[Bug 2041396] Re: gdb 12.1 generates SIGILL on armhf

[Sergio Durigan Junior]

After several hours trying to obtain access to an ARM64 machine where I
could test the fix, vorlon kindly provided me with credentials to a
machine that's capable of launching an armhf container.

I could reproduce the bug:

# gdb -q ./a.out -ex 'b 3' -ex r -ex c
Reading symbols from ./a.out...
Breakpoint 1, thumb_func () at 1.c:3
3         return 42;
Continuing.

Program received signal SIGILL, Illegal instruction.
0x00401004 in ?? ()
...

And also verify that Liu's package fixes the problem:

# gdb -q ./a.out -ex 'b 3' -ex r -ex c
Reading symbols from ./a.out...
Breakpoint 1 at 0x4d8: file 1.c, line 3.
Starting program: /root/a.out 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/arm-linux-gnueabihf/libthread_db.so.1".

Breakpoint 1, thumb_func () at 1.c:3
3         return 42;
Continuing.
[Inferior 1 (process 2666) exited with code 052]

Therefore, I sponsored the upload for him.

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https://bugs.launchpad.net/bugs/2041396

Title:
  gdb 12.1 generates SIGILL on armhf

Status in gdb:
  Fix Released
Status in gdb package in Ubuntu:
  New
Status in gdb source package in Jammy:
  New

Bug description:
  [ Impact ]

   * GDB 12.1 introduced a regression where it will break program execution when the program contains mixed ARM code and THUMB code.
   * Upstream stated they tested the changes on Ubuntu 20.04 and it went okay.

  [ Test Plan ]

  Considering the following C program:

  ```
  __attribute__((target("arm"), noinline))
  int thumb_func() {
    return 42;
  }

  __attribute__((target("thumb")))
  int main() { return thumb_func(); }
  ```

  If you build it using `gcc repro.c -ggdb3 -Og -o repro` and run the
  GDB using the following commands ...

  ```
  b 3
  r
  c
  ```

  (you can save the contents above to a file and run GDB using `gdb -x
  script ./repro`)

  ... you will notice GDB broke the program and threw SIGILL.
  If you run the program without GDB, the program exits normally.

  [ Where problems could occur ]

   * GDB is a complex software. As the patch suggests, it may break other use cases (like single-stepping) entirely.
   * Since this is an ARM-only patch, it's unlikely to affect other CPU architectures. However, it is possible that this fix may break ARM64 execution.

  [ Other Info ]
   
   * This bug has been fixed in GDB 13, but the fix was never backported to GDB 12. You can find the upstream bug in the remote bug watch.

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