Restore GDB backtrace on Linux

[tmcgilchrist]

Small correction to #13241 reported by @kayceesrk

Using the meander example from Retrofitting effect handlers onto OCaml https://dl.acm.org/doi/10.1145/3453483.3454039
Running a GDB session on Ubuntu 24.04/arm64:

$ cat meander.ml 
external ocaml_to_c
         : unit -> int = "ocaml_to_c"
exception E1
exception E2
let c_to_ocaml () = raise E1
let _ = Callback.register
          "c_to_ocaml" c_to_ocaml
let omain () =
  try (* h1 *)
    try (* h2 *) ocaml_to_c ()
    with E2 -> 0
  with E1 -> 42
let _ = assert (omain () = 42)%   

 $ cat meander_c.c 
#include <caml/mlvalues.h>
#include <caml/callback.h>

value ocaml_to_c (value unit) {
    caml_callback(*caml_named_value
                  ("c_to_ocaml"), Val_unit);
    return Val_int(0);
}%                                                        

Compiled using ocamlopt meander_c.c meander.ml -o meander.exe

(gdb) run
Starting program: /home/tsmc/ocaml/meander.exe 

This GDB supports auto-downloading debuginfo from the following URLs:
  <https://debuginfod.ubuntu.com>
Enable debuginfod for this session? (y or [n]) y
Debuginfod has been enabled.
To make this setting permanent, add 'set debuginfod enabled on' to .gdbinit.
Downloading separate debug info for system-supplied DSO at 0xfffff7ffb000
[Thread debugging using libthread_db enabled]                                                                                                             
Using host libthread_db library "/lib/aarch64-linux-gnu/libthread_db.so.1".

Breakpoint 2, <signal handler called>
(gdb) bt
#0  <signal handler called>
#1  0x0000aaaaaaae9138 in caml_startup_common (pooling=0, argv=0xfffffffffc78) at runtime/startup_nat.c:128
#2  caml_startup_common (argv=0xfffffffffc78, pooling=0) at runtime/startup_nat.c:87
#3  0x0000aaaaaaae91b0 in caml_startup_exn (argv=<optimized out>) at runtime/startup_nat.c:135
#4  caml_startup (argv=<optimized out>) at runtime/startup_nat.c:140
#5  caml_main (argv=<optimized out>) at runtime/startup_nat.c:147
#6  0x0000aaaaaaabcfd0 in main (argc=<optimized out>, argv=<optimized out>) at runtime/main.c:37
(gdb) c
Continuing.

Breakpoint 1, 0x0000aaaaaaabd168 in caml_program ()
(gdb) bt
#0  0x0000aaaaaaabd168 in caml_program ()
#1  <signal handler called>
#2  caml_startup (argv=<optimized out>) at runtime/startup_nat.c:141
#3  caml_main (argv=<optimized out>) at runtime/startup_nat.c:147
#4  0x0000aaaaaaae91b0 in caml_startup_exn (argv=<optimized out>) at runtime/startup_nat.c:135
#5  caml_startup (argv=<optimized out>) at runtime/startup_nat.c:140
#6  caml_main (argv=<optimized out>) at runtime/startup_nat.c:147
#7  0x0000000000000001 in ?? ()
Backtrace stopped: previous frame identical to this frame (corrupt stack?)
(gdb) c
Continuing.

Breakpoint 3, ocaml_to_c (unit=1) at meander_c.c:5
5	    caml_callback(*caml_named_value
(gdb) bt
#0  ocaml_to_c (unit=1) at meander_c.c:5
#1  <signal handler called>
#2  0x0000aaaaaaabdab8 in camlMeander.omain_278 () at meander.ml:10
#3  0x0000aaaaaaabdc9c in camlMeander.entry () at meander.ml:13
#4  0x0000aaaaaaabd210 in caml_program ()
#5  <signal handler called>
#6  caml_startup (argv=<optimized out>) at runtime/startup_nat.c:141
#7  caml_main (argv=<optimized out>) at runtime/startup_nat.c:147
#8  0x0000aaaaaaae91b0 in caml_startup_exn (argv=<optimized out>) at runtime/startup_nat.c:135
#9  caml_startup (argv=<optimized out>) at runtime/startup_nat.c:140
#10 caml_main (argv=<optimized out>) at runtime/startup_nat.c:147
#11 0x0000000000000001 in ?? ()
Backtrace stopped: previous frame identical to this frame (corrupt stack?)
(gdb) c
Continuing.

Breakpoint 4, camlMeander.c_to_ocaml_273 () at meander.ml:5
5	let c_to_ocaml () = raise E1
(gdb) bt
#0  camlMeander.c_to_ocaml_273 () at meander.ml:5
#1  <signal handler called>
#2  0x0000aaaaaab20d60 in camlMeander.7 ()
#3  0x0000aaaaaab20d60 in camlMeander.data_begin ()
Backtrace stopped: previous frame identical to this frame (corrupt stack?)
(gdb) info br
Num     Type           Disp Enb Address            What
1       breakpoint     keep y   0x0000aaaaaaabd168 <caml_program>
	breakpoint already hit 1 time
2       breakpoint     keep y   0x0000aaaaaaae9640 <caml_start_program+4>
	breakpoint already hit 1 time
3       breakpoint     keep y   0x0000aaaaaaac3a80 in ocaml_to_c at meander_c.c:5
	breakpoint already hit 1 time
4       breakpoint     keep y   0x0000aaaaaaabda70 in camlMeander.c_to_ocaml_273 at meander.ml:5
	breakpoint already hit 1 time

The bracktrace is corrupted when jumping from the C Runtime into initial OCaml frames, and then again when executing the callback.

