[PATCH 4/6] fs: check FMODE_LSEEK to control internal pipe splicing

["Jason A. Donenfeld" <Jason@zx2c4.com>]

The original direct splicing mechanism from Jens required the input to
be a regular file because it was avoiding the special socket case. It
also recognized blkdevs as being close enough to a regular file. But it
forgot about chardevs, which behave the same way and work fine here.

This is an okayish heuristic, but it doesn't totally work. For example,
a few chardevs should be spliceable here. And a few regular files
shouldn't. This patch fixes this by instead checking whether FMODE_LSEEK
is set, which represents decently enough what we need rewinding for when
splicing to internal pipes.

Fixes: b92ce5589374 ("[PATCH] splice: add direct fd <-> fd splicing support")
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 fs/splice.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/fs/splice.c b/fs/splice.c
index 047b79db8eb5..93a2c9bf6249 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -814,17 +814,15 @@ ssize_t splice_direct_to_actor(struct file *in, struct splice_desc *sd,
 {
 	struct pipe_inode_info *pipe;
 	long ret, bytes;
-	umode_t i_mode;
 	size_t len;
 	int i, flags, more;
 
 	/*
-	 * We require the input being a regular file, as we don't want to
-	 * randomly drop data for eg socket -> socket splicing. Use the
-	 * piped splicing for that!
+	 * We require the input to be seekable, as we don't want to randomly
+	 * drop data for eg socket -> socket splicing. Use the piped splicing
+	 * for that!
 	 */
-	i_mode = file_inode(in)->i_mode;
-	if (unlikely(!S_ISREG(i_mode) && !S_ISBLK(i_mode)))
+	if (unlikely(!(in->f_mode & FMODE_LSEEK)))
 		return -EINVAL;
 
 	/*
-- 
2.35.1