[llvm-dev] [RFC][DWARF] Handling errors in DWARF .debug_line parsing

David Blaikie via llvm-dev llvm-dev at lists.llvm.org
Mon Dec 16 14:47:40 PST 2019

On Mon, Dec 16, 2019 at 2:40 PM Jonas Devlieghere <jonas at devlieghere.com>

> Hey James,
> This sounds really interesting. A few things that come to mind:
>  - I'm curious what kind of errors you'd be okay with in dump mode but
> not in consumer mode. From LLDB's perspective, we probably want to
> extract as much information as possible from a malformed line table,
> as long as we don't end up lying of course.
>  - I need to take another look at the code, but don't we already have
> something similar where the consumer can decide how to deal with these
> kinds of issues? I'm bringing this up because I don't think we really
> need different parsing modes. I think we need to let the consumer
> decide what to do with the potential errors. The verifier in dwarfdump
> would presumably stop after the first error (~strict mode) while the
> dumper would just move on. Would the parsing modes then be a set of
> "presets" with regards to how to handle these issues?
>  - I'm in favor of creating these error messages lazily, especially
> for LLDB where we care about responsiveness. However, that does
> conflict with the behavior you want for the DWARF verifier. We
> probably want a way to control this?

For my part - I'd imagine the verifier would be an aggressive reader of
DWARF - it'd use the same lazy/high quality error API, but the verifier
code itself would try to walk all of the parts of the DWARF to manifest any
lazy error cases, rather than needing any other codepath in the parser.

> Cheers,
> Jonas
> On Mon, Dec 16, 2019 at 11:26 AM David Blaikie via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > (mailing lists can be busy - best to include any known-relevant parties
> on the 'to' line to help highlight the discussion for them)
> >
> > My take on it: I'd be OK with only a strict mode with good error
> messages lazily created. (so you don't get an error if you never needed to
> parse/lookup a file name, for instance - don't want the strict mode to be
> inefficient by aggressively parsing portions of the file you don't end up
> needing) If there are cases where consumers just need to be able to read
> invalid DWARF, yeah, could jump that hurdle when it comes up in a few ways
> I guess.
> >
> > On Mon, Dec 16, 2019 at 3:03 AM James Henderson via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >>
> >> Hi all,
> >>
> >> I'm preparing to propose some additional error checks for the DWARF
> debug line parser which we have locally. However, there have been some
> valid comments in the past that some errors really shouldn't prevent
> parsing for certain situations. For example, a line_range of 0 would only
> be an issue if anything needed that information (and would consequently
> divide by zero), whilst the include directories and file names table should
> be terminated with null bytes, but the parser could use the header length
> field to identify when the tables have ended instead.
> >>
> >> After a brief discussion with Paul Robinson offline, I think it would
> be good to add different parsing modes to the debug line parser. I've got
> three possible categories:
> >> 1) "Dump" mode - this would be the most lenient mode. It is intended
> for tools like llvm-dwarfdump that merely want to print the contents to the
> best of their ability, even if the table is malformed/dodgy in some other
> way. It might print warnings, but these should not prevent it continuing to
> parse what it can.
> >> 2) "Strict" or "Verify" mode - this would be the strictest mode, which
> would emit an error if it sees things like the aforementioned bad
> line_range or filenames tables. Optionally, we could even extend this to
> also cover things like addresses that don't make sense (possibly with or
> without special handling for fixups for GC-sections-processed output), or
> that could be a further mode.
> >> 3) "Consumer" mode - this would be somewhere in between the two above
> modes and would essentially do whatever a consumer such as LLDB wants. It
> might not need to be as strict as the "Strict" mode, but some guidance here
> from the consumers would be best.
> >>
> >> I haven't yet worked out exactly how this would work, but I think it
> would probably fit into the existing error handling mechanism, either with
> some extra conditionals or similar that say whether to report an error or
> not. I will hopefully be putting up some proposed patches later this week
> that illustrate all this.
> >>
> >> I'd appreciate any thoughts people have on the different modes, whether
> we would need more/fewer, what they'd each cover etc.
> >>
> >> Regards,
> >>
> >> James
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> >
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