[RFC PATCH 07/13] fs/userfaultfd: support read_iter to use io_uring

[Nadav Amit]

From: Nadav Amit <namit@xxxxxxxxxx>

iouring with userfaultfd cannot currently be used fixed buffers since
userfaultfd does not provide read_iter(). This is required to allow
asynchronous (queued) reads from userfaultfd.

To support async-reads of userfaultfd provide read_iter() instead of
read().

Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: io-uring@xxxxxxxxxxxxxxx
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
---
 fs/userfaultfd.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index b6a04e526025..6333b4632742 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1195,9 +1195,9 @@ static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
 	return ret;
 }
 
-static ssize_t userfaultfd_read(struct file *file, char __user *buf,
-				size_t count, loff_t *ppos)
+static ssize_t userfaultfd_read_iter(struct kiocb *iocb, struct iov_iter *to)
 {
+	struct file *file = iocb->ki_filp;
 	struct userfaultfd_ctx *ctx = file->private_data;
 	ssize_t _ret, ret = 0;
 	struct uffd_msg msg;
@@ -1207,16 +1207,18 @@ static ssize_t userfaultfd_read(struct file *file, char __user *buf,
 		return -EINVAL;
 
 	for (;;) {
-		if (count < sizeof(msg))
+		if (iov_iter_count(to) < sizeof(msg))
 			return ret ? ret : -EINVAL;
 		_ret = userfaultfd_ctx_read(ctx, no_wait, &msg);
 		if (_ret < 0)
 			return ret ? ret : _ret;
-		if (copy_to_user((__u64 __user *) buf, &msg, sizeof(msg)))
-			return ret ? ret : -EFAULT;
+
+		_ret = copy_to_iter(&msg, sizeof(msg), to);
+		if (_ret != sizeof(msg))
+			return ret ? ret : -EINVAL;
+
 		ret += sizeof(msg);
-		buf += sizeof(msg);
-		count -= sizeof(msg);
+
 		/*
 		 * Allow to read more than one fault at time but only
 		 * block if waiting for the very first one.
@@ -1980,7 +1982,7 @@ static const struct file_operations userfaultfd_fops = {
 #endif
 	.release	= userfaultfd_release,
 	.poll		= userfaultfd_poll,
-	.read		= userfaultfd_read,
+	.read_iter	= userfaultfd_read_iter,
 	.unlocked_ioctl = userfaultfd_ioctl,
 	.compat_ioctl	= compat_ptr_ioctl,
 	.llseek		= noop_llseek,
-- 
2.25.1