[PATCH] D16599: ELF: Define another entry point.

Tue Feb 2 21:57:37 PST 2016

On Tue, Feb 2, 2016 at 3:28 PM, Rui Ueyama <ruiu at google.com> wrote:

> Thank you for the advice. I refined it a bit.
>
> diff --git a/ELF/README.md b/ELF/README.md
> index 49b8167..b71faf4 100644
> --- a/ELF/README.md
> +++ b/ELF/README.md
> @@ -19,3 +19,16 @@ Achieving good performance is one of our goals. It's
> too early to reach a
>  conclusion, but we are optimistic about that as it currently seems to be
> faster
>  than GNU gold. It will be interesting to compare when we are close to
> feature
>  parity.
> +
> +Library Use
> +-----------
> +
> +You can embed LLD to your program by linking against it and calling the
> linker's
> +entry point function lld::elf2::link.
> +
> +The current policy is that it is your reponsibility to give trustworthy
> object
> +files. The function is guaranteed to return as long as you do not pass
> corrupted
> +or malicious object files. A corrupted file could cause a fatal error or
> SEGV.
> +That being said, you don't need to worry too much about it if you create
> object
> +files in a usual way and give it to the linker (it is naturally expected
> to
> +work, or otherwise it's a linker's bug.)
>

I like it!

