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

[Ian Kent]

On Wed, 2019-07-03 at 09:24 +0800, Ian Kent wrote:
> On Wed, 2019-07-03 at 09:09 +0800, Ian Kent wrote:
> > Hi Christian,
> > 
> > About the propagation attributes you mentioned ...
> 
> Umm ... how did you work out if a mount is unbindable from proc
> mountinfo?
> 
> I didn't notice anything that could be used for that when I was
> looking at this.

Oh wait, fs/proc_namespace.c:show_mountinfo() has:
        if (IS_MNT_UNBINDABLE(r))
                seq_puts(m, " unbindable");

I missed that, probably because I didn't have any unbindable mounts
at the time I was looking at it, oops!

That's missing and probably should be added too.

> 
> > 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