[llvm-dev] [DWARF] Handling empty ranges/location lists in ET_REL files

[Robinson, Paul via llvm-dev]

I agree it’s a bug.  An absolute (0, 0) pair is what indicates end-of-list.  You can get pairs of 0 addends with `.quad foo; .quad foo` or `.quad foo; .quad bar` but the former is an empty range and the latter would be a real range.
I’d expect the identical issue to pop up in .debug_ranges, so a patch should address both.
--paulr

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of James Henderson via llvm-dev
Sent: Tuesday, July 21, 2020 4:26 AM
To: llvm-dev <llvm-dev at lists.llvm.org>; David Blaikie <dblaikie at gmail.com>; Alexey Lapshin <a.v.lapshin at mail.ru>
Subject: [llvm-dev] [DWARF] Handling empty ranges/location lists in ET_REL files

Hi all,

I've put this email in a different thread, although it is quite similar to some of the threads on tombstoning etc, with similar underlying structural issues.

Whilst prototyping my fragmented DWARF idea for GC-ing DWARF sections properly, I ran into an object in the game code I was using as my input where a v4 .debug_loc section had a location description that looked something like this:

.quad foo
.quad foo
... # location description

where foo was a section symbol, i.e. the start and end address were the same. Consequently, there would be two relocations with 0 addend patching the start and end offset. When I was using llvm-dwarfdump to dump the .debug_loc section, I ended up with a decoding, and eventually a parsing error, because it saw a 0, 0 pair, so treated the entry as an end of list entry, and assumed the location description was the start of the next list.

The debug_loc parsing code treats 0, 0 pairs as end of list entries, whether or not they are relocated. I think this is a bug - if there are relocations we can be reasonably confident that the compiler did not intend it to be the end of the list, and at link time, this probably won't get resolved to 0, 0 (it's still technically possible it will, if 0 is a valid address, and the corresponding section was put at that address, but that's outside the scope of this email).

I've got a fairly simple change that could solve this, but it would require to check for the presence of a relocation at either address, in the event 0, 0 was read. Should I go ahead with tidying up the change/testing it etc? Or do we want a different solution to this problem (aside from using DWARFv5 of course!)?

Related aside: I haven't checked, but it's quite possible there's a similar problem in .debug_ranges parsing.

James
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200721/503c5088/attachment.html>