Re: GDB-JIT: why would my 'unwind' not be invoked on crash?

On 11/11/25 9:00 PM, K via Gdb wrote:
>>
>> Could you say more about what exactly you're doing?
>>
>> Like are you writing an unwinder in gdb?  Or writing one in Python?
>>
>>
> I'm implementing an unwinder via the GDB-JIT C API for JIT code where
> generating custom ELF debug information is too onerous.
> The first article I read on it, by the implementor Sanjoy D(?) mentioned
> your name as originating the approach!
> So I'm just providing the functions called for by the struct in the API -
> unwind and get-frame-id  (plus the function name/line number info
> separately).
> 
> So gdb has no ELF info to look at (other than of the binary that is
> generating the JIT code), so I was expecting gdb to immediately defer
> to the jit-reader.so loaded into itself with the defined unwind function
> which has a few printf statements so I know when it runs ...
> and it isn't being called until 2 lines of backtrace have already been
> emitted, and only then on garbage data because it has already gone awry
> interpreting the stack contents/frame (with those first 2 lines above).

I don't have first hand experience in writing one of these, so I can't
help you directly.  So as Tom said, I would probably end up debugging
GDB itself as well (which is an easy thing to say, given we're GDB
devs).

The functions involved in finding the appropriate unwinder are
frame_unwind_find_by_frame and frame_unwind_try_unwinder, here:

https://gitlab.com/gnutools/binutils-gdb/-/blob/30f6e34f1fad35f9e8a3230adf2b7268b2a5229f/gdb/frame-unwind.c#L187

The JIT support code prepends its unwinder here:

https://gitlab.com/gnutools/binutils-gdb/-/blob/30f6e34f1fad35f9e8a3230adf2b7268b2a5229f/gdb/jit.c#L1142

So when frame_unwind_try_unwinder runs with "set debug frame 1", trying
to find an unwinder for frame 0, I would expect to see a message like
`Trying unwinder "jit"`.  And then your code should be called.  This is
where I would start investigating.

Simon