linux-kernel - Re: splice() from /dev/zero to a pipe does not work (5.9+)

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Date:   Fri, 7 May 2021 12:29:44 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Kees Cook <keescook@...omium.org>,
        Colin Ian King <colin.king@...onical.com>,
        Christoph Hellwig <hch@....de>,
        Johannes Berg <johannes@...solutions.net>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: splice() from /dev/zero to a pipe does not work (5.9+)

On Fri, May 7, 2021 at 12:17 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> Umm...  That would do wonders to anything that used to do
> copy_to_user()/clear_user()/copy_to_user() and got converted
> to copy_to_iter()/iov_iter_zero()/copy_to_iter()...

I didn't mean for iov_iter_zero doing this - only splice_read_zero().

> Are you sure we can shove zero page into pipe, anyway?
> IIRC, get_page()/put_page() on that is not allowed,

That's what the

    buf->ops = &zero_pipe_buf_ops;

is for. The zero_pipe_buf_ops would have empty get and release
functions, and a 'steal' function that always returns false.

That's how the pipe pages are supposed to work: there are people who
put non-page data (ie things like skbuff allocations etc) into a
splice pipe buffer. It's why we have those "ops" pointers.

              Linus