Line records before symbol address in some cases

[Markus Stange]

It looks like bad debug info can make dump_syms create line records in a function which start before the function address. There is an example in this XUL.sym:

FUNC m 7503d0 94 0 mozilla::storage::Variant_base::Release()
[...]
750458 4 93 459
75045c 8 800 461
FUNC m 750464 8 0 mozilla::storage::NullVariant::GetDataType()
75045c c 800 461
750468 4 0 461
FUNC m 75046c c 0 mozilla::storage::Variant_base::GetAsJSVal(JS::MutableHandle<JS::Value>)
[...]

Here, the line record 75045c c 800 461 shows up twice. And it shows up in GetDataType() even though GetDataType() only starts at 0x750464, which is 8 bytes after 0x75045c.

This causes the symbolication API to return the wrong symbol and negative function offsets.

curl 'https://symbolication.services.mozilla.com/symbolicate/v5' -X POST --data-raw '{"jobs":[{"memoryMap":[["XUL","6117E38C063A30B0BC7B5A0A21F82B0F0"]],"stacks":[[[0,7668831]]]}]}' | jq .

{
  "results": [
    {
      "stacks": [
        [
          {
            "frame": 0,
            "module": "XUL",
            "module_offset": "0x75045f",
            "function": "mozilla::storage::NullVariant::GetDataType()",
            "function_offset": "-0x5",
            "file": "hg:hg.mozilla.org/mozilla-central:mfbt/Span.h:60b4965aa0ca5a7a60c71229600092a65df8bc1d",
            "line": 800
          }
        ]
      ],
      "found_modules": {
        "XUL/6117E38C063A30B0BC7B5A0A21F82B0F0": true
      }
    }
  ]
}

"function_offset": "-0x5" is not good.

[Markus Stange]

This happens when the input information is garbage, which it currently is for Firefox macOS builds.