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.

1 files changed, 19 insertions, 0 deletions

diff --git a/gdb/arm-tdep.c b/gdb/arm-tdep.c
index 157f8a12fb..278f639fe8 100644
--- a/gdb/arm-tdep.c
+++ b/gdb/arm-tdep.c
@@ -8862,6 +8862,22 @@ arm_register_g_packet_guesses (struct gdbarch *gdbarch)
   /* Otherwise we don't have a useful guess.  */
 }
 
+/* Implement the code_of_frame_writable gdbarch method.  */
+
+static int
+arm_code_of_frame_writable (struct gdbarch *gdbarch, struct frame_info *frame)
+{
+  if (gdbarch_tdep (gdbarch)->is_m
+      && get_frame_type (frame) == SIGTRAMP_FRAME)
+    {
+      /* M-profile exception frames return to some magic PCs, where
+	 isn't writable at all.  */
+      return 0;
+    }
+  else
+    return 1;
+}
+
 
 /* Initialize the current architecture based on INFO.  If possible,
    re-use an architecture from ARCHES, which is a list of
@@ -9312,6 +9328,9 @@ arm_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
   set_gdbarch_push_dummy_call (gdbarch, arm_push_dummy_call);
   set_gdbarch_frame_align (gdbarch, arm_frame_align);
 
+  if (is_m)
+    set_gdbarch_code_of_frame_writable (gdbarch, arm_code_of_frame_writable);
+
   set_gdbarch_write_pc (gdbarch, arm_write_pc);
 
   /* Frame handling.  */