Merge sys_clone()/sys_unshare() nsproxy and namespace handling

sys_clone() and sys_unshare() both makes copies of nsproxy and its associated
namespaces.  But they have different code paths.

This patch merges all the nsproxy and its associated namespace copy/clone
handling (as much as possible).  Posted on container list earlier for
feedback.

- Create a new nsproxy and its associated namespaces and pass it back to
  caller to attach it to right process.

- Changed all copy_*_ns() routines to return a new copy of namespace
  instead of attaching it to task->nsproxy.

- Moved the CAP_SYS_ADMIN checks out of copy_*_ns() routines.

- Removed unnessary !ns checks from copy_*_ns() and added BUG_ON()
  just incase.

- Get rid of all individual unshare_*_ns() routines and make use of
  copy_*_ns() instead.

[akpm@osdl.org: cleanups, warning fix]
[clg@fr.ibm.com: remove dup_namespaces() declaration]
[serue@us.ibm.com: fix CONFIG_IPC_NS=n, clone(CLONE_NEWIPC) retval]
[akpm@linux-foundation.org: fix build with CONFIG_SYSVIPC=n]
Signed-off-by: Badari Pulavarty <pbadari@us.ibm.com>
Signed-off-by: Serge Hallyn <serue@us.ibm.com>
Cc: Cedric Le Goater <clg@fr.ibm.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: <containers@lists.osdl.org>
Signed-off-by: Cedric Le Goater <clg@fr.ibm.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/namespace.c

@@ -1441,10 +1441,9 @@ dput_out:
  * Allocate a new namespace structure and populate it with contents
  * copied from the namespace of the passed in task structure.
  */
-struct mnt_namespace *dup_mnt_ns(struct task_struct *tsk,
+static struct mnt_namespace *dup_mnt_ns(struct mnt_namespace *mnt_ns,
 		struct fs_struct *fs)
 {
-	struct mnt_namespace *mnt_ns = tsk->nsproxy->mnt_ns;
 	struct mnt_namespace *new_ns;
 	struct vfsmount *rootmnt = NULL, *pwdmnt = NULL, *altrootmnt = NULL;
 	struct vfsmount *p, *q;
@@ -1509,36 +1508,21 @@ struct mnt_namespace *dup_mnt_ns(struct task_struct *tsk,
 	return new_ns;
 }
 
-int copy_mnt_ns(int flags, struct task_struct *tsk)
+struct mnt_namespace *copy_mnt_ns(int flags, struct mnt_namespace *ns,
+		struct fs_struct *new_fs)
 {
-	struct mnt_namespace *ns = tsk->nsproxy->mnt_ns;
 	struct mnt_namespace *new_ns;
-	int err = 0;
-
-	if (!ns)
-		return 0;
 
+	BUG_ON(!ns);
 	get_mnt_ns(ns);
 
 	if (!(flags & CLONE_NEWNS))
-		return 0;
-
-	if (!capable(CAP_SYS_ADMIN)) {
-		err = -EPERM;
-		goto out;
-	}
-
-	new_ns = dup_mnt_ns(tsk, tsk->fs);
-	if (!new_ns) {
-		err = -ENOMEM;
-		goto out;
-	}
+		return ns;
 
-	tsk->nsproxy->mnt_ns = new_ns;
+	new_ns = dup_mnt_ns(ns, new_fs);
 
-out:
 	put_mnt_ns(ns);
-	return err;
+	return new_ns;
 }
 
 asmlinkage long sys_mount(char __user * dev_name, char __user * dir_name,

include/linux/ipc.h

@@ -92,16 +92,19 @@ extern struct ipc_namespace init_ipc_ns;
 
 #ifdef CONFIG_SYSVIPC
 #define INIT_IPC_NS(ns)		.ns		= &init_ipc_ns,
-extern int copy_ipcs(unsigned long flags, struct task_struct *tsk);
+extern struct ipc_namespace *copy_ipcs(unsigned long flags,
+						struct ipc_namespace *ns);
 #else
 #define INIT_IPC_NS(ns)
-static inline int copy_ipcs(unsigned long flags, struct task_struct *tsk)
-{ return 0; }
+static inline struct ipc_namespace *copy_ipcs(unsigned long flags,
+						struct ipc_namespace *ns)
+{
+	return ns;
+}
 #endif
 
