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

[David Blaikie]

On Mon, Mar 23, 2015 at 1:50 PM, Khilan Gudka <Khilan.Gudka at cl.cam.ac.uk>
wrote:

> Hi all
>
> I appreciate the feedback and it looks like recording the information
> using MD nodes may not have been the right choice.
>
Just to be clear - there's a few different flavors of metadata. One of them
is explicitly typed/built in to LLVM (such as the debug info metadata
you're seeing), but there's a generic unstructured metadata you can use
that doesn't require modifications to LLVM. Though you might still need
changes to whatever bitcode linking tool you're using to insert your custom
metadata, but it shouldn't be too invasive, I'd imagine?

> I quite like the idea of having the build system dump the library
> information.
>
*nod* I'd certainly consider options like this.

> Thanks
> Khilan
> On 23 Mar 2015 20:15, "Duncan P. N. Exon Smith" <dexonsmith at apple.com>
> wrote:
>
>>
>> > On 2015-Mar-23, at 09:52, Eric Christopher <echristo at gmail.com> wrote:
>> >
>> >
>> >
>> > 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?
>> >
>>
>> Seems weird to me too.
>>
>> Moreover, this isn't really debug info, and it's not clear that it's
>> generally useful, so adding first-class support for it via specialized
>> metadata nodes seems premature.
>>
>> > -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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>>
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