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Date:   Thu, 12 Dec 2019 12:15:19 +0100
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Cc:     Jiri Slaby <jslaby@suse.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/2] tty: use tty_init_dev_retry() to workaround a
 race condition
Message-ID: <20191212111519.GA1534818@kroah.com>
References: <20191121152239.28405-1-sudipm.mukherjee@gmail.com>
 <20191121152239.28405-2-sudipm.mukherjee@gmail.com>
 <20191121164138.GD651886@kroah.com>
 <20191121210155.limd7v6cpd5yz2e7@debian>
 <eef58b47-f208-2ac5-6e02-a87f9568c70f@suse.com>
 <20191210114147.ivple4ccr4bj6c4h@debian>
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On Tue, Dec 10, 2019 at 11:41:47AM +0000, Sudip Mukherjee wrote:
> Hi Jiri,
> 
> On Fri, Nov 22, 2019 at 10:05:09AM +0100, 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.
> <snip>
> > >>>
> > >>> 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?
> 
> sorry for the late response, got busy with an out-of-tree driver.
> 
> It fixes the problem if I put tty_port_link_device() before
> uart_configure_port(). Please check the attached patch and that
> completely fixes the problem. Do you want me to send a proper patch for
> it or do you want me to check more into it?

This looks a lot more sane to me, can you resend it in proper format so
that I can apply it?

thanks,

greg k-h