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

James Henderson via llvm-dev llvm-dev at lists.llvm.org
Mon Dec 16 03:03:10 PST 2019

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.


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