On Thu, 7 Feb 2019 17:45:19 +0200 Slavomir Kaslev <kaslevs@xxxxxxxxxx> wrote: > 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]. Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> Does anyone have any issues or comments about this patch? Thanks! -- Steve > > [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@xxxxxxxxxx> > --- > 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.
