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

Tue Feb 2 15:28:01 PST 2016

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.)

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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