With this fix to CFI you get the following (correct) behaviour

(gdb) run
Starting program: /home/tsmc/ocaml/meander.exe 

This GDB supports auto-downloading debuginfo from the following URLs:
  <https://debuginfod.ubuntu.com>
Enable debuginfod for this session? (y or [n]) y
Debuginfod has been enabled.
To make this setting permanent, add 'set debuginfod enabled on' to .gdbinit.
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/aarch64-linux-gnu/libthread_db.so.1".

Breakpoint 2, <signal handler called>
(gdb) bt
#0  <signal handler called>
#1  0x0000aaaaaaae9138 in caml_startup_common (pooling=0, argv=0xfffffffffc78) at runtime/startup_nat.c:128
#2  caml_startup_common (argv=0xfffffffffc78, pooling=0) at runtime/startup_nat.c:87
#3  0x0000aaaaaaae91b0 in caml_startup_exn (argv=<optimized out>) at runtime/startup_nat.c:135
#4  caml_startup (argv=<optimized out>) at runtime/startup_nat.c:140
#5  caml_main (argv=<optimized out>) at runtime/startup_nat.c:147
#6  0x0000aaaaaaabcfd0 in main (argc=<optimized out>, argv=<optimized out>) at runtime/main.c:37
(gdb) c
Continuing.

Breakpoint 1, 0x0000aaaaaaabd168 in caml_program ()
(gdb) bt
#0  0x0000aaaaaaabd168 in caml_program ()
#1  <signal handler called>
#2  0x0000aaaaaaae9138 in caml_startup_common (pooling=0, argv=0xfffffffffc78) at runtime/startup_nat.c:128
#3  caml_startup_common (argv=0xfffffffffc78, pooling=0) at runtime/startup_nat.c:87
#4  0x0000aaaaaaae91b0 in caml_startup_exn (argv=<optimized out>) at runtime/startup_nat.c:135
#5  caml_startup (argv=<optimized out>) at runtime/startup_nat.c:140
#6  caml_main (argv=<optimized out>) at runtime/startup_nat.c:147
#7  0x0000aaaaaaabcfd0 in main (argc=<optimized out>, argv=<optimized out>) at runtime/main.c:37
(gdb) c
Continuing.

Breakpoint 3, ocaml_to_c (unit=1) at meander_c.c:5
5	    caml_callback(*caml_named_value
(gdb) bt
#0  ocaml_to_c (unit=1) at meander_c.c:5
#1  <signal handler called>
#2  0x0000aaaaaaabdab8 in camlMeander.omain_278 () at meander.ml:10
#3  0x0000aaaaaaabdc9c in camlMeander.entry () at meander.ml:13
#4  0x0000aaaaaaabd210 in caml_program ()
#5  <signal handler called>
#6  0x0000aaaaaaae9138 in caml_startup_common (pooling=4, argv=0xaaaaaab24ac8) at runtime/startup_nat.c:128
#7  caml_startup_common (argv=0xaaaaaab24ac8, pooling=4) at runtime/startup_nat.c:87
#8  0x0000aaaaaaae91b0 in caml_startup_exn (argv=<optimized out>) at runtime/startup_nat.c:135
#9  caml_startup (argv=<optimized out>) at runtime/startup_nat.c:140
#10 caml_main (argv=<optimized out>) at runtime/startup_nat.c:147
#11 0x0000aaaaaaabcfd0 in main (argc=<optimized out>, argv=<optimized out>) at runtime/main.c:37
(gdb) c
Continuing.

Breakpoint 4, camlMeander.c_to_ocaml_273 () at meander.ml:5
5	let c_to_ocaml () = raise E1
(gdb) bt
#0  camlMeander.c_to_ocaml_273 () at meander.ml:5
#1  <signal handler called>
#2  0x0000aaaaaaac4d28 in caml_callback_exn (closure=<optimized out>, arg=<optimized out>, arg@entry=1) at runtime/callback.c:206
#3  0x0000aaaaaaac531c in caml_callback (closure=<optimized out>, arg=arg@entry=1) at runtime/callback.c:347
#4  0x0000aaaaaaac3aa0 in ocaml_to_c (unit=<optimized out>) at meander_c.c:5
#5  <signal handler called>
#6  0x0000aaaaaaabdab8 in camlMeander.omain_278 () at meander.ml:10
#7  0x0000aaaaaaabdc9c in camlMeander.entry () at meander.ml:13
#8  0x0000aaaaaaabd210 in caml_program ()
#9  <signal handler called>
#10 0x0000aaaaaaae9138 in caml_startup_common (pooling=4, argv=0xaaaaaab24ac8) at runtime/startup_nat.c:128
#11 caml_startup_common (argv=0xaaaaaab24ac8, pooling=4) at runtime/startup_nat.c:87
#12 0x0000aaaaaaae91b0 in caml_startup_exn (argv=<optimized out>) at runtime/startup_nat.c:135
#13 caml_startup (argv=<optimized out>) at runtime/startup_nat.c:140
#14 caml_main (argv=<optimized out>) at runtime/startup_nat.c:147
#15 0x0000aaaaaaabcfd0 in main (argc=<optimized out>, argv=<optimized out>) at runtime/main.c:37
(gdb) 

[kayceesrk]

The bug was introduced at #13079 (comment). Without the ability to run native debuggers in the testsuite, it is not possible to catch these bugs. Thankfully, this is being worked on: #13199 (comment).

[ghost]

I would have naively expected both versions of arm64.S to be equivalent (i.e. that it would be safe to do CFI_OFFSET after CFI_ADJUST, taking the adjustment offset into account). But then the new flavour is consistent with the use of CFI_ADJUST elsewhere in the file. (e.g. in ENTER_FUNCTION)