Generate a .note.gnu.build-id section in the ELF file and DWARF file

[chingjun]

(From the discussion in https://docs.google.com/document/d/1HjFIJJSo1pRKo6FH84yafKbeEoSjPOXtFwwZfIeQNM0/edit?disco=AAAAJXYigj8)

Existing tools use debug identifier in .note.gnu.build-id section to search for build symbols that corresponds to an existing binary. Dart gen_snapshot should generate this section as well.

The default behavior in breakpad, if this section is not found, is to "locate the .text section of an ELF binary and generate a simple hash by XORing the first page worth of bytes", but in the ELF file that Dart generates, "the first RX page might be often the same because it contains "vm-isolate" stub code which doesn't change very often."

Thanks.

cc @sstrickl @mkustermann

[chingjun]

On a related note, it would be beneficial too if there is DT_SONAME in the DWARF file.

[mkustermann]

@sstrickl Could you take a look at this?

It is probably enough to make a hash out of the contents of kDartVmSnapshotData, kDartVmSnapshotInstructions, kDartCoreIsolateSnapshotData, kDartCoreIsolateSnapshotInstructions

[chingjun]

Just a note that the build-id should be placed in the NOTE segment. Crashpad (the successor of breakpad) is unable to find the build-id if it's not in the NOTE segment since it uses the program headers to find the notes.

[chingjun]

Hi @sstrickl and @mkustermann , any updates or timeline on this? This is currently the only blocking issue for us on the Dart side now. Thanks!

[sstrickl]

Sorry, I was OOO last week, which delayed this a little, but I've got most of this ready now and hope to have it up for review by EOD today, tomorrow at latest.

[chingjun]

Thanks for the update!

[sstrickl]

Okay, now ELF snapshots and their corresponding separate debug info have matching build IDs. This isn't done for separate debug info for assembly snapshots, because we can't know what the text section contents are for an assembly snapshot ahead of time, due to the BSS offset/relocations which are calculated at assembly time. (Similarly, we also don't add our own build ID to assembly snapshots directly, so the one that's there is still added by the assembler, not by us.)

So I'd try out the ELF snapshot side of things and see if it now fits the bill. As for DT_SONAME, I'm unsure what we'd even put there, since we don't have a standard naming for snapshots. I guess we could try something based on the basename of whatever path was provided for --elf, but oof.