From: "Oliver Giles" <ohw.giles@gmail.com>
To: "Johannes Berg" <johannes@sipsolutions.net>,
<linux-kernel@vger.kernel.org>
Cc: "Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
"Christoph Hellwig" <hch@lst.de>
Subject: Re: Splicing to/from a tty
Date: Sun, 17 Jan 2021 19:12:17 +1300 [thread overview]
Message-ID: <C8L7LWQDH7O9.1GHEPFD33W15X@oguc> (raw)
In-Reply-To: <f184764a283bdf3694478fa35ad41d2b3ec38850.camel@sipsolutions.net>
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?
next prev parent reply other threads:[~2021-01-17 6:14 UTC|newest]
Thread overview: 65+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-16 7:35 Splicing to/from a tty Oliver Giles
2021-01-16 16:46 ` Johannes Berg
2021-01-17 6:12 ` Oliver Giles [this message]
2021-01-18 8:53 ` Christoph Hellwig
2021-01-18 8:58 ` Johannes Berg
2021-01-18 19:26 ` Linus Torvalds
2021-01-18 19:45 ` Al Viro
2021-01-18 19:49 ` Linus Torvalds
2021-01-18 19:56 ` Al Viro
2021-01-24 19:11 ` Linus Torvalds
2021-01-25 9:16 ` [PATCH] fs/pipe: allow sendfile() to pipe again Johannes Berg
2021-01-25 10:16 ` Christoph Hellwig
2021-01-25 20:34 ` Linus Torvalds
2021-01-26 6:07 ` Splicing to/from a tty Al Viro
2021-01-26 6:08 ` [PATCH 1/3] do_splice_to(): move the logics for limiting the read length in Al Viro
2021-01-26 6:09 ` [PATCH 2/3] take the guts of file-to-pipe splice into a helper function Al Viro
2021-01-26 6:09 ` [PATCH 3/3] teach sendfile(2) to handle send-to-pipe directly Al Viro
2021-01-26 18:57 ` Linus Torvalds
2021-01-26 19:33 ` Al Viro
2021-01-26 18:49 ` Splicing to/from a tty Linus Torvalds
2021-01-26 19:42 ` Al Viro
2021-01-18 19:34 ` Al Viro
2021-01-18 19:46 ` Linus Torvalds
2021-01-18 19:54 ` Al Viro
2021-01-20 16:26 ` Al Viro
2021-01-20 19:11 ` Al Viro
2021-01-20 19:27 ` Linus Torvalds
2021-01-20 22:25 ` David Laight
2021-01-20 23:02 ` Al Viro
2021-01-20 23:14 ` Al Viro
2021-01-20 23:40 ` Linus Torvalds
2021-01-21 0:38 ` Al Viro
2021-01-21 1:04 ` Linus Torvalds
2021-01-21 1:45 ` Al Viro
2021-01-21 3:38 ` Linus Torvalds
2021-01-21 6:05 ` Willy Tarreau
2021-01-21 8:04 ` Johannes Berg
2021-01-21 10:08 ` David Laight
2021-01-18 8:16 ` Christoph Hellwig
2021-01-18 19:36 ` Linus Torvalds
2021-01-18 20:24 ` Linus Torvalds
2021-01-18 21:35 ` Linus Torvalds
2021-01-18 21:54 ` Linus Torvalds
2021-01-18 22:03 ` Linus Torvalds
2021-01-18 22:20 ` Linus Torvalds
2021-01-19 1:38 ` Linus Torvalds
2021-01-19 11:53 ` Greg Kroah-Hartman
2021-01-19 16:56 ` Robert Karszniewicz
2021-01-19 17:10 ` Robert Karszniewicz
2021-01-19 22:09 ` Oliver Giles
2021-01-19 17:25 ` Linus Torvalds
2021-01-19 20:24 ` Linus Torvalds
2021-01-19 20:38 ` Christoph Hellwig
2021-01-20 1:25 ` Oliver Giles
2021-01-20 4:44 ` Linus Torvalds
2021-01-20 8:15 ` Oliver Giles
2021-01-21 1:18 ` tty splice branch (was "Re: Splicing to/from a tty") Linus Torvalds
2021-01-21 8:44 ` Greg Kroah-Hartman
2021-01-21 8:50 ` Jiri Slaby
2021-01-21 8:58 ` Jiri Slaby
2021-01-21 17:52 ` Linus Torvalds
2021-01-21 8:58 ` Greg Kroah-Hartman
2021-01-21 17:03 ` Splicing to/from a tty Robert Karszniewicz
2021-01-21 18:43 ` Linus Torvalds
2021-01-19 11:52 ` Greg Kroah-Hartman
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