kdb: Switch to use safer dbg_io_ops over console APIs

In kgdb context, calling console handlers aren't safe due to locks used
in those handlers which could in turn lead to a deadlock. Although, using
oops_in_progress increases the chance to bypass locks in most console
handlers but it might not be sufficient enough in case a console uses
more locks (VT/TTY is good example).

Currently when a driver provides both polling I/O and a console then kdb
will output using the console. We can increase robustness by using the
currently active polling I/O driver (which should be lockless) instead
of the corresponding console. For several common cases (e.g. an
embedded system with a single serial port that is used both for console
output and debugger I/O) this will result in no console handler being
used.

In order to achieve this we need to reverse the order of preference to
use dbg_io_ops (uses polling I/O mode) over console APIs. So we just
store "struct console" that represents debugger I/O in dbg_io_ops and
while emitting kdb messages, skip console that matches dbg_io_ops
console in order to avoid duplicate messages. After this change,
"is_console" param becomes redundant and hence removed.

Suggested-by: Daniel Thompson <[e-mail]>
Signed-off-by: Sumit Garg <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Douglas Anderson <[e-mail]>
Reviewed-by: Petr Mladek <[e-mail]>
Acked-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Daniel Thompson <[e-mail]>

drivers/tty/serial/kgdb_nmi.c

···
 	 * I/O utilities that messages sent to the console will automatically
 	 * be displayed on the dbg_io.
 	 */
-	dbg_io_ops->is_console = true;
+	dbg_io_ops->cons = co;
 
 	return 0;
 }

drivers/tty/serial/kgdboc.c

···
 
 #if IS_BUILTIN(CONFIG_KGDB_SERIAL_CONSOLE)
 static struct kgdb_io		kgdboc_earlycon_io_ops;
-static struct console		*earlycon;
 static int                      (*earlycon_orig_exit)(struct console *con);
 #endif /* IS_BUILTIN(CONFIG_KGDB_SERIAL_CONSOLE) */
 
···
 #if IS_BUILTIN(CONFIG_KGDB_SERIAL_CONSOLE)
 static void cleanup_earlycon(void)
 {
-	if (earlycon)
+	if (kgdboc_earlycon_io_ops.cons)
 		kgdb_unregister_io_module(&kgdboc_earlycon_io_ops);
 }
 #else /* !IS_BUILTIN(CONFIG_KGDB_SERIAL_CONSOLE) */
···
 		goto noconfig;
 	}
 
-	kgdboc_io_ops.is_console = 0;
+	kgdboc_io_ops.cons = NULL;
 	kgdb_tty_driver = NULL;
 
 	kgdboc_use_kms = 0;
···
 		int idx;
 		if (cons->device && cons->device(cons, &idx) == p &&
 		    idx == tty_line) {
-			kgdboc_io_ops.is_console = 1;
+			kgdboc_io_ops.cons = cons;
 			break;
 		}
 	}
···
 {
 	char c;
 
-	if (!earlycon->read(earlycon, &c, 1))
+	if (!kgdboc_earlycon_io_ops.cons->read(kgdboc_earlycon_io_ops.cons,
+					       &c, 1))
 		return NO_POLL_CHAR;
 
 	return c;
···
 
 static void kgdboc_earlycon_put_char(u8 chr)
 {
-	earlycon->write(earlycon, &chr, 1);
+	kgdboc_earlycon_io_ops.cons->write(kgdboc_earlycon_io_ops.cons, &chr,
+					   1);
 }
 
 static void kgdboc_earlycon_pre_exp_handler(void)
···
 	 * boot if we detect this case.
 	 */
 	for_each_console(con)
-		if (con == earlycon)
+		if (con == kgdboc_earlycon_io_ops.cons)
 			return;
 
 	already_warned = true;
···
 
 static void kgdboc_earlycon_deinit(void)
 {
-	if (!earlycon)
+	if (!kgdboc_earlycon_io_ops.cons)
 		return;
 
-	if (earlycon->exit == kgdboc_earlycon_deferred_exit)
+	if (kgdboc_earlycon_io_ops.cons->exit == kgdboc_earlycon_deferred_exit)
 		/*
 		 * kgdboc_earlycon is exiting but original boot console exit
 		 * was never called (AKA kgdboc_earlycon_deferred_exit()
 		 * didn't ever run).  Undo our trap.
 		 */
-		earlycon->exit = earlycon_orig_exit;
-	else if (earlycon->exit)
+		kgdboc_earlycon_io_ops.cons->exit = earlycon_orig_exit;
+	else if (kgdboc_earlycon_io_ops.cons->exit)
 		/*
 		 * We skipped calling the exit() routine so we could try to
 		 * keep using the boot console even after it went away.  We're
 		 * finally done so call the function now.
 		 */
-		earlycon->exit(earlycon);
+		kgdboc_earlycon_io_ops.cons->exit(kgdboc_earlycon_io_ops.cons);
 
-	earlycon = NULL;
+	kgdboc_earlycon_io_ops.cons = NULL;
 }
 
 static struct kgdb_io kgdboc_earlycon_io_ops = {
···
 	.write_char		= kgdboc_earlycon_put_char,
 	.pre_exception		= kgdboc_earlycon_pre_exp_handler,
 	.deinit			= kgdboc_earlycon_deinit,
-	.is_console		= true,
 };
 
 #define MAX_CONSOLE_NAME_LEN (sizeof((struct console *) 0)->name)
···
 		goto unlock;
 	}
 
-	earlycon = con;
+	kgdboc_earlycon_io_ops.cons = con;
 	pr_info("Going to register kgdb with earlycon '%s'\n", con->name);
 	if (kgdb_register_io_module(&kgdboc_earlycon_io_ops) != 0) {
-		earlycon = NULL;
+		kgdboc_earlycon_io_ops.cons = NULL;
 		pr_info("Failed to register kgdb with earlycon\n");
 	} else {
 		/* Trap exit so we can keep earlycon longer if needed. */

drivers/usb/early/ehci-dbgp.c

···
 		kgdbdbgp_wait_time = simple_strtoul(ptr, &ptr, 10);
 	}
 	kgdb_register_io_module(&kgdbdbgp_io_ops);
-	kgdbdbgp_io_ops.is_console = early_dbgp_console.index != -1;
+	if (early_dbgp_console.index != -1)
+		kgdbdbgp_io_ops.cons = &early_dbgp_console;
 
 	return 0;
 }

include/linux/kgdb.h

···
  * the I/O driver.
  * @post_exception: Pointer to a function that will do any cleanup work
  * for the I/O driver.
- * @is_console: 1 if the end device is a console 0 if the I/O device is
- * not a console
+ * @cons: valid if the I/O device is a console; else NULL.
  */
 struct kgdb_io {
 	const char		*name;
···
 	void			(*deinit) (void);
 	void			(*pre_exception) (void);
 	void			(*post_exception) (void);
-	int			is_console;
+	struct console		*cons;
 };
 
 extern const struct kgdb_arch		arch_kgdb_ops;

kernel/debug/kdb/kdb_io.c

···
 	if (msg_len == 0)
 		return;
 
-	if (dbg_io_ops && !dbg_io_ops->is_console) {
+	if (dbg_io_ops) {
 		const char *cp = msg;
 		int len = msg_len;
 
···
 
 	for_each_console(c) {
 		if (!(c->flags & CON_ENABLED))
+			continue;
+		if (c == dbg_io_ops->cons)
 			continue;
 		/*
 		 * Set oops_in_progress to encourage the console drivers to