[RFC] splice/tee: len=0 fast path after validity check

[Pavel Begunkov <asml.silence@xxxxxxxxx>]

When len=0, splice() and tee() return 0 even if specified fds are
invalid, hiding errors from users. Move len=0 optimisation later after
basic validity checks.

before:
splice(len=0, fd_in=-1, ...) == 0;

after:
splice(len=0, fd_in=-1, ...) == -EBADF;

Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
---

Totally leaving it at yours judgment, but it'd be nice to have
for io_uring as well.

 fs/splice.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/fs/splice.c b/fs/splice.c
index a1dd54de24d8..8d6fc690f8e9 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -1122,6 +1122,9 @@ long do_splice(struct file *in, loff_t __user *off_in,
 		     !(out->f_mode & FMODE_WRITE)))
 		return -EBADF;
 
+	if (unlikely(!len))
+		return 0;
+
 	ipipe = get_pipe_info(in);
 	opipe = get_pipe_info(out);
 
@@ -1426,9 +1429,6 @@ SYSCALL_DEFINE6(splice, int, fd_in, loff_t __user *, off_in,
 	struct fd in, out;
 	long error;
 
-	if (unlikely(!len))
-		return 0;
-
 	if (unlikely(flags & ~SPLICE_F_ALL))
 		return -EINVAL;
 
@@ -1535,7 +1535,6 @@ static int splice_pipe_to_pipe(struct pipe_inode_info *ipipe,
 	int ret = 0;
 	bool input_wakeup = false;
 
-
 retry:
 	ret = ipipe_prep(ipipe, flags);
 	if (ret)
@@ -1769,6 +1768,9 @@ long do_tee(struct file *in, struct file *out, size_t len, unsigned int flags)
 	 * copying the data.
 	 */
 	if (ipipe && opipe && ipipe != opipe) {
+		if (unlikely(!len))
+			return 0;
+
 		if ((in->f_flags | out->f_flags) & O_NONBLOCK)
 			flags |= SPLICE_F_NONBLOCK;
 
@@ -1795,9 +1797,6 @@ SYSCALL_DEFINE4(tee, int, fdin, int, fdout, size_t, len, unsigned int, flags)
 	if (unlikely(flags & ~SPLICE_F_ALL))
 		return -EINVAL;
 
-	if (unlikely(!len))
-		return 0;
-
 	error = -EBADF;
 	in = fdget(fdin);
 	if (in.file) {
-- 
2.24.0