linux-kernel - Re: [RFC][PATCH] fix short copy handling in copy_mc_pipe_to

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Date:   Tue, 14 Jun 2022 07:36:19 +0100
From:   David Howells <dhowells@...hat.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     dhowells@...hat.com,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        nvdimm@...ts.linux.dev
Subject: Re: [RFC][PATCH] fix short copy handling in copy_mc_pipe_to_iter()

Al Viro <viro@...iv.linux.org.uk> wrote:

> What's wrong with
>         p_occupancy = pipe_occupancy(head, tail);
>         if (p_occupancy >= pipe->max_usage)
>                 return 0;
> 	else
> 		return pipe->max_usage - p_occupancy;

Because "pipe->max_usage - p_occupancy" can be negative.

post_one_notification() is limited by pipe->ring_size, not pipe->max_usage.

The idea is to allow some slack in a watch pipe for the watch_queue code to
use that userspace can't.

David