[PATCH 1/2] gdbserver: catch fetch registers error

[Markus Metzger <markus dot t dot metzger at intel dot com>]

When the PTRACE_PEEKUSER ptrace request to read registers fails, gdbserer throws
an error that is caught in captured_main, where it causes a E01 error packet to
be sent and gdbserer to quit (if --once was specified) or the event loop to be
re-started (otherwise).

We may get such ptrace errors when trying to fetch registers for an exited or
running thread.  There are checks in GDB that check those conditions and throw
meaningful error messages before we could run into the above ptrace error,
e.g. thread.c:validate_registers_access.

I ran into a new case and, rather than adding another call to
validate_registers_access in GDB, I propose to catch the error already when
handling the 'g' packet in gdbserver and reply with an error packet - assuming
that gdbserver's internal state is still intact.

To not replace a meaningful error message with E01, I'm trying to generate a
useful error message when the error is detected and the exception is thrown.

It would look like this ...

gdb) PASS: gdb.btrace/enable-running.exp: continue to breakpoint: cont to 44
cont&
Continuing.
(gdb) PASS: gdb.btrace/enable-running.exp: cont&
record btrace
warning: Remote failure reply: E.Selected thread is running.
warning: Remote failure reply: E.Selected thread is running.

... although in this particular case, I'm going to suppress the warning.

To make this look a bit nicer, we could consider stripping the "E." or the
entire "Remote failure reply: E." when (re-)throwing the error inside GDB in
remote.c.

CC:  Daniel Jacobowitz	<drow@false.org>

2016-12-06  Markus Metzger  <markus.t.metzger@intel.com>

gdbserver/
	* server.c (process_serial_event): Add TRY/CATCH.
	* linux-low.c (fetch_register): Improve error message.
---
 gdb/gdbserver/linux-low.c | 19 ++++++++++++++++++-
 gdb/gdbserver/server.c    | 18 ++++++++++++++++--
 2 files changed, 34 insertions(+), 3 deletions(-)

diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
index e3e372c..a942b87 100644
--- a/gdb/gdbserver/linux-low.c
+++ b/gdb/gdbserver/linux-low.c
@@ -5692,7 +5692,24 @@ fetch_register (const struct usrregs_info *usrregs,
 		(PTRACE_TYPE_ARG3) (uintptr_t) regaddr, (PTRACE_TYPE_ARG4) 0);
       regaddr += sizeof (PTRACE_XFER_TYPE);
       if (errno != 0)
-	error ("reading register %d: %s", regno, strerror (errno));
+	{
+	  /* ESRCH could mean that the thread is not traced, exited, or is not
+	     stopped.  */
+	  if (errno == ESRCH)
+	    {
+	      struct lwp_info *lwp = get_thread_lwp (current_thread);
+
+	      if (!lwp_is_stopped (lwp))
+		error (_("Selected thread is running."));
+
+	      if (lwp_is_marked_dead (lwp))
+		error (_("Selected thread has terminated."));
+	    }
+
+	  /* Report a generic error if we could not determine the exact
+	     reason.  */
+	  error (_("Could not read register %d: %s."), regno, strerror (errno));
+	}
     }
 
   if (the_low_target.supply_ptrace_register)
diff --git a/gdb/gdbserver/server.c b/gdb/gdbserver/server.c
index ef8dd03..3064b4f 100644
--- a/gdb/gdbserver/server.c
+++ b/gdb/gdbserver/server.c
@@ -4132,8 +4132,22 @@ process_serial_event (void)
 	    write_enn (own_buf);
 	  else
 	    {
-	      regcache = get_thread_regcache (current_thread, 1);
-	      registers_to_string (regcache, own_buf);
+	      TRY
+		{
+		  regcache = get_thread_regcache (current_thread, 1);
+		  registers_to_string (regcache, own_buf);
+		}
+	      CATCH (exception, RETURN_MASK_ALL)
+		{
+		  const char *message;
+
+		  message = exception.message;
+		  if (message == NULL)
+		    message = _("Reading registers failed.");
+
+		  sprintf (own_buf, "E.%s", message);
+		}
+	      END_CATCH
 	    }
 	}
       break;
-- 
1.8.3.1