 #ifdef CONFIG_IPC_NS
 extern void free_ipc_ns(struct kref *kref);
-extern int unshare_ipcs(unsigned long flags, struct ipc_namespace **ns);
 #endif
 
 static inline struct ipc_namespace *get_ipc_ns(struct ipc_namespace *ns)

include/linux/mnt_namespace.h

@@ -14,10 +14,9 @@ struct mnt_namespace {
 	int event;
 };
 
-extern int copy_mnt_ns(int, struct task_struct *);
-extern void __put_mnt_ns(struct mnt_namespace *ns);
-extern struct mnt_namespace *dup_mnt_ns(struct task_struct *,
+extern struct mnt_namespace *copy_mnt_ns(int, struct mnt_namespace *,
 		struct fs_struct *);
+extern void __put_mnt_ns(struct mnt_namespace *ns);
 
 static inline void put_mnt_ns(struct mnt_namespace *ns)
 {

include/linux/nsproxy.h

@@ -31,10 +31,11 @@ struct nsproxy {
 };
 extern struct nsproxy init_nsproxy;
 
-struct nsproxy *dup_namespaces(struct nsproxy *orig);
 int copy_namespaces(int flags, struct task_struct *tsk);
 void get_task_namespaces(struct task_struct *tsk);
 void free_nsproxy(struct nsproxy *ns);
+int unshare_nsproxy_namespaces(unsigned long, struct nsproxy **,
+	struct fs_struct *);
 
 static inline void put_nsproxy(struct nsproxy *ns)
 {

include/linux/pid_namespace.h

@@ -29,7 +29,7 @@ static inline void get_pid_ns(struct pid_namespace *ns)
 	kref_get(&ns->kref);
 }
 
-extern int copy_pid_ns(int flags, struct task_struct *tsk);
+extern struct pid_namespace *copy_pid_ns(int flags, struct pid_namespace *ns);
 extern void free_pid_ns(struct kref *kref);
 
 static inline void put_pid_ns(struct pid_namespace *ns)

include/linux/utsname.h

@@ -49,9 +49,7 @@ static inline void get_uts_ns(struct uts_namespace *ns)
 }
 
 #ifdef CONFIG_UTS_NS
-extern int unshare_utsname(unsigned long unshare_flags,
-				struct uts_namespace **new_uts);
-extern int copy_utsname(int flags, struct task_struct *tsk);
+extern struct uts_namespace *copy_utsname(int flags, struct uts_namespace *ns);
 extern void free_uts_ns(struct kref *kref);
 
 static inline void put_uts_ns(struct uts_namespace *ns)
@@ -59,21 +57,12 @@ static inline void put_uts_ns(struct uts_namespace *ns)
 	kref_put(&ns->kref, free_uts_ns);
 }
 #else
-static inline int unshare_utsname(unsigned long unshare_flags,
-			struct uts_namespace **new_uts)
+static inline struct uts_namespace *copy_utsname(int flags,
+						struct uts_namespace *ns)
 {
-	if (unshare_flags & CLONE_NEWUTS)
-		return -EINVAL;
-
-	return 0;
+	return ns;
 }
 
-static inline int copy_utsname(int flags, struct task_struct *tsk)
-{
-	if (flags & CLONE_NEWUTS)
-		return -EINVAL;
-	return 0;
-}
 static inline void put_uts_ns(struct uts_namespace *ns)
 {
 }

ipc/util.c

@@ -85,53 +85,20 @@ err_mem:
 	return ERR_PTR(err);
 }
 
-int unshare_ipcs(unsigned long unshare_flags, struct ipc_namespace **new_ipc)
+struct ipc_namespace *copy_ipcs(unsigned long flags, struct ipc_namespace *ns)
 {
-	struct ipc_namespace *new;
-
-	if (unshare_flags & CLONE_NEWIPC) {
-		if (!capable(CAP_SYS_ADMIN))
-			return -EPERM;
-
-		new = clone_ipc_ns(current->nsproxy->ipc_ns);
-		if (IS_ERR(new))
-			return PTR_ERR(new);
-
-		*new_ipc = new;
-	}
-
-	return 0;
-}
-
-int copy_ipcs(unsigned long flags, struct task_struct *tsk)
-{
-	struct ipc_namespace *old_ns = tsk->nsproxy->ipc_ns;
 	struct ipc_namespace *new_ns;
-	int err = 0;
 
