[llvm-dev] DWARF debug line error handling changes

[James Henderson via llvm-dev]

Hi all,

Since December, I've made several changes to the DWARF debug line parser to
improve its error handling. Some examples include removing redundant error
checks[1] and making address size mismatches non-fatal to parsing[2], with
several more about to land or being planned.

David Blaikie recommended I post something to give a bit more context to
these changes. I am a member of Sony's Linker and Binutils team, and as
part of the linking process, we make updates to the debug line section in
order to handle GC-section processing and similar modifications. In order
for these operations to be safe, we need the input line program to be in a
good state. As a result, in addition to the existing LLVM error checks, we
have added several additional downstream checks. These were added
downstream at the time for speed, but we've always had the intention of
bringing these to the wider community. We now have the time to do this.

There are broadly-speaking two kinds of changes we are making:

1) Adding additional new checks to make sure the table is valid. Where
possible, these are made as late as reasonable. That way, we don't emit
errors or warnings until it is necessary, and potentially therefore not at
all in some cases.

2) Making existing error checks less fatal, i.e. changing the code to allow
the parser to continue even though something doesn't look right in the
code. This allows consumers to gather more information, whether for
displaying or otherwise. In several cases, the change is to assume that the
length recorded in a field is correct, even if it doesn't match what either
the standard says should be the length. This won't always be the right
thing to do (it might be that just the length field is correct), but
providing more information, together with a warning that should indicate
that something may not be quite right, is surely more useful than refusing
to give the extra information. I picked the recorded length because other
areas of the code already assume it to be correct, and use it to iterate
onto other parts of the line table section.

I hope this provides greater context for these changes!

Regards,

James

[1]
https://github.com/llvm/llvm-project/commit/418cd8216b41f4c08e0e1b22feda381d9b2345da
[2]
https://github.com/llvm/llvm-project/commit/07804f75a6cc506fada40c474f1e60840ce737d8
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