linux-kernel - Re: [PATCH 4/6] vfs: Allow mount information to be queried by fsinfo() [ver #15]

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Date:   Wed, 03 Jul 2019 09:09:19 +0800
From:   Ian Kent <raven@...maw.net>
To:     christian@...uner.io, David Howells <dhowells@...hat.com>,
        viro@...iv.linux.org.uk
Cc:     mszeredi@...hat.com, linux-api@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] vfs: Allow mount information to be queried by
 fsinfo() [ver #15]

Hi Christian,

About the propagation attributes you mentioned ...

On Fri, 2019-06-28 at 16:47 +0100, David Howells wrote:

snip ...

> +
> +#ifdef CONFIG_FSINFO
> +int fsinfo_generic_mount_info(struct path *path, struct fsinfo_kparams
> *params)
> +{
> +	struct fsinfo_mount_info *p = params->buffer;
> +	struct super_block *sb;
> +	struct mount *m;
> +	struct path root;
> +	unsigned int flags;
> +
> +	if (!path->mnt)
> +		return -ENODATA;
> +
> +	m = real_mount(path->mnt);
> +	sb = m->mnt.mnt_sb;
> +
> +	p->f_sb_id		= sb->s_unique_id;
> +	p->mnt_id		= m->mnt_id;
> +	p->parent_id		= m->mnt_parent->mnt_id;
> +	p->change_counter	= atomic_read(&m->mnt_change_counter);
> +
> +	get_fs_root(current->fs, &root);
> +	if (path->mnt == root.mnt) {
> +		p->parent_id = p->mnt_id;
> +	} else {
> +		rcu_read_lock();
> +		if (!are_paths_connected(&root, path))
> +			p->parent_id = p->mnt_id;
> +		rcu_read_unlock();
> +	}
> +	if (IS_MNT_SHARED(m))
> +		p->group_id = m->mnt_group_id;
> +	if (IS_MNT_SLAVE(m)) {
> +		int master = m->mnt_master->mnt_group_id;
> +		int dom = get_dominating_id(m, &root);
> +		p->master_id = master;
> +		if (dom && dom != master)
> +			p->from_id = dom;

This provides information about mount propagation (well mostly).

My understanding of this was that:
"If a mount is propagation private (or slave) the group_id will
be zero otherwise it's propagation shared and it's group id will
be non-zero.

If a mount is propagation slave and propagation peers exist then
the mount field mnt_master will be non-NULL. Then mnt_master
(slave's master) can be used to set master_id. If the group id
of the propagation source is not that of the master then set
the from_id group as well."

This parallels the way in which these values are reported in
the proc pseudo file system.

Perhaps adding flags as well as setting the fields would be
useful too, since interpreting the meaning of the structure
fields isn't obvious, ;)

David, Al, thoughts?

Ian