-	if (!old_ns)
-		return 0;
-
-	get_ipc_ns(old_ns);
+	BUG_ON(!ns);
+	get_ipc_ns(ns);
 
 	if (!(flags & CLONE_NEWIPC))
-		return 0;
+		return ns;
 
-	if (!capable(CAP_SYS_ADMIN)) {
-		err = -EPERM;
-		goto out;
-	}
+	new_ns = clone_ipc_ns(ns);
 
-	new_ns = clone_ipc_ns(old_ns);
-	if (!new_ns) {
-		err = -ENOMEM;
-		goto out;
-	}
-
-	tsk->nsproxy->ipc_ns = new_ns;
-out:
-	put_ipc_ns(old_ns);
-	return err;
+	put_ipc_ns(ns);
+	return new_ns;
 }
 
 void free_ipc_ns(struct kref *kref)
@@ -145,11 +112,11 @@ void free_ipc_ns(struct kref *kref)
 	kfree(ns);
 }
 #else
-int copy_ipcs(unsigned long flags, struct task_struct *tsk)
+struct ipc_namespace *copy_ipcs(unsigned long flags, struct ipc_namespace *ns)
 {
 	if (flags & CLONE_NEWIPC)
-		return -EINVAL;
-	return 0;
+		return ERR_PTR(-EINVAL);
+	return ns;
 }
 #endif
 

kernel/fork.c

@@ -1515,26 +1515,6 @@ static int unshare_fs(unsigned long unshare_flags, struct fs_struct **new_fsp)
 	return 0;
 }
 
-/*
- * Unshare the mnt_namespace structure if it is being shared
- */
-static int unshare_mnt_namespace(unsigned long unshare_flags,
-		struct mnt_namespace **new_nsp, struct fs_struct *new_fs)
-{
-	struct mnt_namespace *ns = current->nsproxy->mnt_ns;
-
-	if ((unshare_flags & CLONE_NEWNS) && ns) {
-		if (!capable(CAP_SYS_ADMIN))
-			return -EPERM;
-
-		*new_nsp = dup_mnt_ns(current, new_fs ? new_fs : current->fs);
-		if (!*new_nsp)
-			return -ENOMEM;
-	}
-
-	return 0;
-}
-
 /*
  * Unsharing of sighand is not supported yet
  */
@@ -1593,16 +1573,6 @@ static int unshare_semundo(unsigned long unshare_flags, struct sem_undo_list **n
 	return 0;
 }
 
-#ifndef CONFIG_IPC_NS
-static inline int unshare_ipcs(unsigned long flags, struct ipc_namespace **ns)
-{
-	if (flags & CLONE_NEWIPC)
-		return -EINVAL;
-
-	return 0;
-}
-#endif
-
 /*
  * unshare allows a process to 'unshare' part of the process
  * context which was originally shared using clone.  copy_*
@@ -1615,14 +1585,11 @@ asmlinkage long sys_unshare(unsigned long unshare_flags)
 {
 	int err = 0;
 	struct fs_struct *fs, *new_fs = NULL;
-	struct mnt_namespace *ns, *new_ns = NULL;
 	struct sighand_struct *new_sigh = NULL;
 	struct mm_struct *mm, *new_mm = NULL, *active_mm = NULL;
 	struct files_struct *fd, *new_fd = NULL;
 	struct sem_undo_list *new_ulist = NULL;
 	struct nsproxy *new_nsproxy = NULL, *old_nsproxy = NULL;
-	struct uts_namespace *uts, *new_uts = NULL;
-	struct ipc_namespace *ipc, *new_ipc = NULL;
 
 	check_unshare_flags(&unshare_flags);
 
