kdb,kgdb: Allow arbitrary kgdb magic knock sequences

The first packet that gdb sends when the kernel is in kdb mode seems
to change with every release of gdb.  Instead of continuing to add
many different gdb packets, change kdb to automatically look for any
thing that looks like a gdb packet.

Example 1 cold start test:
echo g > /proc/sysrq-trigger
$D#44+

Example 2 cold start test:
echo g > /proc/sysrq-trigger
$3#33

The second one should re-enter kdb's shell right away and is purely a
test.
Signed-off-by: NJason Wessel <jason.wessel@windriver.com>

kernel/debug/kdb/kdb_io.c

@@ -31,15 +31,21 @@ char kdb_prompt_str[CMD_BUFLEN];
 
 int kdb_trap_printk;
 
-static void kgdb_transition_check(char *buffer)
+static int kgdb_transition_check(char *buffer)
 {
-	int slen = strlen(buffer);
-	if (strncmp(buffer, "$?#3f", slen) != 0 &&
-	    strncmp(buffer, "$qSupported", slen) != 0 &&
-	    strncmp(buffer, "+$qSupported", slen) != 0) {
+	if (buffer[0] != '+' && buffer[0] != '$') {
 		KDB_STATE_SET(KGDB_TRANS);
 		kdb_printf("%s", buffer);
+	} else {
+		int slen = strlen(buffer);
+		if (slen > 3 && buffer[slen - 3] == '#') {
+			kdb_gdb_state_pass(buffer);
+			strcpy(buffer, "kgdb");
+			KDB_STATE_SET(DOING_KGDB);
+			return 1;
 		}
+	}
+	return 0;
 }
 
 static int kdb_read_get_key(char *buffer, size_t bufsize)
@@ -251,6 +257,10 @@ static char *kdb_read(char *buffer, size_t bufsize)
 	case 13: /* enter */
 		*lastchar++ = '\n';
 		*lastchar++ = '\0';
+		if (!KDB_STATE(KGDB_TRANS)) {
+			KDB_STATE_SET(KGDB_TRANS);
+			kdb_printf("%s", buffer);
+		}
 		kdb_printf("\n");
 		return buffer;
 	case 4: /* Del */
@@ -382,11 +392,13 @@ static char *kdb_read(char *buffer, size_t bufsize)
 				 * printed characters if we think that
 				 * kgdb is connecting, until the check
 				 * fails */
-				if (!KDB_STATE(KGDB_TRANS))
-					kgdb_transition_check(buffer);
-				else
+				if (!KDB_STATE(KGDB_TRANS)) {
+					if (kgdb_transition_check(buffer))
+						return buffer;
+				} else {
 					kdb_printf("%c", key);
 				}
+			}
 			/* Special escape to kgdb */
 			if (lastchar - buffer >= 5 &&
 			    strcmp(lastchar - 5, "$?#3f") == 0) {