[PATCH for-5.7] io_uring: fix zero len do_splice()

[Pavel Begunkov <asml.silence@gmail.com>]

do_splice() doesn't expect len to be 0. Just always return 0 in this
case as splice(2) do.

Fixes: 7d67af2c0134 ("io_uring: add splice(2) support")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 fs/io_uring.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 65458eda2127..d53a1ef2a205 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2772,16 +2772,19 @@ static int io_splice(struct io_kiocb *req, bool force_nonblock)
 	struct file *out = sp->file_out;
 	unsigned int flags = sp->flags & ~SPLICE_F_FD_IN_FIXED;
 	loff_t *poff_in, *poff_out;
-	long ret;
+	long ret = 0;
 
 	if (force_nonblock)
 		return -EAGAIN;
 
 	poff_in = (sp->off_in == -1) ? NULL : &sp->off_in;
 	poff_out = (sp->off_out == -1) ? NULL : &sp->off_out;
-	ret = do_splice(in, poff_in, out, poff_out, sp->len, flags);
-	if (force_nonblock && ret == -EAGAIN)
-		return -EAGAIN;
+
+	if (sp->len) {
+		ret = do_splice(in, poff_in, out, poff_out, sp->len, flags);
+		if (force_nonblock && ret == -EAGAIN)
+			return -EAGAIN;
+	}
 
 	io_put_file(req, in, (sp->flags & SPLICE_F_FD_IN_FIXED));
 	req->flags &= ~REQ_F_NEED_CLEANUP;
-- 
2.24.0