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* [RFC PATCH] fs: Make splice() and tee() take into account O_NONBLOCK flag on pipes
@ 2019-02-07 15:45 Slavomir Kaslev
  2019-02-13 14:44 ` Steven Rostedt
  0 siblings, 1 reply; 7+ messages in thread
From: Slavomir Kaslev @ 2019-02-07 15:45 UTC (permalink / raw)
  To: Alexander Viro; +Cc: Jens Axboe, Steven Rostedt, linux-fsdevel, linux-kernel

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].


Signed-off-by: Slavomir Kaslev <>
 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;
 		ret = do_splice_from(ipipe, out, &offset, len, flags);
@@ -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;
 		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.

^ permalink raw reply	[flat|nested] 7+ messages in thread

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Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-02-07 15:45 [RFC PATCH] fs: Make splice() and tee() take into account O_NONBLOCK flag on pipes Slavomir Kaslev
2019-02-13 14:44 ` Steven Rostedt
2019-02-19 17:12   ` Steven Rostedt
2019-03-04 15:58     ` Steven Rostedt
2019-03-05  0:04       ` Linus Torvalds
2019-03-05  0:09         ` Linus Torvalds
2019-03-05  1:54           ` Steven Rostedt

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