Messages in this thread | | | Subject | Re: [PATCH v2 2/2] tty: use tty_init_dev_retry() to workaround a race condition | From | Jiri Slaby <> | Date | Fri, 22 Nov 2019 10:11:26 +0100 |
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On 22. 11. 19, 10:05, Jiri Slaby wrote: > On 21. 11. 19, 22:01, Sudip Mukherjee wrote: >> Hi Greg, >> >> On Thu, Nov 21, 2019 at 05:41:38PM +0100, Greg Kroah-Hartman wrote: >>> On Thu, Nov 21, 2019 at 03:22:39PM +0000, Sudip Mukherjee wrote: >>>> There seems to be a race condition in tty drivers and I could see on >>>> many boot cycles a NULL pointer dereference as tty_init_dev() tries to >>>> do 'tty->port->itty = tty' even though tty->port is NULL. >>>> 'tty->port' will be set by the driver and if the driver has not yet done >>>> it before we open the tty device we can get to this situation. By adding >>>> some extra debug prints, I noticed that: >>>> >>>> 6.650130: uart_add_one_port >>>> 6.663849: register_console >>>> 6.664846: tty_open >>>> 6.674391: tty_init_dev >>>> 6.675456: tty_port_link_device >>>> >>>> uart_add_one_port() registers the console, as soon as it registers, the >>>> userspace tries to use it and that leads to tty_open() but >>>> uart_add_one_port() has not yet done tty_port_link_device() and so >>>> tty->port is not yet configured when control reaches tty_init_dev(). >>> >>> Shouldn't we do tty_port_link_device() before uart_add_one_port() to >>> remove that race? Once you register the console, yes, tty_open() can >>> happen, so the driver had better be ready to go at that point in time. >>> >> >> But tty_port_link_device() is done by uart_add_one_port() itself. >> After registering the console uart_add_one_port() will call >> tty_port_register_device_attr_serdev() and tty_port_link_device() is >> called from this. Thats still tty core. > > Interferences of console vs tty code are ugly. Does it help to simply > put tty_port_link_device to uart_add_one_port before uart_configure_port?
Alternatively, you can try setting tty_port in uart_install by: tty->port = &state->port.
BTW do you see the warning from tty_init_dev: "driver does not set tty->port. This will crash the kernel later. Fix the driver!\n" ? Maybe not given console is registered already, but crashes.
> thanks, -- js suse labs
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