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

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Message-Id: <C8L7LWQDH7O9.1GHEPFD33W15X@oguc>
Date:   Sun, 17 Jan 2021 19:12:17 +1300
From:   "Oliver Giles" <ohw.giles@...il.com>
To:     "Johannes Berg" <johannes@...solutions.net>,
        <linux-kernel@...r.kernel.org>
Cc:     "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "Christoph Hellwig" <hch@....de>
Subject: Re: Splicing to/from a tty

On Sun Jan 17, 2021 at 5:46 AM NZDT, Johannes Berg wrote:
> On Sat, 2021-01-16 at 20:35 +1300, Oliver Giles wrote:
> > Commit 36e2c7421f02 (fs: don't allow splice read/write without
> > explicit ops) broke my userspace application which talks to an SSL VPN
> > by splice()ing between "openssl s_client" and "pppd". The latter
> > operates over a pty, and since that commit there is no fallback for
> > splice()ing between a pipe and a pty, or any tty for that matter.
> > 
> > The above commit mentions switching them to the iter ops and using
> > generic_file_splice_read. IIUC, this would require implementing iter
> > ops also on the line disciplines, which sounds pretty disruptive.
> > 
> > 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().
>
> 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;
> }

I can confirm the behaviour you see, and that it starts occurring from the same
commit 36e2c7421f02a22 (fs: don't allow splice read/write without explicit ops).

In my tty case, it is clear that removing the default fallback would cause this
to fail, but assuming the sendfile() source is on a regular filesystem, I am
unsure why splice cannot find the appropriate splice_write method. Could be
connected to the fact that the message from warn_unsupported in fs/splice.c
outputs "splice write not supported for file  (pid: 983819 comm: test-sendfile)",
i.e. the file path is blank. In my case of directly calling splice on a pty, I
do see a path such as /ptyp0 in that error before implementing splice_(read/write)
callbacks. Maybe splice is getting a bogus file pointer from sendfile?