kernfs: implement ->read_iter

Switch kernfs to implement the read_iter method instead of plain old
read to prepare to supporting splice and sendfile again.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210120204631.274206-2-hch@lst.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/kernfs/file.c

@@ -14,6 +14,7 @@
 #include <linux/pagemap.h>
 #include <linux/sched/mm.h>
 #include <linux/fsnotify.h>
+#include <linux/uio.h>
 
 #include "kernfs-internal.h"
 
@@ -180,11 +181,10 @@ static const struct seq_operations kernfs_seq_ops = {
  * it difficult to use seq_file.  Implement simplistic custom buffering for
  * bin files.
  */
-static ssize_t kernfs_file_direct_read(struct kernfs_open_file *of,
-				       char __user *user_buf, size_t count,
-				       loff_t *ppos)
+static ssize_t kernfs_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 {
-	ssize_t len = min_t(size_t, count, PAGE_SIZE);
+	struct kernfs_open_file *of = kernfs_of(iocb->ki_filp);
+	ssize_t len = min_t(size_t, iov_iter_count(iter), PAGE_SIZE);
 	const struct kernfs_ops *ops;
 	char *buf;
 
@@ -210,7 +210,7 @@ static ssize_t kernfs_file_direct_read(struct kernfs_open_file *of,
 	of->event = atomic_read(&of->kn->attr.open->event);
 	ops = kernfs_ops(of->kn);
 	if (ops->read)
-		len = ops->read(of, buf, len, *ppos);
+		len = ops->read(of, buf, len, iocb->ki_pos);
 	else
 		len = -EINVAL;
 
@@ -220,12 +220,12 @@ static ssize_t kernfs_file_direct_read(struct kernfs_open_file *of,
 	if (len < 0)
 		goto out_free;
 
-	if (copy_to_user(user_buf, buf, len)) {
+	if (copy_to_iter(buf, len, iter) != len) {
 		len = -EFAULT;
 		goto out_free;
 	}
 
-	*ppos += len;
+	iocb->ki_pos += len;
 
  out_free:
 	if (buf == of->prealloc_buf)
@@ -235,22 +235,11 @@ static ssize_t kernfs_file_direct_read(struct kernfs_open_file *of,
 	return len;
 }
 
-/**
- * kernfs_fop_read - kernfs vfs read callback
- * @file: file pointer
- * @user_buf: data to write
- * @count: number of bytes
- * @ppos: starting offset
- */
-static ssize_t kernfs_fop_read(struct file *file, char __user *user_buf,
-			       size_t count, loff_t *ppos)
+static ssize_t kernfs_fop_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 {
-	struct kernfs_open_file *of = kernfs_of(file);
-
-	if (of->kn->flags & KERNFS_HAS_SEQ_SHOW)
-		return seq_read(file, user_buf, count, ppos);
-	else
-		return kernfs_file_direct_read(of, user_buf, count, ppos);
+	if (kernfs_of(iocb->ki_filp)->kn->flags & KERNFS_HAS_SEQ_SHOW)
+		return seq_read_iter(iocb, iter);
+	return kernfs_file_read_iter(iocb, iter);
 }
 
 /**
@@ -960,7 +949,7 @@ void kernfs_notify(struct kernfs_node *kn)
 EXPORT_SYMBOL_GPL(kernfs_notify);
 
 const struct file_operations kernfs_file_fops = {
-	.read		= kernfs_fop_read,
+	.read_iter	= kernfs_fop_read_iter,
 	.write		= kernfs_fop_write,
 	.llseek		= generic_file_llseek,
 	.mmap		= kernfs_fop_mmap,