Print the "file" command suggestion in exec_file_locate_attach

currently:
	$ gdbserver-7.9 :1234 true &
	$ gdb -q -ex 'target remote :1234' # that -q is not relevant here
	Remote debugging using :1234
	warning: Could not load vsyscall page because no executable was specified
	try using the "file" command first.
	0x00007ffff7ddcc80 in ?? ()
	(gdb) b main
	No symbol table is loaded.  Use the "file" command.
	Make breakpoint pending on future shared library load? (y or [n]) _

Provide more suggestive message to use the "file" command.

gdb/ChangeLog
2016-04-06  Jan Kratochvil  <jan.kratochvil@redhat.com>
	    Pedro Alves  <palves@redhat.com>

	* exec.c (exec_file_locate_attach): Print warning for unsupported
	target_pid_to_exec_file.
	* symfile-mem.c (add_vsyscall_page): Remove the "file" command
	message part.

4 files changed, 43 insertions, 3 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index fc9448d..c59249f 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,11 @@
+2016-04-06  Jan Kratochvil  <jan.kratochvil@redhat.com>
+	    Pedro Alves  <palves@redhat.com>
+
+	* exec.c (exec_file_locate_attach): Print warning for unsupported
+	target_pid_to_exec_file.
+	* symfile-mem.c (add_vsyscall_page): Remove the "file" command
+	message part.
+
 2016-04-04  Simon Marchi  <simon.marchi@ericsson.com>
 
 	* cli/cli-decode.c (help_cmd_list): Fix function doc and remove
diff --git a/gdb/exec.c b/gdb/exec.c
index 90811c0..a10ab9b 100644
--- a/gdb/exec.c
+++ b/gdb/exec.c
@@ -151,7 +151,13 @@ exec_file_locate_attach (int pid, int from_tty)
   /* Try to determine a filename from the process itself.  */
   exec_file = target_pid_to_exec_file (pid);
   if (exec_file == NULL)
-    return;
+    {
+      warning (_("No executable has been specified and target does not "
+		 "support\n"
+		 "determining executable automatically.  "
+		 "Try using the \"file\" command."));
+      return;
+    }
 
   /* If gdb_sysroot is not empty and the discovered filename
      is absolute then prefix the filename with gdb_sysroot.  */
diff --git a/gdb/remote.c b/gdb/remote.c
index 5c407b6..a88f4cd 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -1496,6 +1496,15 @@ enum {
 
 static struct packet_config remote_protocol_packets[PACKET_MAX];
 
+/* gdbserver < 7.7 (before its fix from 2013-12-11) did reply to any
+   unknown 'v' packet with string "OK".  "OK" gets interpreted by GDB
+   as a reply to known packet.  For packet "vFile:setfs:" it is an
+   invalid reply and GDB would return error in
+   remote_hostio_set_filesystem, making remote files access impossible.
+   If this variable is non-zero it means the remote gdbserver is buggy
+   and any not yet detected packets are assumed as unsupported.  */
+static int unknown_v_replies_ok;
+
 /* Returns the packet's corresponding "set remote foo-packet" command
    state.  See struct packet_config for more details.  */
 
@@ -1519,6 +1528,9 @@ packet_config_support (struct packet_config *config)
     case AUTO_BOOLEAN_FALSE:
       return PACKET_DISABLE;
     case AUTO_BOOLEAN_AUTO:
+      if (unknown_v_replies_ok && config->name != NULL
+	  && config->name[0] == 'v')
+	return PACKET_DISABLE;
       return config->support;
     default:
       gdb_assert_not_reached (_("bad switch"));
@@ -4023,6 +4035,21 @@ remote_start_remote (int from_tty, struct target_ops *target, int extended_p)
   if (packet_support (PACKET_QAllow) != PACKET_DISABLE)
     remote_set_permissions (target);
 
+  /* See unknown_v_replies_ok description.  */
+  {
+    const char v_mustreplyempty[] = "vMustReplyEmpty";
+
+    putpkt (v_mustreplyempty);
+    getpkt (&rs->buf, &rs->buf_size, 0);
+    if (strcmp (rs->buf, "OK") == 0)
+      unknown_v_replies_ok = 1;
+    else if (strcmp (rs->buf, "") == 0)
+      unknown_v_replies_ok = 0;
+    else
+      error (_("Remote replied unexpectedly to '%s': %s"), v_mustreplyempty,
+	     rs->buf);
+  }
+
   /* Next, we possibly activate noack mode.
 
      If the QStartNoAckMode packet configuration is set to AUTO,
diff --git a/gdb/symfile-mem.c b/gdb/symfile-mem.c
index 8eb5176..79739a6 100644
--- a/gdb/symfile-mem.c
+++ b/gdb/symfile-mem.c
@@ -214,8 +214,7 @@ add_vsyscall_page (struct target_ops *target, int from_tty)
 	  format should fix this.  */
 	{
 	  warning (_("Could not load vsyscall page "
-		     "because no executable was specified\n"
-		     "try using the \"file\" command first."));
+		     "because no executable was specified"));
 	  return;
 	}
       args.bfd = bfd;