linux-kernel - Re: [RFC][PATCH 1/3] vfs: Keep a list of mounts on a mount point

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Message-ID: <20131008134453.GG14242@tucsk.piliscsaba.szeredi.hu>
Date:	Tue, 8 Oct 2013 15:44:53 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Rob Landley <rob@...dley.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC][PATCH 1/3] vfs: Keep a list of mounts on a mount point

On Fri, Oct 04, 2013 at 03:42:37PM -0700, Eric W. Biederman wrote:
> 
> Signed-off-by: Eric W. Biederman <ebiederman@...tter.com>
> ---
>  fs/mount.h     |    2 ++
>  fs/namespace.c |    5 +++++
>  2 files changed, 7 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/mount.h b/fs/mount.h
> index 64a858143ff9..e4342b8dfab1 100644
> --- a/fs/mount.h
> +++ b/fs/mount.h
> @@ -21,6 +21,7 @@ struct mnt_pcp {
>  struct mountpoint {
>  	struct list_head m_hash;
>  	struct dentry *m_dentry;
> +	struct list_head m_list;
>  	int m_count;
>  };
>  
> @@ -47,6 +48,7 @@ struct mount {
>  	struct mount *mnt_master;	/* slave is on master->mnt_slave_list */
>  	struct mnt_namespace *mnt_ns;	/* containing namespace */
>  	struct mountpoint *mnt_mp;	/* where is it mounted */
> +	struct list_head mnt_mp_list;	/* list mounts with the same mountpoint */
>  #ifdef CONFIG_FSNOTIFY
>  	struct hlist_head mnt_fsnotify_marks;
>  	__u32 mnt_fsnotify_mask;
> diff --git a/fs/namespace.c b/fs/namespace.c
> index da5c49483430..d092964fe7f9 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -197,6 +197,7 @@ static struct mount *alloc_vfsmnt(const char *name)
>  		INIT_LIST_HEAD(&mnt->mnt_share);
>  		INIT_LIST_HEAD(&mnt->mnt_slave_list);
>  		INIT_LIST_HEAD(&mnt->mnt_slave);
> +		INIT_LIST_HEAD(&mnt->mnt_mp_list);
>  #ifdef CONFIG_FSNOTIFY
>  		INIT_HLIST_HEAD(&mnt->mnt_fsnotify_marks);
>  #endif
> @@ -636,6 +637,7 @@ static struct mountpoint *new_mountpoint(struct dentry *dentry)
>  	mp->m_dentry = dentry;
>  	mp->m_count = 1;
>  	list_add(&mp->m_hash, chain);
> +	INIT_LIST_HEAD(&mp->m_list);
>  	return mp;
>  }
>  
> @@ -691,6 +693,7 @@ static void detach_mnt(struct mount *mnt, struct path *old_path)
>  	list_del_init(&mnt->mnt_hash);
>  	put_mountpoint(mnt->mnt_mp);
>  	mnt->mnt_mp = NULL;
> +	list_del_init(&mnt->mnt_mp_list);

Should be done *before* put_mountpoint(), for obvious reasons.

And a BUG_ON(!list_empty(&mp->m_list)) in put_mountpoint() for good measure (and
no, WARN_ON() is not better here, since use-after-free is definitely worse than
a BUG).

>  }
>  
>  /*
> @@ -705,6 +708,7 @@ void mnt_set_mountpoint(struct mount *mnt,
>  	child_mnt->mnt_mountpoint = dget(mp->m_dentry);
>  	child_mnt->mnt_parent = mnt;
>  	child_mnt->mnt_mp = mp;
> +	list_add_tail(&child_mnt->mnt_mp_list, &mp->m_list);
>  }
>  
>  /*
> @@ -1193,6 +1197,7 @@ void umount_tree(struct mount *mnt, int propagate)
>  			p->mnt_parent->mnt_ghosts++;
>  			put_mountpoint(p->mnt_mp);
>  			p->mnt_mp = NULL;
> +			list_del_init(&mnt->mnt_mp_list);

Ditto.

>  		}
>  		change_mnt_propagation(p, MS_PRIVATE);
>  	}
> -- 
> 1.7.5.4
> 
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