allow non-block splice only when in file is seekable.

do_splice_to() is split to two parts: read data from in file to spd, and move
data from spd to pipe. If there isn't much space in pipe for the data in spd,
and the splice is called with non-block flag, the data dropped due to non
enough space in pipe will be lost forever when in file isn't seekable.

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
----
 fs/splice.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/fs/splice.c b/fs/splice.c
index 9e52de5..8137e23 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -1346,8 +1346,12 @@ static long do_splice(struct file *in, loff_t __user *off_in,
 			if (copy_from_user(&offset, off_in, sizeof(loff_t)))
 				return -EFAULT;
 			off = &offset;
-		} else
+		} else {
+			if ((flags & SPLICE_F_NONBLOCK) &&
+			    !(in->f_mode & FMODE_PREAD))
+				return -EINVAL;
 			off = &in->f_pos;
+		}
 
 		ret = do_splice_to(in, off, opipe, len, flags);
 
--
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