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

[K via Gdb <gdb@sourceware.org>]

the debug shows gdb's own "aarch64 prologue" is matching first.
and it isn't wrong - I verified the first backtrace entry as correct.
I can't get into the gdb codebase though to find out why it unravels from
that point on.
I have the immediate source of my segfault and will run with that.

In the matter of aarch64's program counter: anyone know how to get this out
of the gdb-jit interface
given that the pc is not in a register?


On Wed, 12 Nov 2025 at 16:50, Simon Marchi <simark@simark.ca> wrote:

> 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
>