[PATCH v6 4/4] kdb: Switch to use safer dbg_io_ops over console APIs

[Sumit Garg]

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 <daniel.thompson@xxxxxxxxxx>
Signed-off-by: Sumit Garg <sumit.garg@xxxxxxxxxx>
---
 drivers/tty/serial/kgdb_nmi.c |  2 +-
 drivers/tty/serial/kgdboc.c   | 32 ++++++++++++++++----------------
 drivers/usb/early/ehci-dbgp.c |  3 ++-
 include/linux/kgdb.h          |  5 ++---
 kernel/debug/kdb/kdb_io.c     |  4 +++-
 5 files changed, 24 insertions(+), 22 deletions(-)

diff --git a/drivers/tty/serial/kgdb_nmi.c b/drivers/tty/serial/kgdb_nmi.c
index 5022447..6004c0c 100644
--- a/drivers/tty/serial/kgdb_nmi.c
+++ b/drivers/tty/serial/kgdb_nmi.c
@@ -50,7 +50,7 @@ static int kgdb_nmi_console_setup(struct console *co, char *options)
 	 * 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;
 }
diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c
index 4139698..84ffede 100644
--- a/drivers/tty/serial/kgdboc.c
+++ b/drivers/tty/serial/kgdboc.c
@@ -45,7 +45,6 @@ static struct platform_device *kgdboc_pdev;
 
 #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) */
 
@@ -145,7 +144,7 @@ static void kgdboc_unregister_kbd(void)
 #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) */
@@ -178,7 +177,7 @@ static int configure_kgdboc(void)
 		goto noconfig;
 	}
 
-	kgdboc_io_ops.is_console = 0;
+	kgdboc_io_ops.cons = NULL;
 	kgdb_tty_driver = NULL;
 
 	kgdboc_use_kms = 0;
@@ -198,7 +197,7 @@ static int configure_kgdboc(void)
 		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;
 		}
 	}
@@ -433,7 +432,8 @@ static int kgdboc_earlycon_get_char(void)
 {
 	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;
@@ -441,7 +441,8 @@ static int kgdboc_earlycon_get_char(void)
 
 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)
@@ -461,7 +462,7 @@ 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;
@@ -484,25 +485,25 @@ static int kgdboc_earlycon_deferred_exit(struct console *con)
 
 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 = {
@@ -511,7 +512,6 @@ 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)
@@ -557,10 +557,10 @@ static int __init kgdboc_earlycon_init(char *opt)
 		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. */
diff --git a/drivers/usb/early/ehci-dbgp.c b/drivers/usb/early/ehci-dbgp.c
index ea0d531..775cf70 100644
--- a/drivers/usb/early/ehci-dbgp.c
+++ b/drivers/usb/early/ehci-dbgp.c
@@ -1058,7 +1058,8 @@ static int __init kgdbdbgp_parse_config(char *str)
 		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;
 }
diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
index c62d764..529116b 100644
--- a/include/linux/kgdb.h
+++ b/include/linux/kgdb.h
@@ -276,8 +276,7 @@ struct kgdb_arch {
  * 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;
@@ -288,7 +287,7 @@ struct kgdb_io {
 	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;
diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
index 0e4f2ed..683a799 100644
--- a/kernel/debug/kdb/kdb_io.c
+++ b/kernel/debug/kdb/kdb_io.c
@@ -549,7 +549,7 @@ static void kdb_msg_write(const char *msg, int msg_len)
 	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;
 
@@ -562,6 +562,8 @@ static void kdb_msg_write(const char *msg, int 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
 		 * disregard their internal spin locks: in the current calling
-- 
2.7.4