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SubjectRe: 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---

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