[LLVMdev] New kind of metadata to capture LLVM IR linking structure

[Eric Christopher]

On Mon, Mar 23, 2015 at 9:50 AM David Blaikie <dblaikie at gmail.com> wrote:

> On Mon, Mar 23, 2015 at 8:15 AM, Khilan Gudka <Khilan.Gudka at cl.cam.ac.uk>
> wrote:
>
>> Hi David
>>
>> Thanks for your email.
>>
>> What's the benefit/purpose of the MDLLVMModule over just having the
>>> MDCompileUnits themselves? I would imagine the user cares about which
>>> source file the problem was in (obtained from the MDCompileUnit), not the
>>> sequence of BC modules that may've been built into?
>>>
>>
>> We envisage it to be useful when an analysis tool built using LLVM needs
>> to know which MDCompileUnits were part of a particular library that has
>> been linked in. For instance, we're currently analysing the sandboxing
>> behaviour within the Chromium web browser, which comprises hundreds of
>> internal libraries and many external ones. To be able to perform this
>> analysis  we have to link them all together into a single .bc/.ll file.
>>
>> Having the module structure allows us to model interactions between
>> different modules (without manually (and sometimes unreliably) having to
>> work out which source file corresponds to which library (e.g. libssl,
>> libpci, libpolicy, librenderer, etc)). It also allows an analysis tool to
>> support turning on/off output warnings for particular libraries (as they
>> can lead to a lot of analysis output).
>>
>
> Fair enough - I've no idea/opinion on whether that's the right abstraction
> (other people with more domain knowledge of analysis infrastructure might
> chime in with some thoughts).
>
> Practically speaking: would directory paths be sufficient? The
> MDCompileUnits already have information about where the source file was.
>
>
I agree, this seems very weird. You have very good source location
information down to directory/file/line/column for individual instructions
in the existing metadata scheme, I'm not sure what this is getting you over
that?

-eric


>
> - David
>
>
>>
>>
>>>
>>>
>>>>
>>>> I would be very grateful if someone could review this.
>>>>
>>>> Thanks
>>>>
>>>> --
>>>> Khilan Gudka
>>>> Research Associate
>>>> Security Group
>>>> Computer Laboratory
>>>> University of Cambridge
>>>> http://www.cl.cam.ac.uk/~kg365/
>>>>
>>>>
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>>>>
>>>>
>>>
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