[RFC] Support command "catch syscall" properly on different targets

[Yao Qi]

Sergio Durigan Junior <sergiodj@redhat.com> writes:

> So, as I said in my previous messages, I don't think that making the
> "catch syscall" command to fail on the exec target.
>
> What do you think of the attached patch (applies on top of your patch,
> rebased to the current HEAD)?  It implements what I proposed, but in a
> different way.  If the target is "None" (no binary loaded) or "exec"
> (inferior loaded but never started), then it displays a warning but
> still creates the catchpoint.  The actual check for these targets
> happens in the insert_catch_syscall function, which is called when we
> already know if the target actually supports the syscall catchpoint.
>
> Maybe I forgot to cover some corner case, but I still think we should
> support "catch syscall" when no inferior has been started.

I don't have a strong opinion against your approach.  Since "catch
point" is only supported on some arches on linux native target, I think
it is OK to leave gdbarch_get_syscall_number_p checking in
catch_syscall_command_1, so I withdraw my patch.

However, when I play with your patch, I find GDB can disable catch point if it
isn't inserted successfully, in breakpoint.c:insert_bp_location,

  else if (bl->owner->type == bp_catchpoint)
    {
      int val;

      gdb_assert (bl->owner->ops != NULL
		  && bl->owner->ops->insert_location != NULL);

      val = bl->owner->ops->insert_location (bl);
      if (val)
	{
	  bl->owner->enable_state = bp_disabled;

	  if (val == 1)
	    warning (_("\
Error inserting catchpoint %d: Your system does not support this type\n\
of catchpoint."), bl->owner->number);
	  else
	    warning (_("Error inserting catchpoint %d."), bl->owner->number);
	}

as shown below,

(gdb) target remote :1234
Remote debugging using :1234
Reading symbols from /lib64/ld-linux-x86-64.so.2...(no debugging symbols found)...done.
0x00007ffff7ddb2d0 in ?? () from /lib64/ld-linux-x86-64.so.2
(gdb) c
Continuing.
warning: Error inserting catchpoint 1: Your system does not support this type
of catchpoint.
....

(gdb) info breakpoints 
Num     Type           Disp Enb Address            What
1       catchpoint     keep n                      syscall "open"

According this observation, I don't see the need check
gdbarch_get_syscall_number_p in catch_syscall_command_1.  Probably we
can remove it.

-- 
Yao (齐尧)