[RFC PATCH] fs: Make splice() and tee() take into account O_NONBLOCK flag on pipes

[Slavomir Kaslev <kaslevs@vmware.com>]

The current implementation of splice() and tee() ignores O_NONBLOCK set on pipe
file descriptors and checks only the SPLICE_F_NONBLOCK flag for blocking on pipe
arguments. This is inconsistent since splice()-ing from/to non-pipe file
descriptors does take O_NONBLOCK into consideration.

Fix this by promoting O_NONBLOCK, when set on a pipe, to SPLICE_F_NONBLOCK.

Some context for how the current implementation of splice() leads to
inconsistent behavior. In the ongoing work[1] to add VM tracing capability to
trace-cmd we stream tracing data over named FIFOs or vsockets from guests back
to the host.

When we receive SIGINT from user to stop tracing, we set O_NONBLOCK on the input
file descriptor and set SPLICE_F_NONBLOCK for the next call to splice(). If
splice() was blocked waiting on data from the input FIFO, after SIGINT splice()
restarts with the same arguments (no SPLICE_F_NONBLOCK) and blocks again instead
of returning -EAGAIN when no data is available.

This differs from the splice() behavior when reading from a vsocket or when
we're doing a traditional read()/write() loop (trace-cmd's --nosplice argument).

With this patch applied we get the same behavior in all situations after setting
O_NONBLOCK which also matches the behavior of doing a read()/write() loop
instead of splice().

This change does have potential of breaking users who don't expect EAGAIN from
splice() when SPLICE_F_NONBLOCK is not set. OTOH programs that set O_NONBLOCK
and don't anticipate EAGAIN are arguably buggy[2].

[1] https://github.com/skaslev/trace-cmd/tree/vsock
[2] https://github.com/torvalds/linux/blob/d47e3da1759230e394096fd742aad423c291ba48/fs/read_write.c#L1425

Signed-off-by: Slavomir Kaslev <kaslevs@vmware.com>
---
 fs/splice.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/fs/splice.c b/fs/splice.c
index de2ede048473..6a1761b74f8d 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -1123,6 +1123,9 @@ static long do_splice(struct file *in, loff_t __user *off_in,
 		if (ipipe == opipe)
 			return -EINVAL;
 
+		if ((in->f_flags | out->f_flags) & O_NONBLOCK)
+			flags |= SPLICE_F_NONBLOCK;
+
 		return splice_pipe_to_pipe(ipipe, opipe, len, flags);
 	}
 
@@ -1148,6 +1151,9 @@ static long do_splice(struct file *in, loff_t __user *off_in,
 		if (unlikely(ret < 0))
 			return ret;
 
+		if (in->f_flags & O_NONBLOCK)
+			flags |= SPLICE_F_NONBLOCK;
+
 		file_start_write(out);
 		ret = do_splice_from(ipipe, out, &offset, len, flags);
 		file_end_write(out);
@@ -1172,6 +1178,9 @@ static long do_splice(struct file *in, loff_t __user *off_in,
 			offset = in->f_pos;
 		}
 
+		if (out->f_flags & O_NONBLOCK)
+			flags |= SPLICE_F_NONBLOCK;
+
 		pipe_lock(opipe);
 		ret = wait_for_space(opipe, flags);
 		if (!ret)
@@ -1717,6 +1726,9 @@ static long do_tee(struct file *in, struct file *out, size_t len,
 	 * copying the data.
 	 */
 	if (ipipe && opipe && ipipe != opipe) {
+		if ((in->f_flags | out->f_flags) & O_NONBLOCK)
+			flags |= SPLICE_F_NONBLOCK;
+
 		/*
 		 * Keep going, unless we encounter an error. The ipipe/opipe
 		 * ordering doesn't really matter.
-- 
2.19.1