linux-kernel - Re: potential null pointer deference (or maybe invalid null check) in io_uring io_poll_remove

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Date:   Fri, 9 Jul 2021 08:19:48 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Colin Ian King <colin.king@...onical.com>
Cc:     Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: potential null pointer deference (or maybe invalid null check) in
 io_uring io_poll_remove_double()

On 7/9/21 5:55 AM, Colin Ian King wrote:
> Hi Jens,
> 
> I was triaging some outstanding Coverity static analysis warnings and
> found a potential issue in the following commit:
> 
> commit 807abcb0883439af5ead73f3308310453b97b624
> Author: Jens Axboe <axboe@...nel.dk>
> Date:   Fri Jul 17 17:09:27 2020 -0600
> 
>     io_uring: ensure double poll additions work with both request types
> 
> The analysis from Coverity is as follows:
> 
> 4962 static int io_poll_double_wake(struct wait_queue_entry *wait,
> unsigned mode,
> 4963                               int sync, void *key)
> 4964 {
> 4965        struct io_kiocb *req = wait->private;
> 4966        struct io_poll_iocb *poll = io_poll_get_single(req);
> 4967        __poll_t mask = key_to_poll(key);
> 4968
> 4969        /* for instances that support it check for an event match
> first: */
> 
>     deref_ptr: Directly dereferencing pointer poll.
> 
> 4970        if (mask && !(mask & poll->events))
> 4971                return 0;
> 4972        if (!(poll->events & EPOLLONESHOT))
> 4973                return poll->wait.func(&poll->wait, mode, sync, key);
> 4974
> 4975        list_del_init(&wait->entry);
> 4976
> 
>   Dereference before null check (REVERSE_INULL)
>   check_after_deref: Null-checking poll suggests that it may be null,
> but it has already been dereferenced on all paths leading to the check.
> 
> 4977        if (poll && poll->head) {
> 4978                bool done;
> 
> pointer poll is being dereferenced on line 4970, however, on line 4977
> it is being null checked. Either the null check is redundant (because it
> can never be null) or it needs to be performed before the poll->events
> read on line 4970.

I think it's dead code, originally copied from the single poll wake
side. The 'poll' non-zero check should just go.

-- 
Jens Axboe