io_uring: explicitly count entries for poll reqs

If __io_queue_proc() fails to add a second poll entry, e.g. kmalloc()
failed, but it goes on with a third waitqueue, it may succeed and
overwrite the error status. Count the number of poll entries we added,
so we can set pt->error to zero at the beginning and find out when the
mentioned scenario happens.

Cc: stable@vger.kernel.org
Fixes: 18bceab101add ("io_uring: allow POLL_ADD with double poll_wait() users")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/9d6b9e561f88bcc0163623b74a76c39f712151c3.1626774457.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 0cac361bf6b8de27cb5e16cf4a0a4d757219f198..6668902cf50c0c61851b8660663880505dff4180 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -4802,6 +4802,7 @@ IO_NETOP_FN(recv);
 struct io_poll_table {
        struct poll_table_struct pt;
        struct io_kiocb *req;
+       int nr_entries;
        int error;
 };
 
@@ -4995,11 +4996,11 @@ static void __io_queue_proc(struct io_poll_iocb *poll, struct io_poll_table *pt,
        struct io_kiocb *req = pt->req;
 
        /*
-        * If poll->head is already set, it's because the file being polled
-        * uses multiple waitqueues for poll handling (eg one for read, one
-        * for write). Setup a separate io_poll_iocb if this happens.
+        * The file being polled uses multiple waitqueues for poll handling
+        * (e.g. one for read, one for write). Setup a separate io_poll_iocb
+        * if this happens.
         */
-       if (unlikely(poll->head)) {
+       if (unlikely(pt->nr_entries)) {
                struct io_poll_iocb *poll_one = poll;
 
                /* already have a 2nd entry, fail a third attempt */
@@ -5027,7 +5028,7 @@ static void __io_queue_proc(struct io_poll_iocb *poll, struct io_poll_table *pt,
                *poll_ptr = poll;
        }
 
-       pt->error = 0;
+       pt->nr_entries++;
        poll->head = head;
 
        if (poll->events & EPOLLEXCLUSIVE)
@@ -5104,9 +5105,12 @@ static __poll_t __io_arm_poll_handler(struct io_kiocb *req,
 
        ipt->pt._key = mask;
        ipt->req = req;
-       ipt->error = -EINVAL;
+       ipt->error = 0;
+       ipt->nr_entries = 0;
 
        mask = vfs_poll(req->file, &ipt->pt) & poll->events;
+       if (unlikely(!ipt->nr_entries) && !ipt->error)
+               ipt->error = -EINVAL;
 
        spin_lock_irq(&ctx->completion_lock);
        if (likely(poll->head)) {