Re: Splicing to/from a tty

[Johannes Berg]

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;
}