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

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Date:   Mon, 18 Jan 2021 09:53:11 +0100
From:   Christoph Hellwig <hch@....de>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     Oliver Giles <ohw.giles@...il.com>, linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Christoph Hellwig <hch@....de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: Splicing to/from a tty

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?

> So I wrote a simple test program (below) and that errors out on kernel
> 5.10.4, while it works fine on the 5.9.16 I currently have. Haven't
> tried reverting anything yet, but now that I haev a test program it
> should be simple to even bisect.
> 
> johannes
> 
> 
> #include <unistd.h>
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> #include <sys/sendfile.h>
> #include <stdio.h>
> #include <assert.h>
> 
> int main(int argc, char **argv)
> {
> 	int in = open(argv[0], O_RDONLY);
> 	int p[2], out;
> 	off_t off = 0;
> 	int err;
> 
> 	assert(in >= 0);
> 	assert(pipe(p) >= 0);
> 	out = p[1];
> 	err = sendfile(out, in, &off, 1024);
> 	if (err < 0)
> 		perror("sendfile");
> 	assert(err == 1024);
> 
> 	return 0;
> }
> 
---end quoted text---