Re: [PATCH v2 00/12] make rpc_pipefs be mountable multiple time

[Kirill A. Shutemov]

 On Thu, Dec 30, 2010 at 06:52:43AM -0600, Rob Landley wrote:
> On 12/30/2010 05:45 AM, Kirill A. Shutemov wrote:
> > Currently, there is no association between rpc_pipefs and mount namespace,
> 
> There is in that the root context doesn't need to have this mounted, and 
> new namespaces do.  So there's an existing association between a LACK of 
> a namespace and a different default behavior.
>
> My understanding (correct me if I'm wrong) is that the historical 
> behavior is that there's only one, and it doesn't actually live anywhere 
> in the filesystem tree.  You're adding a special location.  I'm 
> wondering if there's any way for that location not to be special.

/var/lib/net/rpc_pipefs is default path where userspace part of NFS stack
(gssd, idmapd) want to see rpc_pipefs

> > so I don't see simple way to restrict number of rpc_pipefs per mount
> > namespace. Associating mount namespace with rpc_pipefs is not a good idea,
> > I think.
> 
> I'm talking about associating a default rpc_pipefs instance with a 
> namespace, which it seems to me you're already doing by emulating the 
> legacy behavior.  Before you CLONE_NEWNS you get a magic default mount 
> that doesn't exist in the tree.  After you CLONE_NEWNS you get something 
> like -EINVAL unless you supply your own default.

Root namespace is special. In case of nfsroot you need rpc_pipefs before
root available.

> (I'm actually not sure 
> why new namespaces don't fall back to the magic global one...)

It breaks isolation. Container should not use host's rpc_pipefs without
host's permission.
 
> I'm suggesting that if the user doesn't specify -o rpcmount then the 
> default could be the first rpc_pipefs mount visible to the current 
> process context, rather than a specific path.  Logic to do that exists 
> in the proc/self/mounts code (which I'm reading through now...).

static int check_rpc_pipefs(struct vfsmount *mnt, void *arg)
{
        struct vfsmount **rpcmount = arg;
        struct path path = {
                .mnt = mnt,
                .dentry = mnt->mnt_root,
        };

        if (!mnt->mnt_sb)
                return 0;
        if (mnt->mnt_sb->s_magic != RPCAUTH_GSSMAGIC)
                return 0;

        if (!path_is_under(&path, &current->fs->root))
                return 0;

        *rpcmount = mntget(mnt);
        return 1;
}

struct vfsmount *get_rpc_pipefs(const char *p)
{
        int error;
        struct vfsmount *rpcmount = ERR_PTR(-EINVAL);
        struct path path;

        if (!p) {
                iterate_mounts(check_rpc_pipefs, &rpcmount,
                                current->nsproxy->mnt_ns->root);

                if (IS_ERR(rpcmount) && (current->nsproxy->mnt_ns ==
                                        init_task.nsproxy->mnt_ns))
                        return mntget(init_rpc_pipefs);

                return rpcmount;
        }

        error = kern_path(p, LOOKUP_FOLLOW | LOOKUP_DIRECTORY, &path);
        if (error)
                return ERR_PTR(error);

        check_rpc_pipefs(path.mnt, &rpcmount);
        path_put(&path);

        return rpcmount;
}
EXPORT_SYMBOL_GPL(get_rpc_pipefs);

Something like this? Patch to replace patch #10 attached.

-- 
 Kirill A. Shutemov