On 2024-02-22, Peter Collingbourne pcc@google.com wrote:

During the handoff from earlycon to the real console driver, we have two separate drivers operating on the same device concurrently. In the case of the 8250 driver these concurrent accesses cause problems due to the driver's use of banked registers, controlled by LCR.DLAB. It is possible for the setup(), config_port(), pm() and set_mctrl() callbacks to set DLAB, which can cause the earlycon code that intends to access TX to instead access DLL, leading to missed output and corruption on the serial line due to unintended modifications to the baud rate.

In particular, for setup() we have:

univ8250_console_setup() -> serial8250_console_setup() -> uart_set_options() -> serial8250_set_termios() -> serial8250_do_set_termios() -> serial8250_do_set_divisor()

For config_port() we have:

serial8250_config_port() -> autoconfig()

For pm() we have:

serial8250_pm() -> serial8250_do_pm() -> serial8250_set_sleep()

For set_mctrl() we have (for some devices):

serial8250_set_mctrl() -> omap8250_set_mctrl() -> __omap8250_set_mctrl()

To avoid such problems, let's make it so that the console is locked during pre-registration calls to these callbacks, which will prevent the earlycon driver from running concurrently.

Even after the current atomic/threaded print rework is completed, the console lock will be used to synchronize boot consoles with non-boot consoles. I am fine with this solution.

Comments below...

Remove the partial solution to this problem in the 8250 driver that locked the console only during autoconfig_irq(), as this would result in a deadlock with the new approach. The console continues to be locked during autoconfig_irq() because it can only be called through uart_configure_port().

Although this patch introduces more locking than strictly necessary (and in particular it also locks during the call to rs485_config() which is not affected by this issue as far as I can tell), it follows the principle that it is the responsibility of the generic console code to manage the earlycon handoff by ensuring that earlycon and real console driver code cannot run concurrently, and not the individual drivers.

Signed-off-by: Peter Collingbourne pcc@google.com Link: https://linux-review.googlesource.com/id/I7cf8124dcebf8618e6b2ee543fa5b25532... Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Cc: stable@vger.kernel.org

---

drivers/tty/serial/8250/8250_port.c | 6 ------ drivers/tty/serial/serial_core.c | 10 ++++++++++ kernel/printk/printk.c | 20 +++++++++++++++++--- 3 files changed, 27 insertions(+), 9 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c index 8ca061d3bbb9..1d65055dde27 100644 --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -1329,9 +1329,6 @@ static void autoconfig_irq(struct uart_8250_port *up) inb_p(ICP); }

• if (uart_console(port))

• console_lock();
  

• /* forget possible initially masked and pending IRQ */ probe_irq_off(probe_irq_on()); save_mcr = serial8250_in_MCR(up);

@@ -1371,9 +1368,6 @@ static void autoconfig_irq(struct uart_8250_port *up) if (port->flags & UPF_FOURPORT) outb_p(save_ICP, ICP);

• if (uart_console(port))

• console_unlock();
  

• port->irq = (irq > 0) ? irq : 0;

} diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c index d6a58a9e072a..128aa0e0ae24 100644 --- a/drivers/tty/serial/serial_core.c +++ b/drivers/tty/serial/serial_core.c @@ -2608,7 +2608,11 @@ uart_configure_port(struct uart_driver *drv, struct uart_state *state, port->type = PORT_UNKNOWN; flags |= UART_CONFIG_TYPE; }

It would be nice to add a comment here mentioning why the console_lock is taken. Even if it is something brief like:

/* Sychronize with possible boot console. */

• if (uart_console(port))
  

• 	console_lock();
  

  port->ops->config_port(port, flags);

• if (uart_console(port))
  

• 	console_unlock();
  

  }

if (port->type != PORT_UNKNOWN) { @@ -2616,6 +2620,9 @@ uart_configure_port(struct uart_driver *drv, struct uart_state *state, uart_report_port(drv, port);

Also, here a brief comment.

• if (uart_console(port))
  

• 	console_lock();
  

• /* Power up port for set_mctrl() */ uart_change_pm(state, UART_PM_STATE_ON);

@@ -2632,6 +2639,9 @@ uart_configure_port(struct uart_driver *drv, struct uart_state *state, uart_rs485_config(port);

• if (uart_console(port))
  

• 	console_unlock();
  

• /*
  • If this driver supports console, and it hasn't been
  • successfully registered yet, try to re-register it.

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index f2444b581e16..db69545e6250 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -3263,6 +3263,20 @@ static int __init keep_bootcon_setup(char *str) early_param("keep_bootcon", keep_bootcon_setup);

And here. Maybe slightly more verbose since there is an entire wrapper function created for the purpose.

+static int console_call_setup(struct console *newcon, char *options) +{

• int err;
• if (!newcon->setup)

• return 0;
  

• console_lock();
• err = newcon->setup(newcon, options);
• console_unlock();
• return err;

+}

/*

• This is called by register_console() to try to match
• the newly registered console with any of the ones selected

@@ -3298,8 +3312,8 @@ static int try_enable_preferred_console(struct console *newcon, if (_braille_register_console(newcon, c)) return 0;

• 	if (newcon->setup &&
  

• 	    (err = newcon->setup(newcon, c->options)) != 0)
  

• 	err = console_call_setup(newcon, c->options);
  

• 	if (err != 0)
  	return err;
  

  } newcon->flags |= CON_ENABLED;

@@ -3325,7 +3339,7 @@ static void try_enable_default_console(struct console *newcon) if (newcon->index < 0) newcon->index = 0;

• if (newcon->setup && newcon->setup(newcon, NULL) != 0)

• if (console_call_setup(newcon, NULL) != 0) return;

newcon->flags |= CON_ENABLED; -- 2.44.0.rc1.240.g4c46232300-goog

With comments added:

Reviewed-by: John Ogness john.ogness@linutronix.de