Re: [PATCH] tty: fix pid refcount leak in tty_signal_session_leader

[Andy Shevchenko]

On Sun, Jul 26, 2020 at 8:30 AM Xin Xiong <xiongx18@xxxxxxxxxxxx> wrote:
>
> In the loop, every time when p->signal->leader is true, the function
> tty_signal_session_leader() will invoke get_pid() and return a
> reference of tty->pgrp with increased refcount to the local variable
> tty_pgrp or return NULL if it fails. After finishing the loop, the
> function invokes put_pid() for only once, decreasing the refcount that
> tty_pgrp keeps.
>
> Refcount leaks may occur when the scenario that p->signal->leader is
> true happens more than once. In this assumption, if the above scenario
> happens n times in the loop, the function forgets to decrease the
> refcount for n-1 times, which causes refcount leaks.
>
> Fix the issue by decreasing the current refcount of the local variable
> tty_pgrp before assigning new objects to it.
>
> Signed-off-by: Xiyu Yang <xiyuyang19@xxxxxxxxxxxx>
> Signed-off-by: Xin Tan <tanxin.ctf@xxxxxxxxx>
> Signed-off-by: Xin Xiong <xiongx18@xxxxxxxxxxxx>

This SoB chain is out of order. If you are the author, your SoB should
go first, if you are a commiter, the From line should correspond to
the first SoB (not yours), if it's a group of authors (funny for
one-/twoliner) then you consider to use Co-developed-by. Please, read
Submitting Patches document.

...

>                         put_pid(p->signal->tty_old_pgrp);  /* A noop */
>                         spin_lock(&tty->ctrl_lock);
> +                       if (tty_pgrp)
> +                               put_pid(tty_pgrp);
>                         tty_pgrp = get_pid(tty->pgrp);
>                         if (tty->pgrp)
>                                 p->signal->tty_old_pgrp = get_pid(tty->pgrp);

I guess this patch wasn't thought thru. You see the get_pid for it
happens twice in a row. Perhaps you have to get the logic behind all
these first?

P.S. ...on top of what Greg said.

-- 
With Best Regards,
Andy Shevchenko