[pushed] gdbserver: redo stepping over breakpoint that was on top of a permanent breakpoint

[Pedro Alves <palves at redhat dot com>]

I'm going to add an alternate mechanism of breakpoint trap
identification to 'check_stopped_by_breakpoint' that does not rely on
checking the instruction at PC.  The mechanism currently used to tell
whether we're stepping over a permanent breakpoint doesn't fit in that
new method.  This patch redoes the whole logic in a different way that
works with both old and new methods, in essence moving the "stepped
permanent breakpoint" detection "one level up".  It makes lower level
check_stopped_by_breakpoint always the adjust the PC, and then has
linux_wait_1 advance the PC past the breakpoint if necessary.  This
ends up being better also because this now handles
non-decr_pc_after_break targets too.  Before, such targets would get
stuck forever reexecuting the breakpoint instruction.

Tested on x86_64 Fedora 20.

gdb/gdbserver/ChangeLog:
2015-02-23  Pedro Alves  <palves@redhat.com>

	* linux-low.c (check_stopped_by_breakpoint): Don't check if the
	thread was doing a step-over; always adjust the PC if
	we stepped over a permanent breakpoint.
	(linux_wait_1): If we stepped over breakpoint that was on top of a
	permanent breakpoint, manually advance the PC past it.
---
 gdb/gdbserver/ChangeLog   |  8 ++++++++
 gdb/gdbserver/linux-low.c | 45 +++++++++++++++++++++++++++++++++++++--------
 2 files changed, 45 insertions(+), 8 deletions(-)

diff --git a/gdb/gdbserver/ChangeLog b/gdb/gdbserver/ChangeLog
index d724e6c..a130aab 100644
--- a/gdb/gdbserver/ChangeLog
+++ b/gdb/gdbserver/ChangeLog
@@ -1,5 +1,13 @@
 2015-02-23  Pedro Alves  <palves@redhat.com>
 
+	* linux-low.c (check_stopped_by_breakpoint): Don't check if the
+	thread was doing a step-over; always adjust the PC if
+	we stepped over a permanent breakpoint.
+	(linux_wait_1): If we stepped over breakpoint that was on top of a
+	permanent breakpoint, manually advance the PC past it.
+
+2015-02-23  Pedro Alves  <palves@redhat.com>
+
 	* linux-x86-low.c (REGSIZE): Define in both 32-bit and 64-bit
 	modes.
 	(x86_fill_gregset, x86_store_gregset): Use it when handling
diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
index a6e1e3d..1c66985 100644
--- a/gdb/gdbserver/linux-low.c
+++ b/gdb/gdbserver/linux-low.c
@@ -507,14 +507,8 @@ check_stopped_by_breakpoint (struct lwp_info *lwp)
   /* We may have just stepped a breakpoint instruction.  E.g., in
      non-stop mode, GDB first tells the thread A to step a range, and
      then the user inserts a breakpoint inside the range.  In that
-     case, we need to report the breakpoint PC.  But, when we're
-     trying to step past one of our own breakpoints, that happens to
-     have been placed on top of a permanent breakpoint instruction, we
-     shouldn't adjust the PC, otherwise the program would keep
-     trapping the permanent breakpoint forever.  */
-  if ((!lwp->stepping
-       || (!ptid_equal (ptid_of (current_thread), step_over_bkpt)
-	   && lwp->stop_pc == sw_breakpoint_pc))
+     case we need to report the breakpoint PC.  */
+  if ((!lwp->stepping || lwp->stop_pc == sw_breakpoint_pc)
       && (*the_low_target.breakpoint_at) (sw_breakpoint_pc))
     {
       if (debug_threads)
@@ -2552,6 +2546,41 @@ linux_wait_1 (ptid_t ptid,
       return ptid_of (current_thread);
     }
 
+  /* If step-over executes a breakpoint instruction, it means a
+     gdb/gdbserver breakpoint had been planted on top of a permanent
+     breakpoint.  The PC has been adjusted by
+     check_stopped_by_breakpoint to point at the breakpoint address.
+     Advance the PC manually past the breakpoint, otherwise the
+     program would keep trapping the permanent breakpoint forever.  */
+  if (!ptid_equal (step_over_bkpt, null_ptid)
+      && event_child->stop_reason == LWP_STOPPED_BY_SW_BREAKPOINT)
+    {
+      unsigned int increment_pc;
+
+      if (the_low_target.breakpoint_len > the_low_target.decr_pc_after_break)
+	increment_pc = the_low_target.breakpoint_len;
+      else
+	increment_pc = the_low_target.decr_pc_after_break;
+
+      if (debug_threads)
+	{
+	  debug_printf ("step-over for %s executed software breakpoint\n",
+			target_pid_to_str (ptid_of (current_thread)));
+	}
+
+      if (increment_pc != 0)
+	{
+	  struct regcache *regcache
+	    = get_thread_regcache (current_thread, 1);
+
+	  event_child->stop_pc += increment_pc;
+	  (*the_low_target.set_pc) (regcache, event_child->stop_pc);
+
+	  if (!(*the_low_target.breakpoint_at) (event_child->stop_pc))
+	    event_child->stop_reason = LWP_STOPPED_BY_NO_REASON;
+	}
+    }
+
   /* If this event was not handled before, and is not a SIGTRAP, we
      report it.  SIGILL and SIGSEGV are also treated as traps in case
      a breakpoint is inserted at the current PC.  If this target does
-- 
1.9.3