[PATCH 5.4 75/77] io_uring: disable polling pollfree files

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Pavel Begunkov <asml.silence@gmail.com>

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: 221c5eb233823 ("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 |    1 +
 fs/io_uring.c            |    3 +++
 fs/signalfd.c            |    1 +
 include/linux/fs.h       |    1 +
 4 files changed, 6 insertions(+)

--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -6083,6 +6083,7 @@ 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)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1908,6 +1908,9 @@ static int io_poll_add(struct io_kiocb *
 	__poll_t mask;
 	u16 events;
 
+	if (req->file->f_op->may_pollfree)
+		return -EOPNOTSUPP;
+
 	if (unlikely(req->ctx->flags & IORING_SETUP_IOPOLL))
 		return -EINVAL;
 	if (sqe->addr || sqe->ioprio || sqe->off || sqe->len || sqe->buf_index)
--- a/fs/signalfd.c
+++ b/fs/signalfd.c
@@ -248,6 +248,7 @@ static const struct file_operations sign
 	.poll		= signalfd_poll,
 	.read		= signalfd_read,
 	.llseek		= noop_llseek,
+	.may_pollfree	= true,
 };
 
 static int do_signalfd4(int ufd, sigset_t *mask, int flags)
--- a/include/linux/fs.h
+++ b/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 {