dev] Need help with code generation
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Tue, Mar 22, 2016 at 4:27 AM, Rafael Espíndola <
>>>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>>>
>>>>>>>> > Maybe not, but it's not impossible either - browsers manage to
>>>>>>>> harden themselves against malicious input and they operate in a far hostile
>>>>>>>> environment with many more input formats than we do.
>>>>>>>>
>>>>>>>> It is important to note how different they are. Both Firefox and
>>>>>>>> Chromium have people working just to try to make them more secure.
>>>>>>>> Compare that with LLVM: One week ago I pointed out that your patch
>>>>>>>> (r263521) introduces a crash. It still hasn't been reverted or even
>>>>>>>> acknowledge yet.
>>>>>>>>
>>>>>>>>
>>>>>>>> > I'm not trying to shift your personal goal, or to direct the
>>>>>>>> features that you choose to put your time into, but I am interested in
>>>>>>>> project policy.
>>>>>>>>
>>>>>>>> Why do you care about policy that is not followed? A policy saying
>>>>>>>> llvm should not crash on any input is as relevant as one that says
>>>>>>>> that clang should keep bootstrapping in under one second.
>>>>>>>>
>>>>>>>
>>>>>>> It's pretty different when you say, essentially, that patches to
>>>>>>> address these things are unlikely to be accepted. It doesn't seem
>>>>>>> surprising that people wouldn't try to provide those patches and would
>>>>>>> choose not to use the project if that's the expressed policy of the
>>>>>>> developers on the project and doesn't line up with the needs of other
>>>>>>> people.
>>>>>>>
>>>>>>>
>>>>>>> +1
>>>>>>>
>>>>>>>  -Hal
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> So, if we stick to reality, what we have is that lld (ELF and COFF)
>>>>>>>> are already the most reliable parts of the toolchain. If not for Rui
>>>>>>>> and I being upfront about it most people would not even know that
>>>>>>>> you
>>>>>>>> could crash it. So please, just let us keep working on the most
>>>>>>>> reliable part of the toolchain.
>>>>>>>>
>>>>>>>> Cheers,
>>>>>>>> Rafael
>>>>>>>> _______________________________________________
>>>>>>>> LLVM Developers mailing list
>>>>>>>> llvm-dev at lists.llvm.org
>>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> LLVM Developers mailing list
>>>>>>> llvm-dev at lists.llvm.org
>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> --
>>>>>>> Hal Finkel
>>>>>>> Assistant Computational Scientist
>>>>>>> Leadership Computing Facility
>>>>>>> Argonne National Laboratory
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> LLVM Developers mailing list
>>>>>>> llvm-dev at lists.llvm.org
>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
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