-- Sean Silva

>
> On Tue, Feb 2, 2016 at 3:21 PM, Sean Silva <chisophugis at gmail.com> wrote:
>
>>
>>
>> On Tue, Feb 2, 2016 at 3:04 PM, Rui Ueyama <ruiu at google.com> wrote:
>>
>>> I'm going to add to the linker.
>>>
>>> +// Entry point of the ELF linker. Returns true on success. It is
>>> +// guaranteed to return as long as you do not pass corrupted or malicious
>>> +// object files. A corrupted file could cause a fatal error or SEGV.
>>> +// That being said, you don't need to worry too much about it if you
>>> +// create object files in a usual way and feed it to the linker
>>> +// (it is naturally expected to work, or otherwise that's a linker's bug.)
>>>  bool link(ArrayRef<const char *> Args, llvm::raw_ostream &Error = llvm::errs());
>>>
>>>
>>>
>> That sounds fine to me. I would consider adding it to README.txt instead,
>> and to phrase it as "this is our current policy" instead of casual advice
>> (otherwise it is difficult to use as a starting point for discussion IMO).
>> Whatever you think makes sense though.
>>
>> -- Sean Silva
>>
>>
>>> On Tue, Feb 2, 2016 at 2:25 PM, Rui Ueyama <ruiu at google.com> wrote:
>>>
>>>> On Tue, Feb 2, 2016 at 1:42 PM, Sean Silva <chisophugis at gmail.com>
>>>> wrote:
>>>>
>>>>>
>>>>>
>>>>> On Tue, Feb 2, 2016 at 8:44 AM, David Blaikie <dblaikie at gmail.com>
>>>>> wrote:
>>>>>
>>>>>>
>>>>>>
>>>>>> On Mon, Feb 1, 2016 at 11:05 PM, Sean Silva via llvm-commits <
>>>>>> llvm-commits at lists.llvm.org> wrote:
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Mon, Feb 1, 2016 at 12:27 PM, Rui Ueyama <ruiu at google.com> wrote:
>>>>>>>
>>>>>>>> Even if a file is technically sane, you can craft a malicious one;
>>>>>>>> for example, you can probably crash the linker by OOM by setting a very
>>>>>>>> large number as an alignment requirement for each section so that the size
>>>>>>>> of output becomes huge. It is easily doable using assembly. So my answer
>>>>>>>> is "any clang or gcc produced .o not including inline asm". (It does not
>>>>>>>> mean that we do not try to recover from errors caused by bad assembly code,
>>>>>>>> but we don't/can't guarantee 100% recovery.)
>>>>>>>>
>>>>>>>
>>>>>>> You can probably find some way to set the alignment using an
>>>>>>> attribute or whatever even from clang (and without inlineasm).
>>>>>>>
>>>>>>> I don't think there is a platonically-ideal answer for this. It's
>>>>>>> more about goals:
>>>>>>> - as a command line tool, we don't want legitimate users to see us
>>>>>>> crashing during normal use (if a user is intentionally trying to kill LLD,
>>>>>>> it is not as embarrassing though, so we don't need to worry much about that
>>>>>>> case).
>>>>>>> - we want to be useful (someday) as a library that can be safely
>>>>>>> used in-process, so we need to provide certain guarantees (but these are
>>>>>>> not hugely constraining, because we can assume that the calling code is
>>>>>>> programmatically generating the file in good faith).
>>>>>>>
>>>>>>
>>>>>> I don't think this is a valid assumption for all programmatic users
>>>>>> (& indeed Clang and LLVM both have ways of accepting untrusted inputs - the
>>>>>> assumption in LLVM is "if it's not already in the in-memory representation,
>>>>>> it's not trusted" (parsing bitcode, reading files, etc) and I think the
>>>>>> same would probably be reasonable in lld - callers with object contents in
>>>>>> memory (or even a higher level representation - the same as the difference
>>>>>> between LLVM IR and LLVM bitcode in a memory buffer) can choose to have lld
>>>>>> assume validity (if they produced it from an API they trust/are willing to
>>>>>> bugfix if it's ever wrong) or ask for verification (if they got the object
>>>>>> over a network connection or other untrusted source (perhaps read it out of
>>>>>> a compressed archive, etc))). An API integration of LLD into the Clang
>>>>>> driver wouldn't be a sound place to make this assumption - some objects may
>>>>>> be passed to Clang (not generated by it) from some other compilation or
>>>>>> source, for example.
>>>>>>
>>>>>
>>>>> I think these can serve as a baseline that we can document / elaborate
>>>>> on down the road though.
>>>>> For the moment, we can document our current intentions/policies. That
>>>>> way people can either a) concretely file bug reports against us for
>>>>> violating our intentions or b) we can have a concrete discussion on
>>>>> llvm-dev about changing those documented policies/intentions.
>>>>>
>>>>
>>>> Good point. We need to document the current policy whatever it is. And
>>>> the current policy after I submit these pending patches is that "the linker
>>>> doesn't crash or exit (or it is a bug) as long as you don't give
>>>> corrupted/malicious object files." I will write that to the Driver file
>>>> which all people who wants to use will see.
>>>>
>>>> It seems our current situation is that any time anything related to
>>>>> this comes up, everybody and their dog start talking about different
>>>>> hypothetical situations that nobody is actively working on using LLD for
>>>>> (since there are other, higher priorities right now). These may or may not
>>>>> be true, or the parallels to clang/LLVM may or may not be true, but
>>>>> currently we don't have a starting point for a useful discussion. It is all
>>>>> ad-hoc. We need a fixed point of reference for future discussion and what I
>>>>> posted (in this thread and others) seems like a sweet spot to start with;
>>>>> it provides reasonable guarantees and avoids overcommitting our development
>>>>> effort at an early stage.
>>>>>
>>>> I actually have points to say in response to what you said, but here in
>>>>> an llvm-commits discussion is not the right place to discuss it.
>>>>>
>>>>> -- Sean Silva
>>>>>
>>>>>
>>>>>>
>>>>>>> -- Sean Silva
>>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> On Mon, Feb 1, 2016 at 12:11 PM, Rafael Espíndola <
>>>>>>>> rafael.espindola at gmail.com> wrote:
>>>>>>>>
>>>>>>>>> On 1 February 2016 at 15:06, Rui Ueyama <ruiu at google.com> wrote:
>>>>>>>>> > On Mon, Feb 1, 2016 at 11:57 AM, Rafael Espíndola
>>>>>>>>> > <rafael.espindola at gmail.com> wrote:
>>>>>>>>> >>
>>>>>>>>> >> On 1 February 2016 at 14:46, Sean Silva <chisophugis at gmail.com>
>>>>>>>>> wrote:
>>>>>>>>> >> > I think one of the main use cases that has been requested is
>>>>>>>>> to be able
>>>>>>>>> >> > to
>>>>>>>>> >> > programmatically call the linker with "known good" object
>>>>>>>>> files (i.e.
>>>>>>>>> >> > produced by the compiler). That simplifies things a lot.
>>>>>>>>> Rui's recent
>>>>>>>>> >> > patches that are thread_local'izing existing globals seems
>>>>>>>>> like a
>>>>>>>>> >> > satisfactory approach. Or am I missing something?
>>>>>>>>> >>
>>>>>>>>> >> Yes, known good files are a lot easier to handle. We just have
>>>>>>>>> to be
>>>>>>>>> >> clear what "known good" is.
>>>>>>>>> >>
>>>>>>>>> >> > The R_X86_64_REX_GOTPCRELX situation can probably be likened
>>>>>>>>> to someone
>>>>>>>>> >> > giving clang a piece of source code with an inline asm that
>>>>>>>>> has:
>>>>>>>>> >> >
>>>>>>>>> >> > .text
>>>>>>>>> >> > .byte <some garbage>
>>>>>>>>> >> >
>>>>>>>>> >> > in it. We don't guarantee that the output "makes sense"
>>>>>>>>> because there's
>>>>>>>>> >> > really no way for us to know what "makes sense" in a precise
>>>>>>>>> way (i.e.,
>>>>>>>>> >> > a
>>>>>>>>> >> > way that we can program).
>>>>>>>>> >>
>>>>>>>>> >> Would we still be required to check the offsets so we don't
>>>>>>>>> crash? An
>>>>>>>>> >> assembly file can contain
>>>>>>>>> >>
>>>>>>>>> >> .reloc 0, R_X86_64_REX_GOTPCRELX, foo
>>>>>>>>> >> .long 4
>>>>>>>>> >>
>>>>>>>>> >> which would put that relocation in an invalid location. In
>>>>>>>>> general, is
>>>>>>>>> >> an arbitrary assembly file to be considered "known good"? Is
>>>>>>>>> that true
>>>>>>>>> >> even for things like
>>>>>>>>> >>
>>>>>>>>> >> .section .eh_frame, ....
>>>>>>>>> >> garbage
>>>>>>>>> >>
>>>>>>>>> >> that the linker has to parse?
>>>>>>>>> >
>>>>>>>>> >
>>>>>>>>> > I think the answer is case-by-case, but I don't think we have to
>>>>>>>>> guarantee
>>>>>>>>> > to recover from errors caused by carefully-crafted malicious
>>>>>>>>> object files.
>>>>>>>>> > (Is there anyone who disagrees with that?)
>>>>>>>>>
>>>>>>>>> It is definitely not a use case *I* have an interest in. I just
>>>>>>>>> want
>>>>>>>>> to be an agreement on what use case we want to support at the
>>>>>>>>> moment.
>>>>>>>>> Is it "any .o file", "any llvm-mc or gas produced .o", "any clang
>>>>>>>>> or
>>>>>>>>> gcc produced .o not including inline asm"?
>>>>>>>>>
>>>>>>>>> Cheers,
>>>>>>>>> Rafael
>>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> llvm-commits mailing list
>>>>>>> llvm-commits at lists.llvm.org
>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
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