Disable displaced stepping if trying it fails

Running the testsuite with "maint set target-non-stop on" shows:

 (gdb) PASS: gdb.base/valgrind-infcall.exp: continue #98 (false warning)
 continue
 Continuing.
 dl_main (phdr=<optimized out>..., auxv=<optimized out>) at rtld.c:2302
 2302      LIBC_PROBE (init_complete, 2, LM_ID_BASE, r);
 Cannot access memory at address 0x400532
 (gdb) PASS: gdb.base/valgrind-infcall.exp: continue #99 (false warning)
 p gdb_test_infcall ()
 $1 = 1
 (gdb) FAIL: gdb.base/valgrind-infcall.exp: p gdb_test_infcall ()

Even though that was a native GNU/Linux test run, this test spawns
Valgrind and connects to it with "target remote".  The error above is
actually orthogonal to target-non-stop.  The real issue is that that
enables displaced stepping, and displaced stepping doesn't work with
Valgrind, because we can't write to the inferior memory (thus can't
copy the instruction to the scratch pad area).

I'm sure there will be other targets with the same issue, so trying to
identify Valgrind wouldn't be sufficient.  The fix is to try setting
up the displaced step anyway.  If we get a MEMORY_ERROR, we disable
displaced stepping for that inferior, and fall back to doing an
in-line step-over.  If "set displaced-stepping" is "on" (as opposed to
"auto), GDB warns displaced stepping failed ("on" is mainly useful for
the testsuite, not for users).

Tested on x86_64 Fedora 20.

gdb/ChangeLog:
2015-08-07  Pedro Alves  <palves@redhat.com>

	* inferior.h (struct inferior) <displaced_stepping_failed>: New
	field.
	* infrun.c (use_displaced_stepping_now_p): New parameter 'inf'.
	Return false if dispaced stepping failed before.
	(resume): Pass the current inferior to
	use_displaced_stepping_now_p.  Wrap displaced_step_prepare in
	TRY/CATCH.  If we get a MEMORY_ERROR, set the inferior's
	displaced_stepping_failed flag, and fall back to an in-line
	step-over.

gdb/testsuite/ChangeLog:
2015-08-07  Pedro Alves  <palves@redhat.com>

	* gdb.base/valgrind-disp-step.c: New file.
	* gdb.base/valgrind-disp-step.exp: New file.

1 files changed, 92 insertions, 20 deletions

diff --git a/gdb/infrun.c b/gdb/infrun.c
index f1d8e7c..157c12c 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -1468,6 +1468,10 @@ struct displaced_step_inferior_state
   /* The process this displaced step state refers to.  */
   int pid;
 
+  /* True if preparing a displaced step ever failed.  If so, we won't
+     try displaced stepping for this inferior again.  */
+  int failed_before;
+
   /* If this is not null_ptid, this is the thread carrying out a
      displaced single-step in process PID.  This thread's state will
      require fixing up once it has completed its step.  */
@@ -1638,16 +1642,24 @@ show_can_use_displaced_stepping (struct ui_file *file, int from_tty,
 }
 
 /* Return non-zero if displaced stepping can/should be used to step
-   over breakpoints.  */
+   over breakpoints of thread TP.  */
 
 static int
-use_displaced_stepping (struct gdbarch *gdbarch)
+use_displaced_stepping (struct thread_info *tp)
 {
+  struct regcache *regcache = get_thread_regcache (tp->ptid);
+  struct gdbarch *gdbarch = get_regcache_arch (regcache);
+  struct displaced_step_inferior_state *displaced_state;
+
+  displaced_state = get_displaced_stepping_state (ptid_get_pid (tp->ptid));
+
   return (((can_use_displaced_stepping == AUTO_BOOLEAN_AUTO
 	    && target_is_non_stop_p ())
 	   || can_use_displaced_stepping == AUTO_BOOLEAN_TRUE)
 	  && gdbarch_displaced_step_copy_insn_p (gdbarch)
-	  && find_record_target () == NULL);
+	  && find_record_target () == NULL
+	  && (displaced_state == NULL
+	      || !displaced_state->failed_before));
 }
 
 /* Clean out any stray displaced stepping state.  */
@@ -1701,7 +1713,7 @@ displaced_step_dump_bytes (struct ui_file *file,
    Returns 1 if preparing was successful -- this thread is going to be
    stepped now; or 0 if displaced stepping this thread got queued.  */
 static int
-displaced_step_prepare (ptid_t ptid)
+displaced_step_prepare_throw (ptid_t ptid)
 {
   struct cleanup *old_cleanups, *ignore_cleanups;
   struct thread_info *tp = find_thread_ptid (ptid);
@@ -1811,6 +1823,50 @@ displaced_step_prepare (ptid_t ptid)
   return 1;
 }
 
