Re: [PATCH] Only use 32-bit thumb-2 breakpoint instruction if necessary

[Yao Qi <qiyaoltc at gmail dot com>]

Antoine Tremblay <antoine.tremblay@ericsson.com> writes:

>> We can't keep GDB and GDBserver in sync because we may have old gdb
>> talks with new gdbserver or new gdb talks with old gdbserver.  If an old
>> gdb talks with a new gdbserver, gdb checks program_breakpoint_here_p by
>> reading an instruction and see if it is a breakpoint instruction.  If
>> the original instruction is a 32-bit thumb instruction, gdbserver uses
>> 16-bit thumb breakpoint, but gdb expects to match 32-bit thumb
>> breakpoint.  IOW, gdb and gdbserver may see/use different kinds of
>> breakpoint on the same address (instruction), but gdb and gdbserver
>> can't do that.
>
> Right. Do you see any possible solution to this ?
>

I don't have any solutions off hand.  Nowadays, GDB chooses the
breakpoint kind by address and the instruction on that address
in arm_breakpoint_kind_from_pc,

	  if (target_read_memory (*pcptr, buf, 2) == 0)
	    {
	      unsigned short inst1;

	      inst1 = extract_unsigned_integer (buf, 2, byte_order_for_code);
	      if (thumb_insn_size (inst1) == 4)
		return ARM_BP_KIND_THUMB2;
	    }

we can change it to show breakpoints in target_read_memory,

  /* Make sure we see the memory breakpoints.  */
  cleanup = make_show_memory_breakpoints_cleanup (1);
  if (target_read_memory (*pcptr, buf, 2) == 0)
    {
       do_cleanups (cleanup);

       /* The remote stub may choose 16-bit thumb breakpoint for
          32-bit thumb instruction if the instruction is out side
          of IT block.   */
       if (memcmp (buf, tdep->thumb_breakpoint) == 0)
          return ARM_BP_KIND_THUMB;

       unsigned short inst1;

       inst1 = extract_unsigned_integer (buf, 2, byte_order_for_code);
       // Suppose thumb_insn_size returns 4 for the first half of 32-bit
       // thumb-2 breakpoint.
       if (thumb_insn_size (inst1) == 4)
	 return ARM_BP_KIND_THUMB2;
    }
   do_cleanups (cleanup);

> The only thing I would see is to accept inconsistent breakpoint
> sizes GDBServer.

In GDBserver, all breakpoints (gdb breakpoints, single-step breakpoints)
are organized via raw_breakpoint, is it possible that 32-bit thumb
breakpoint is use for gdb breakpoint and 16-bit thumb breakpoint is used
for single-step breakpoint for the same address?  It makes troubles to
raw_breakpoint management, as we find a raw_breakpoint for a given kind
on a given address.  I don't know what GDBserver should do, so I can't
give much help here, sorry.  I would like to change GDBserver code, and
fix the regressions caused by the change.

>
> I'm not sure anyway why that size/check is there assuming
> GDB/GDBServer is the only one writing breakpoints in the program ?

Because so far, arm is the only target in which we can use different
kind breakpoint instructions for a given address.

-- 
Yao (齐尧)