io_uring: disable polling pollfree files

Older kernels lack io_uring POLLFREE handling. As only affected files
are signalfd and android binder the safest option would be to disable
polling those files via io_uring and hope there are no users.

Fixes: 221c5eb233 ("io_uring: add support for IORING_OP_POLL")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder.c

@@ -6069,6 +6069,7 @@ const struct file_operations binder_fops = {
 	.open = binder_open,
 	.flush = binder_flush,
 	.release = binder_release,
+	.may_pollfree = true,
 };
 
 static int __init init_binder_device(const char *name)

fs/io_uring.c

@@ -5198,6 +5198,11 @@ static __poll_t __io_arm_poll_handler(struct io_kiocb *req,
 	struct io_ring_ctx *ctx = req->ctx;
 	bool cancel = false;
 
+	if (req->file->f_op->may_pollfree) {
+		spin_lock_irq(&ctx->completion_lock);
+		return -EOPNOTSUPP;
+	}
+
 	INIT_HLIST_NODE(&req->hash_node);
 	io_init_poll_iocb(poll, mask, wake_func);
 	poll->file = req->file;

fs/signalfd.c

@@ -248,6 +248,7 @@ static const struct file_operations signalfd_fops = {
 	.poll		= signalfd_poll,
 	.read		= signalfd_read,
 	.llseek		= noop_llseek,
+	.may_pollfree	= true,
 };
 
 static int do_signalfd4(int ufd, sigset_t *mask, int flags)

include/linux/fs.h

@@ -1859,6 +1859,7 @@ struct file_operations {
 				   struct file *file_out, loff_t pos_out,
 				   loff_t len, unsigned int remap_flags);
 	int (*fadvise)(struct file *, loff_t, loff_t, int);
+	bool may_pollfree;
 } __randomize_layout;
 
 struct inode_operations {