Subject: Re: [PATCH v2] fail dentry revalidation after namespace change

From: [email-masked] (Eric W. Biederman)
Date: Fri, 06 Jul 2012 02:37:00 -0700

Glauber Costa <[email-masked]> writes:

> When we change the namespace tag of a sysfs entry, the associated dentry
> is still kept around. readdir() will work correctly and not display the
> old entries, but open() will still succeed, so will reads and writes.

Note reads and writes of file handles open before the move should
continue to work.

> This will no longer happen if sysfs is remounted, hinting that this is a
> cache-related problem.
>
> I am using the following sequence to demonstrate that:
>
> shell1:
> ip link add type veth
> unshare -nm
>
> shell2:
> ip link set veth1 <pid_of_shell_1>
> cat /sys/devices/virtual/net/veth1/ifindex
>
> Before that patch, this will succeed (fail to fail). After it, it will
> correctly return an error. Differently from a normal rename, which we
> handle fine, changing the object namespace will keep it's path intact.
> So this check seems necessary as well.
>
> [ v2: get type from parent, as suggested by Eric Biederman ]

Reviewed-by: "Eric W. Biederman" <[email-masked]>

> Signed-off-by: Glauber Costa <[email-masked]>
> CC: Tejun Heo <[email-masked]>
> CC: Eric W. Biederman <[email-masked]>
> CC: Greg Kroah-Hartman <[email-masked]>
> ---
>  fs/sysfs/dir.c |    8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/fs/sysfs/dir.c b/fs/sysfs/dir.c
> index e6bb9b2..c0bf38a 100644
> --- a/fs/sysfs/dir.c
> +++ b/fs/sysfs/dir.c
> @@ -307,6 +307,7 @@ static int sysfs_dentry_revalidate(struct dentry *dentry, struct nameidata *nd)
>  {
>  	struct sysfs_dirent *sd;
>  	int is_dir;
> +	int type;
>  
>  	if (nd->flags & LOOKUP_RCU)
>  		return -ECHILD;
> @@ -326,6 +327,13 @@ static int sysfs_dentry_revalidate(struct dentry *dentry, struct nameidata *nd)
>  	if (strcmp(dentry->d_name.name, sd->s_name) != 0)
>  		goto out_bad;
>  
> +	/* The sysfs dirent has been moved to a different namespace */
> +	type = KOBJ_NS_TYPE_NONE;
> +	if (sd->s_parent)
> +		type = sysfs_ns_type(sd->s_parent);
> +	if (type && (sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns))
> +		goto out_bad;
> +
>  	mutex_unlock(&sysfs_mutex);
>  out_valid:
>  	return 1;