Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Message-ID: <ece3af8e4512517da220bdd2f43119ca889f6c61.camel@sipsolutions.net>
Subject: Re: Splicing to/from a tty
From:   Johannes Berg <johannes@sipsolutions.net>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Oliver Giles <ohw.giles@gmail.com>, linux-kernel@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>
Date:   Mon, 18 Jan 2021 09:58:49 +0100
In-Reply-To: <20210118085311.GA2735@lst.de>
References: <C8KER7U60WXE.25UFD8RE6QZQK@oguc>
         <f184764a283bdf3694478fa35ad41d2b3ec38850.camel@sipsolutions.net>
         <20210118085311.GA2735@lst.de>
Content-Type: text/plain; charset="UTF-8"
User-Agent: Evolution 3.36.5 (3.36.5-2.fc32) 
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Precedence: bulk

On Mon, 2021-01-18 at 09:53 +0100, Christoph Hellwig wrote:
> On Sat, Jan 16, 2021 at 05:46:33PM +0100, Johannes Berg wrote:
> > > For my case, I attempted to instead implement splice_write and
> > > splice_read in tty_fops; I managed to get splice_write working calling
> > > ld->ops->write, but splice_read is not so simple because the
> > > tty_ldisc_ops read method expects a userspace buffer. So I cannot see
> > > how to implement this without either (a) using set_fs, or (b)
> > > implementing iter ops on all line disciplines.
> > > 
> > > Is splice()ing between a tty and a pipe worth supporting at all? Not a
> > > big deal for my use case at least, but it used to work.
> > 
> > Is it even strictly related to the tty?
> > 
> > I was just now looking into why my cgit/fcgi/nginx setup no longer
> > works, and the reason is getting -EINVAL from sendfile() when the input
> > is a file and the output is a pipe().
> 
> Yes, pipes do not support ->splice_write currenly.

Well, it clearly used to work :-)

>    I think just wiring
> up iter_file_splice_write would work.  Al?

Seems to work for the simple test case that I had, at least:

diff --git a/fs/pipe.c b/fs/pipe.c
index c5989cfd564d..39c96845a72f 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -1206,6 +1206,7 @@ const struct file_operations pipefifo_fops = {
 	.unlocked_ioctl	= pipe_ioctl,
 	.release	= pipe_release,
 	.fasync		= pipe_fasync,
+	.splice_write	= iter_file_splice_write,
 };
 
 /*

johannes