linux-kernel - Re: Splicing to/from a tty

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Date:   Mon, 18 Jan 2021 19:34:57 +0000
From:   Al Viro <viro@...iv-ca>
To:     Christoph Hellwig <hch@....de>
Cc:     Johannes Berg <johannes@...solutions.net>,
        Oliver Giles <ohw.giles@...il.com>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: Splicing to/from a tty

On Mon, Jan 18, 2021 at 09:53:11AM +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.   I think just wiring
> up iter_file_splice_write would work.  Al?

I'd rather have sendfile(2) do what splice(2) does and handle pipes
directly.  Let me take a look,,,