linux-kernel - Re: [PATCH 2/6] Adjust watch_queue documentation to mention mount and superblock watches. [ver #5]

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Date:   Mon, 01 Jul 2019 09:52:50 +0100
From:   David Howells <dhowells@...hat.com>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     dhowells@...hat.com, viro@...iv.linux.org.uk,
        Casey Schaufler <casey@...aufler-ca.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        nicolas.dichtel@...nd.com, raven@...maw.net,
        Christian Brauner <christian@...uner.io>,
        keyrings@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] Adjust watch_queue documentation to mention mount and superblock watches. [ver #5]

Randy Dunlap <rdunlap@...radead.org> wrote:

> I'm having a little trouble parsing that sentence.
> Could you clarify it or maybe rewrite/modify it?
> Thanks.

How about:

  * ``info_filter`` and ``info_mask`` act as a filter on the info field of the
    notification record.  The notification is only written into the buffer if::

	(watch.info & info_mask) == info_filter

    This could be used, for example, to ignore events that are not exactly on
    the watched point in a mount tree by specifying NOTIFY_MOUNT_IN_SUBTREE
    must not be set, e.g.::

	{
		.type = WATCH_TYPE_MOUNT_NOTIFY,
		.info_filter = 0,
		.info_mask = NOTIFY_MOUNT_IN_SUBTREE,
		.subtype_filter = ...,
	}

    as an event would be only permissible with this filter if::

    	(watch.info & NOTIFY_MOUNT_IN_SUBTREE) == 0

David