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Subject: Re: [PATCH v2 2/2] tty: use tty_init_dev_retry() to workaround a race
 condition
From:   Jiri Slaby <jslaby@suse.com>
To:     Sudip Mukherjee <sudipm.mukherjee@gmail.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     linux-kernel@vger.kernel.org
References: <20191121152239.28405-1-sudipm.mukherjee@gmail.com>
 <20191121152239.28405-2-sudipm.mukherjee@gmail.com>
 <20191121164138.GD651886@kroah.com> <20191121210155.limd7v6cpd5yz2e7@debian>
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Message-ID: <674a05c6-67d7-f337-f529-ca1c62c35624@suse.com>
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