[cfe-dev] RFC: ODR checker for Clang and LLD

[David Blaikie via cfe-dev]

Does this need LLVM support - or is there some generic representation that
could be used instead? (I guess LLVM would want to be aware of it when
merging modules though, so maybe it's worth having a first-class
representation - though LLVM module linking could special case a section
the same way the linker could/would - not sure what's the better choice
there)

I was thinking (hand-wavingly vague since I don't know that much about
object files, etc) one of those auto-appending sections and an array of
constchar*+hash attributed to that section. (then even without an
odr-checking aware linker (which would compare and discard these sections)
the data could be merged & a post-processing pass on the binary could still
point out ODR violations without anything in the toolchain (except clang)
needing to support this extra info)

On Tue, Jun 6, 2017 at 10:41 PM Peter Collingbourne via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi all,
>
> I'd like to propose an ODR checker feature for Clang and LLD. The feature
> would be similar to gold's --detect-odr-violations feature, but better: we
> can rely on integration with clang to avoid relying on debug info and to
> perform more precise matching.
>
> The basic idea is that we use clang's ability to create ODR hashes for
> declarations. ODR hashes are computed using all information about a
> declaration that is ODR-relevant. If the flag -fdetect-odr-violations is
> passed, Clang will store the ODR hashes in a so-called ODR table in each
> object file. Each ODR table will contain a mapping from mangled declaration
> names to ODR hashes. At link time, the linker will read the ODR table and
> report any mismatches.
>
> To make this work:
> - LLVM will be extended with the ability to represent ODR tables in the IR
> and emit them to object files
> - Clang will be extended with the ability to emit ODR tables using ODR
> hashes
> - LLD will be extended to read ODR tables from object files
>
> I have implemented a prototype of this feature. It is available here:
> https://github.com/pcc/llvm-project/tree/odr-checker and some results
> from applying it to chromium are here: crbug.com/726071
> As you can see it did indeed find a number of real ODR violations in
> Chromium, including some that wouldn't be detectable using debug info.
>
> If you're interested in what the format of the ODR table would look like,
> that prototype shows pretty much what I had in mind, but I expect many
> other aspects of the implementation to change as it is upstreamed.
>
> Thanks,
> --
> --
> Peter
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