[14/14] fs: don't change the address limit for ->read_iter in __kernel_read

Commit Message

Christoph Hellwig May 28, 2020, 5:40 a.m. UTC

If we read to a file that implements ->read_iter there is no need
to change the address limit if we send a kvec down.  Implement that
case, and prefer it over using plain ->read with a changed address
limit if available.

Signed-off-by: Christoph Hellwig <[e-mail]>
---
 fs/read_write.c | 24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

Comments

Al Viro May 28, 2020, 7:03 p.m. UTC | #1

On Thu, May 28, 2020 at 07:40:43AM +0200, Christoph Hellwig wrote:
> If we read to a file that implements ->read_iter there is no need
> to change the address limit if we send a kvec down.  Implement that
> case, and prefer it over using plain ->read with a changed address
> limit if available.

Ditto re never converting something that bails out on uaccess_kernel()
in ->read() over to ->read_iter().

Patch

diff --git a/fs/read_write.c b/fs/read_write.c
index 46ddfce17e839..c93acbd8bf5a3 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -421,7 +421,6 @@  static ssize_t new_sync_read(struct file *filp, char __user *buf, size_t len, lo
 
 ssize_t __kernel_read(struct file *file, void *buf, size_t count, loff_t *pos)
 {
-	mm_segment_t old_fs = get_fs();
 	ssize_t ret;
 
 	if (!(file->f_mode & FMODE_CAN_READ))
@@ -429,14 +428,25 @@  ssize_t __kernel_read(struct file *file, void *buf, size_t count, loff_t *pos)
 
 	if (count > MAX_RW_COUNT)
 		count =  MAX_RW_COUNT;
-	set_fs(KERNEL_DS);
-	if (file->f_op->read)
+	if (file->f_op->read_iter) {
+		struct kvec iov = { .iov_base = buf, .iov_len = count };
+		struct kiocb kiocb;
+		struct iov_iter iter;
+
+		init_sync_kiocb(&kiocb, file);
+		kiocb.ki_pos = *pos;
+		iov_iter_kvec(&iter, READ, &iov, 1, count);
+		ret = file->f_op->read_iter(&kiocb, &iter);
+		*pos = kiocb.ki_pos;
+	} else if (file->f_op->read) {
+		mm_segment_t old_fs = get_fs();
+
+		set_fs(KERNEL_DS);
 		ret = file->f_op->read(file, (void __user *)buf, count, pos);
-	else if (file->f_op->read_iter)
-		ret = new_sync_read(file, (void __user *)buf, count, pos);
-	else
+		set_fs(old_fs);
+	} else {
 		ret = -EINVAL;
-	set_fs(old_fs);
+	}
 	if (ret > 0) {
 		fsnotify_access(file);
 		add_rchar(current, ret);