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

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Message-ID: <YJWjpPoJYK6IeF7C@zeniv-ca.linux.org.uk>
Date:   Fri, 7 May 2021 20:31:32 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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 07, 2021 at 12:29:44PM -0700, Linus Torvalds wrote:
> 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.

Supposed to - sure, but I'd like to verify that they actually do work
that way before we go there.  Let me RTFS a bit...