Skip unwritable frames in command "finish"

Skip unwritable frames in command "finish"

Nowadays, GDB can't insert breakpoint on the return address of the
exception handler on ARM M-profile, because the address is a magic
one 0xfffffff9,

 (gdb) bt
 #0  CT32B1_IRQHandler () at ../src/timer.c:67
 #1  <signal handler called>
 #2  main () at ../src/timer.c:127

(gdb) info frame
Stack level 0, frame at 0x200ffa8:
 pc = 0x4ec in CT32B1_IRQHandler (../src/timer.c:67); saved pc = 0xfffffff9
 called by frame at 0x200ffc8
 source language c.
 Arglist at 0x200ffa0, args:
 Locals at 0x200ffa0, Previous frame's sp is 0x200ffa8
 Saved registers:
  r7 at 0x200ffa0, lr at 0x200ffa4

(gdb) x/x 0xfffffff9
0xfffffff9:     Cannot access memory at address 0xfffffff9

(gdb) finish
Run till exit from #0  CT32B1_IRQHandler () at ../src/timer.c:67
Ed:15: Target error from Set break/watch: Et:96: Pseudo-address (0xFFFFFFxx) for EXC_RETURN is invalid (GDB error?)

Warning:
Cannot insert hardware breakpoint 0.
Could not insert hardware breakpoints:
You may have requested too many hardware breakpoints/watchpoints.

Command aborted.

even some debug probe can't set hardware breakpoint on the magic
address too,

(gdb) hbreak *0xfffffff9
Hardware assisted breakpoint 2 at 0xfffffff9
(gdb) c
Continuing.
Ed:15: Target error from Set break/watch: Et:96: Pseudo-address (0xFFFFFFxx) for EXC_RETURN is invalid (GDB error?)

Warning:
Cannot insert hardware breakpoint 2.
Could not insert hardware breakpoints:
You may have requested too many hardware breakpoints/watchpoints.

Command aborted.

The problem described above is quite similar to PR 8841, in which GDB
can't set breakpoint on signal trampoline, which is mapped to a read-only
page by kernel.  The rationale of this patch is to skip "unwritable"
frames when looking for caller frames in command "finish", and a new
gdbarch method code_of_frame_writable is added.  This patch fixes
the problem on ARM cortex-m target, but it can be used to fix
PR 8841 too.

gdb:

2016-05-10  Yao Qi  <yao.qi@arm.com>

	* arch-utils.c (default_code_of_frame_writable): New function.
	* arch-utils.h (default_code_of_frame_writable): Declare.
	* arm-tdep.c (arm_code_of_frame_writable): New function.
	(arm_gdbarch_init): Install gdbarch method
	code_of_frame_writable if the target is M-profile.
	* frame.c (skip_unwritable_frames): New function.
	* frame.h (skip_unwritable_frames): Declare.
	* gdbarch.sh (code_of_frame_writable): New.
	* gdbarch.c, gdbarch.h: Re-generated.
	* infcmd.c (finish_command): Call skip_unwritable_frames.