linux-kernel - Re: [PATCH 2/2] debugfs: Make automount point inodes permanently empty

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Date:	Wed, 9 Mar 2016 14:32:24 -0600
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Seth Forshee <seth.forshee@...onical.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 2/2] debugfs: Make automount point inodes permanently
 empty

Quoting Seth Forshee (seth.forshee@...onical.com):
> Starting with 4.1 the tracing subsystem has its own filesystem
> which is automounted in the tracing subdirectory of debugfs.
> Prior to this debugfs could be bind mounted in a cloned mount
> namespace, but if tracefs has been mounted under debugfs this
> now fails because there is a locked child mount. This creates
> a regression for container software which bind mounts debugfs
> to satisfy the assumption of some userspace software.
> 
> In other pseudo filesystems such as proc and sysfs we're already
> creating mountpoints like this in such a way that no dirents can
> be created in the directories, allowing them to be exceptions to
> some MNT_LOCKED tests. In fact we're already do this for the
> tracefs mountpoint in sysfs.
> 
> Do the same in debugfs_create_automount(), since the intention
> here is clearly to create a mountpoint. This fixes the regression,
> as locked child mounts on permanently empty directories do not
> cause a bind mount to fail.
> 
> Cc: stable@...r.kernel.org # v4.1+
> Signed-off-by: Seth Forshee <seth.forshee@...onical.com>

Acked-by: Serge Hallyn <serge.hallyn@...onical.com>

> ---
>  fs/debugfs/inode.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
> index bece948b363d..8580831ed237 100644
> --- a/fs/debugfs/inode.c
> +++ b/fs/debugfs/inode.c
> @@ -457,7 +457,7 @@ struct dentry *debugfs_create_automount(const char *name,
>  	if (unlikely(!inode))
>  		return failed_creating(dentry);
>  
> -	inode->i_mode = S_IFDIR | S_IRWXU | S_IRUGO | S_IXUGO;
> +	make_empty_dir_inode(inode);
>  	inode->i_flags |= S_AUTOMOUNT;
>  	inode->i_private = data;
>  	dentry->d_fsdata = (void *)f;
> -- 
> 1.9.1