@@ -1637,36 +1604,24 @@ asmlinkage long sys_unshare(unsigned long unshare_flags)
 		goto bad_unshare_out;
 	if ((err = unshare_fs(unshare_flags, &new_fs)))
 		goto bad_unshare_cleanup_thread;
-	if ((err = unshare_mnt_namespace(unshare_flags, &new_ns, new_fs)))
-		goto bad_unshare_cleanup_fs;
 	if ((err = unshare_sighand(unshare_flags, &new_sigh)))
-		goto bad_unshare_cleanup_ns;
+		goto bad_unshare_cleanup_fs;
 	if ((err = unshare_vm(unshare_flags, &new_mm)))
 		goto bad_unshare_cleanup_sigh;
 	if ((err = unshare_fd(unshare_flags, &new_fd)))
 		goto bad_unshare_cleanup_vm;
 	if ((err = unshare_semundo(unshare_flags, &new_ulist)))
 		goto bad_unshare_cleanup_fd;
-	if ((err = unshare_utsname(unshare_flags, &new_uts)))
+	if ((err = unshare_nsproxy_namespaces(unshare_flags, &new_nsproxy,
+			new_fs)))
 		goto bad_unshare_cleanup_semundo;
-	if ((err = unshare_ipcs(unshare_flags, &new_ipc)))
-		goto bad_unshare_cleanup_uts;
-
-	if (new_ns || new_uts || new_ipc) {
-		old_nsproxy = current->nsproxy;
-		new_nsproxy = dup_namespaces(old_nsproxy);
-		if (!new_nsproxy) {
-			err = -ENOMEM;
-			goto bad_unshare_cleanup_ipc;
-		}
-	}
 
-	if (new_fs || new_ns || new_mm || new_fd || new_ulist ||
-				new_uts || new_ipc) {
+	if (new_fs ||  new_mm || new_fd || new_ulist || new_nsproxy) {
 
 		task_lock(current);
 
 		if (new_nsproxy) {
+			old_nsproxy = current->nsproxy;
 			current->nsproxy = new_nsproxy;
 			new_nsproxy = old_nsproxy;
 		}
@@ -1677,12 +1632,6 @@ asmlinkage long sys_unshare(unsigned long unshare_flags)
 			new_fs = fs;
 		}
 
-		if (new_ns) {
-			ns = current->nsproxy->mnt_ns;
-			current->nsproxy->mnt_ns = new_ns;
-			new_ns = ns;
-		}
-
 		if (new_mm) {
 			mm = current->mm;
 			active_mm = current->active_mm;
@@ -1698,32 +1647,12 @@ asmlinkage long sys_unshare(unsigned long unshare_flags)
 			new_fd = fd;
 		}
 
-		if (new_uts) {
-			uts = current->nsproxy->uts_ns;
-			current->nsproxy->uts_ns = new_uts;
-			new_uts = uts;
-		}
-
-		if (new_ipc) {
-			ipc = current->nsproxy->ipc_ns;
-			current->nsproxy->ipc_ns = new_ipc;
-			new_ipc = ipc;
-		}
-
 		task_unlock(current);
 	}
 
 	if (new_nsproxy)
 		put_nsproxy(new_nsproxy);
 
-bad_unshare_cleanup_ipc:
-	if (new_ipc)
-		put_ipc_ns(new_ipc);
-
-bad_unshare_cleanup_uts:
-	if (new_uts)
-		put_uts_ns(new_uts);
-
 bad_unshare_cleanup_semundo:
 bad_unshare_cleanup_fd:
 	if (new_fd)
@@ -1738,10 +1667,6 @@ bad_unshare_cleanup_sigh:
 		if (atomic_dec_and_test(&new_sigh->count))
 			kmem_cache_free(sighand_cachep, new_sigh);
 
-bad_unshare_cleanup_ns:
-	if (new_ns)
-		put_mnt_ns(new_ns);
-
 bad_unshare_cleanup_fs:
 	if (new_fs)
 		put_fs_struct(new_fs);

kernel/nsproxy.c

@@ -38,10 +38,8 @@ void get_task_namespaces(struct task_struct *tsk)
 
 /*
  * creates a copy of "orig" with refcount 1.
- * This does not grab references to the contained namespaces,
- * so that needs to be done by dup_namespaces.
  */
-static inline struct nsproxy *clone_namespaces(struct nsproxy *orig)
+static inline struct nsproxy *clone_nsproxy(struct nsproxy *orig)
 {
 	struct nsproxy *ns;
 
@@ -52,26 +50,49 @@ static inline struct nsproxy *clone_namespaces(struct nsproxy *orig)
 }
 
