fs: Make splice() and tee() take into account O_NONBLOCK flag on pipes

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>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/splice.c b/fs/splice.c
index ec8b54b..6489fb9 100644 (file)
--- 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.