+/* Wrapper for displaced_step_prepare_throw that disabled further
+   attempts at displaced stepping if we get a memory error.  */
+
+static int
+displaced_step_prepare (ptid_t ptid)
+{
+  int prepared = -1;
+
+  TRY
+    {
+      prepared = displaced_step_prepare_throw (ptid);
+    }
+  CATCH (ex, RETURN_MASK_ERROR)
+    {
+      struct displaced_step_inferior_state *displaced_state;
+
+      if (ex.error != MEMORY_ERROR)
+	throw_exception (ex);
+
+      if (debug_infrun)
+	{
+	  fprintf_unfiltered (gdb_stdlog,
+			      "infrun: disabling displaced stepping: %s\n",
+			      ex.message);
+	}
+
+      /* Be verbose if "set displaced-stepping" is "on", silent if
+	 "auto".  */
+      if (can_use_displaced_stepping == AUTO_BOOLEAN_TRUE)
+	{
+	  warning (_("disabling displaced stepping: %s\n"),
+		   ex.message);
+	}
+
+      /* Disable further displaced stepping attempts.  */
+      displaced_state
+	= get_displaced_stepping_state (ptid_get_pid (ptid));
+      displaced_state->failed_before = 1;
+    }
+  END_CATCH
+
+  return prepared;
+}
+
 static void
 write_memory_ptid (ptid_t ptid, CORE_ADDR memaddr,
 		   const gdb_byte *myaddr, int len)
@@ -1941,6 +1997,7 @@ static void keep_going_pass_signal (struct execution_control_state *ecs);
 static void prepare_to_wait (struct execution_control_state *ecs);
 static int keep_going_stepped_thread (struct thread_info *tp);
 static int thread_still_needs_step_over (struct thread_info *tp);
+static void stop_all_threads (void);
 
 /* Are there any pending step-over requests?  If so, run all we can
    now and return true.  Otherwise, return false.  */
@@ -1961,8 +2018,6 @@ start_step_over (void)
       struct execution_control_state *ecs = &ecss;
       enum step_over_what step_what;
       int must_be_in_line;
-      struct regcache *regcache = get_thread_regcache (tp->ptid);
-      struct gdbarch *gdbarch = get_regcache_arch (regcache);
 
       next = thread_step_over_chain_next (tp);
 
@@ -1974,7 +2029,7 @@ start_step_over (void)
       step_what = thread_still_needs_step_over (tp);
       must_be_in_line = ((step_what & STEP_OVER_WATCHPOINT)
 			 || ((step_what & STEP_OVER_BREAKPOINT)
-			     && !use_displaced_stepping (gdbarch)));
+			     && !use_displaced_stepping (tp)));
 
       /* We currently stop all threads of all processes to step-over
 	 in-line.  If we need to start a new in-line step-over, let
@@ -2434,15 +2489,15 @@ resume (enum gdb_signal sig)
      We can't use displaced stepping when we are waiting for vfork_done
      event, displaced stepping breaks the vfork child similarly as single
      step software breakpoint.  */
-  if (use_displaced_stepping (gdbarch)
-      && tp->control.trap_expected
+  if (tp->control.trap_expected
+      && use_displaced_stepping (tp)
       && !step_over_info_valid_p ()
       && sig == GDB_SIGNAL_0
       && !current_inferior ()->waiting_for_vfork_done)
     {
-      struct displaced_step_inferior_state *displaced;
+      int prepared = displaced_step_prepare (inferior_ptid);
 
-      if (!displaced_step_prepare (inferior_ptid))
+      if (prepared == 0)
 	{
 	  if (debug_infrun)
 	    fprintf_unfiltered (gdb_stdlog,
@@ -2452,14 +2507,32 @@ resume (enum gdb_signal sig)
 	  discard_cleanups (old_cleanups);
 	  return;
 	}
+      else if (prepared < 0)
+	{
+	  /* Fallback to stepping over the breakpoint in-line.  */
+
+	  if (target_is_non_stop_p ())
+	    stop_all_threads ();
+
+	  set_step_over_info (get_regcache_aspace (regcache),
+			      regcache_read_pc (regcache), 0);
+
+	  step = maybe_software_singlestep (gdbarch, pc);
+
+	  insert_breakpoints ();
+	}
+      else if (prepared > 0)
+	{
+	  struct displaced_step_inferior_state *displaced;
 
-      /* Update pc to reflect the new address from which we will execute
-	 instructions due to displaced stepping.  */
-      pc = regcache_read_pc (get_thread_regcache (inferior_ptid));
+	  /* Update pc to reflect the new address from which we will
+	     execute instructions due to displaced stepping.  */
+	  pc = regcache_read_pc (get_thread_regcache (inferior_ptid));
 
-      displaced = get_displaced_stepping_state (ptid_get_pid (inferior_ptid));
-      step = gdbarch_displaced_step_hw_singlestep (gdbarch,
-						   displaced->step_closure);
+	  displaced = get_displaced_stepping_state (ptid_get_pid (inferior_ptid));
+	  step = gdbarch_displaced_step_hw_singlestep (gdbarch,
+						       displaced->step_closure);
+	}
     }
 
   /* Do we need to do it the hard way, w/temp breakpoints?  */
@@ -2578,8 +2651,8 @@ resume (enum gdb_signal sig)
     }
 
   if (debug_displaced
-      && use_displaced_stepping (gdbarch)
       && tp->control.trap_expected
+      && use_displaced_stepping (tp)
       && !step_over_info_valid_p ())
     {
       struct regcache *resume_regcache = get_thread_regcache (tp->ptid);
@@ -7367,8 +7440,7 @@ keep_going_pass_signal (struct execution_control_state *ecs)
 	 watchpoint.  The instruction copied to the scratch pad would
 	 still trigger the watchpoint.  */
       if (remove_bp
-	  && (remove_wps
-	      || !use_displaced_stepping (get_regcache_arch (regcache))))
+	  && (remove_wps || !use_displaced_stepping (ecs->event_thread)))
 	{
 	  set_step_over_info (get_regcache_aspace (regcache),
 			      regcache_read_pc (regcache), remove_wps);