 /*
- * copies the nsproxy, setting refcount to 1, and grabbing a
- * reference to all contained namespaces.  Called from
- * sys_unshare()
+ * Create new nsproxy and all of its the associated namespaces.
+ * Return the newly created nsproxy.  Do not attach this to the task,
+ * leave it to the caller to do proper locking and attach it to task.
  */
-struct nsproxy *dup_namespaces(struct nsproxy *orig)
+static struct nsproxy *create_new_namespaces(int flags, struct task_struct *tsk,
+			struct fs_struct *new_fs)
 {
-	struct nsproxy *ns = clone_namespaces(orig);
+	struct nsproxy *new_nsp;
 
-	if (ns) {
-		if (ns->mnt_ns)
-			get_mnt_ns(ns->mnt_ns);
-		if (ns->uts_ns)
-			get_uts_ns(ns->uts_ns);
-		if (ns->ipc_ns)
-			get_ipc_ns(ns->ipc_ns);
-		if (ns->pid_ns)
-			get_pid_ns(ns->pid_ns);
-	}
+	new_nsp = clone_nsproxy(tsk->nsproxy);
+	if (!new_nsp)
+		return ERR_PTR(-ENOMEM);
 
-	return ns;
+	new_nsp->mnt_ns = copy_mnt_ns(flags, tsk->nsproxy->mnt_ns, new_fs);
+	if (IS_ERR(new_nsp->mnt_ns))
+		goto out_ns;
+
+	new_nsp->uts_ns = copy_utsname(flags, tsk->nsproxy->uts_ns);
+	if (IS_ERR(new_nsp->uts_ns))
+		goto out_uts;
+
+	new_nsp->ipc_ns = copy_ipcs(flags, tsk->nsproxy->ipc_ns);
+	if (IS_ERR(new_nsp->ipc_ns))
+		goto out_ipc;
+
+	new_nsp->pid_ns = copy_pid_ns(flags, tsk->nsproxy->pid_ns);
+	if (IS_ERR(new_nsp->pid_ns))
+		goto out_pid;
+
+	return new_nsp;
+
+out_pid:
+	if (new_nsp->ipc_ns)
+		put_ipc_ns(new_nsp->ipc_ns);
+out_ipc:
+	if (new_nsp->uts_ns)
+		put_uts_ns(new_nsp->uts_ns);
+out_uts:
+	if (new_nsp->mnt_ns)
+		put_mnt_ns(new_nsp->mnt_ns);
+out_ns:
+	kfree(new_nsp);
+	return ERR_PTR(-ENOMEM);
 }
 
 /*
@@ -92,47 +113,21 @@ int copy_namespaces(int flags, struct task_struct *tsk)
 	if (!(flags & (CLONE_NEWNS | CLONE_NEWUTS | CLONE_NEWIPC)))
 		return 0;
 
-	new_ns = clone_namespaces(old_ns);
-	if (!new_ns) {
-		err = -ENOMEM;
+	if (!capable(CAP_SYS_ADMIN)) {
+		err = -EPERM;
 		goto out;
 	}
 
-	tsk->nsproxy = new_ns;
-
-	err = copy_mnt_ns(flags, tsk);
-	if (err)
-		goto out_ns;
-
-	err = copy_utsname(flags, tsk);
-	if (err)
-		goto out_uts;
-
-	err = copy_ipcs(flags, tsk);
-	if (err)
-		goto out_ipc;
-
-	err = copy_pid_ns(flags, tsk);
-	if (err)
-		goto out_pid;
+	new_ns = create_new_namespaces(flags, tsk, tsk->fs);
+	if (IS_ERR(new_ns)) {
+		err = PTR_ERR(new_ns);
+		goto out;
+	}
 
