TemporaryBreakpoint and FInishingbreakpoint issues

[Ena Irtnanoelec <ena.irtnanoelec@gmail.com>]

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  Dear all,

I've got some troubles with both FinishingBreakpoint and
TemporaryBreakpoints I'm trying to set using the Python's API.

My use case is as follows : I've added some breakpoints on particular code
of the linux kernel, on a syscall definition. This is the code in which I'm
placing a BP:

SYSCALL_DEFINE3
<https://elixir.bootlin.com/linux/latest/C/ident/COMPAT_SYSCALL_DEFINE3>(execve
<https://elixir.bootlin.com/linux/latest/C/ident/execve>, const char
__user <https://elixir.bootlin.com/linux/latest/C/ident/__user> *,
filename <https://elixir.bootlin.com/linux/latest/C/ident/filename>,
	const compat_uptr_t
<https://elixir.bootlin.com/linux/latest/C/ident/compat_uptr_t> __user
<https://elixir.bootlin.com/linux/latest/C/ident/__user> *, argv
<https://elixir.bootlin.com/linux/latest/C/ident/argv>,
	const compat_uptr_t
<https://elixir.bootlin.com/linux/latest/C/ident/compat_uptr_t> __user
<https://elixir.bootlin.com/linux/latest/C/ident/__user> *, envp
<https://elixir.bootlin.com/linux/latest/C/ident/envp>){
	return compat_do_execve
<https://elixir.bootlin.com/linux/latest/C/ident/compat_do_execve>(getname
<https://elixir.bootlin.com/linux/latest/C/ident/getname>(filename
<https://elixir.bootlin.com/linux/latest/C/ident/filename>), argv
<https://elixir.bootlin.com/linux/latest/C/ident/argv>, envp
<https://elixir.bootlin.com/linux/latest/C/ident/envp>);}

So far no problems, thus, once there, I would like to have the exit code of
this function. This is where I have some difficulties. I first tried with a
FinishingBreakpoint as follows. Thus, it never each the "normal finish" and
always ends up in "out of scope", despite the fact it is placed at the
correct place (in an assembly file - entry-common.S). When I'm manually
stepping thru, I got there.
class ExecveBreakpoint(gdb.Breakpoint):

    def stop(self):
        MyFinishBreakpoint(gdb.selected_frame())


class MyFinishBreakpoint (gdb.FinishBreakpoint):
    def stop (self):
        print ("normal finish")

    def out_of_scope ():
        print ("abnormal finish")




I was wondering if this was because there were no ret instruction at the
end of the block ? Anyway, I've tried to get around by computing the last
instruction before continuing in assembly
        pc = gdb.selected_frame().pc()
        addr = gdb.block_for_pc(pc).end
        addr = addr - \
            gdb.selected_frame().architecture().disassemble(addr)[0][
'length']

        TestBp(f"*0x{addr:02x}", temporary=True)

class TestBp(gdb.Breakpoint):
  def stop(self):
  print(f"count = {self.hit_count}")
        #self.delete()# <- this crashed gdb
        return False

It happens that even though I made it temporary, it is never deleted.
Furthermore, the hit_count is always 0 even though I've got multiple print
calls on the console.

Any idea ?
Kind regards,
Mathieu