potential null pointer deference (or maybe invalid null check) in io_uring io_poll_remove_double()

From: Colin Ian King
Date: Fri Jul 09 2021 - 08:04:50 EST

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@xxxxxxxxx>
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);
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);
4975 list_del_init(&wait->entry);

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.