Re: [PATCH] tty: fix data race in n_tty_receive_buf_common

[Kohli, Gaurav]

Hi Alan,


On 1/5/2018 7:45 PM, Alan Cox wrote:
> > But in above case , there we can hit another race, if we have a sequence
> > like this
> > tty_init_dev->alloc_tty_struct -> tty_ldisc_init -> this will initialize
> > ldisc ,
> > but at this moment disc_data is still NULL
> >
> > And if flush_to_ldisc comes in between, it will take ldisc reference and
> > proceeds receive buffer.
> So you need to move the lock up one line to protect the assignment to
> tty->port->itty. We can do that.
>
> At that point your flush_to_ldisc should see either port->itty = NULL or a
> valid initialized ldisc.

Yes , with little modification this should work.

+retval =Â tty_ldisc_lock(tty, 5 * HZ);
+if (retval)
+ÂÂÂÂ ÂÂÂ goto err_release_lock;
tty->port->itty = tty;
/*
* Structures all installed ... call the ldisc open routines.
* If we fail here just call release_tty to clean up. No need
* to decrement the use counts, as release_tty doesn't care.
*/
retval = tty_ldisc_setup(tty, tty->link);
ÂÂÂ ÂÂÂ if (retval)
ÂÂÂ ÂÂÂ ÂÂÂ goto err_release_tty;
tty_ldisc_unlock(tty);
err_release_tty:
tty_unlock(tty);
tty_info_ratelimited(tty, "ldisc open failed (%d), clearing slot %d\n",
ÂÂÂ retval, idx);
ÂÂÂ release_tty(tty, idx);
ÂÂÂ return ERR_PTR(retval);

+err_release_lock;
+tty_ldisc_unlock(tty);
+return ERR_PTR(retval);

Please let me know if above modification seems fine , then we can upload 
this and try reproduction of Bug.

Regards
Gaurav

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