[Lldb-commits] [lldb] [lldb] Add utility to create Mach-O corefile from YAML desc (PR #153911)

[Jason Molenda via lldb-commits]

jasonmolenda wrote:

> > You could imagine someone making an ELF corefile output capability, for instance.
> 
> Yes I'd love to be generating the core files eventually. Maybe we can with yaml2obj but I never seriously looked into it. As right now, patching in notes by hand makes for smaller files, but it's not reproducible or self documenting.
> 
> I assume the existing yaml2obj cannot make core files, or you wouldn't be writing this.

I don't think yaml2obj does make corefiles, but I believe this utility serves a different purpose.  yaml2obj could if you already have a small corefile (they're rarely small) that only contains the bytes needed to reproduce a bug that you want to test.  You may need to hand-modify an existing corefile to remove unnecessary memory segments, or to change values to elicit some behavior you want to test.

The idea with this tool is that you have the memory bytes that are critical to show an issue, or register values, and you want to construct a corefile based on those bytes. You may not be starting with a real corefile to begin with.  For that matter, you might construct stack memory values manually to elicit an unwinder bug, for instance.  (nb until I find a way for ObjectFileJSON to specify unwind rules for functions, this is unlikely to work, unless you copy the bytes of the relevant functions into the YAML so lldb can do its instruction emulation unwind plan).

Last week I had a bug report of someone seeing an issue with lldb in a firmware environment, where it would be difficult for me to set up the environment and corefiles were not an option.  I got the register values & 400 bytes of stack memory (lit. just "reg read" and "mem read"), and a copy of the relevant binary + dSYM, reformatted it all into YAML and created a Mach-O corefile that was able to reproduce the issue on my desktop.  Pretty handy!  Agree this is not common, but it was funny it came up only a couple weeks after I wrote the tool.

I think it fills a niche distinct from obj2yaml / yaml2obj, regardless of corefile support in that tool.

https://github.com/llvm/llvm-project/pull/153911