+	tsk->nsproxy = new_ns;
 out:
 	put_nsproxy(old_ns);
 	return err;
-
-out_pid:
-	if (new_ns->ipc_ns)
-		put_ipc_ns(new_ns->ipc_ns);
-out_ipc:
-	if (new_ns->uts_ns)
-		put_uts_ns(new_ns->uts_ns);
-out_uts:
-	if (new_ns->mnt_ns)
-		put_mnt_ns(new_ns->mnt_ns);
-out_ns:
-	tsk->nsproxy = old_ns;
-	kfree(new_ns);
-	goto out;
 }
 
 void free_nsproxy(struct nsproxy *ns)
@@ -147,3 +142,41 @@ void free_nsproxy(struct nsproxy *ns)
 		put_pid_ns(ns->pid_ns);
 	kfree(ns);
 }
+
+/*
+ * Called from unshare. Unshare all the namespaces part of nsproxy.
+ * On sucess, returns the new nsproxy and a reference to old nsproxy
+ * to make sure it stays around.
+ */
+int unshare_nsproxy_namespaces(unsigned long unshare_flags,
+		struct nsproxy **new_nsp, struct fs_struct *new_fs)
+{
+	struct nsproxy *old_ns = current->nsproxy;
+	int err = 0;
+
+	if (!(unshare_flags & (CLONE_NEWNS | CLONE_NEWUTS | CLONE_NEWIPC)))
+		return 0;
+
+#ifndef CONFIG_IPC_NS
+	if (unshare_flags & CLONE_NEWIPC)
+		return -EINVAL;
+#endif
+
+#ifndef CONFIG_UTS_NS
+	if (unshare_flags & CLONE_NEWUTS)
+		return -EINVAL;
+#endif
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	get_nsproxy(old_ns);
+
+	*new_nsp = create_new_namespaces(unshare_flags, current,
+				new_fs ? new_fs : current->fs);
+	if (IS_ERR(*new_nsp)) {
+		err = PTR_ERR(*new_nsp);
+		put_nsproxy(old_ns);
+	}
+	return err;
+}

kernel/pid.c

@@ -360,16 +360,11 @@ struct pid *find_ge_pid(int nr)
 }
 EXPORT_SYMBOL_GPL(find_get_pid);
 
-int copy_pid_ns(int flags, struct task_struct *tsk)
+struct pid_namespace *copy_pid_ns(int flags, struct pid_namespace *old_ns)
 {
-	struct pid_namespace *old_ns = tsk->nsproxy->pid_ns;
-	int err = 0;
-
-	if (!old_ns)
-		return 0;
-
+	BUG_ON(!old_ns);
 	get_pid_ns(old_ns);
-	return err;
+	return old_ns;
 }
 
 void free_pid_ns(struct kref *kref)

kernel/utsname.c

@@ -31,59 +31,26 @@ static struct uts_namespace *clone_uts_ns(struct uts_namespace *old_ns)
 	return ns;
 }
 
-/*
- * unshare the current process' utsname namespace.
- * called only in sys_unshare()
- */
-int unshare_utsname(unsigned long unshare_flags, struct uts_namespace **new_uts)
-{
-	if (unshare_flags & CLONE_NEWUTS) {
-		if (!capable(CAP_SYS_ADMIN))
-			return -EPERM;
-
-		*new_uts = clone_uts_ns(current->nsproxy->uts_ns);
-		if (!*new_uts)
-			return -ENOMEM;
-	}
-
-	return 0;
-}
-
 /*
  * Copy task tsk's utsname namespace, or clone it if flags
  * specifies CLONE_NEWUTS.  In latter case, changes to the
  * utsname of this process won't be seen by parent, and vice
  * versa.
  */
-int copy_utsname(int flags, struct task_struct *tsk)
+struct uts_namespace *copy_utsname(int flags, struct uts_namespace *old_ns)
 {
-	struct uts_namespace *old_ns = tsk->nsproxy->uts_ns;
 	struct uts_namespace *new_ns;
-	int err = 0;
-
-	if (!old_ns)
-		return 0;
 
+	BUG_ON(!old_ns);
 	get_uts_ns(old_ns);
 
 	if (!(flags & CLONE_NEWUTS))
-		return 0;
-
-	if (!capable(CAP_SYS_ADMIN)) {
-		err = -EPERM;
-		goto out;
-	}
+		return old_ns;
 
 	new_ns = clone_uts_ns(old_ns);
-	if (!new_ns) {
-		err = -ENOMEM;
-		goto out;
-	}
-	tsk->nsproxy->uts_ns = new_ns;
 
-out:
 	put_uts_ns(old_ns);
-	return err;
+	return new_ns;
 }
 
 void free_uts_ns(struct kref *kref)