Re: [PATCHv7 01/33] ns: Introduce Time Namespace

[Vincenzo Frascino]

Hi Andrei and Dmitry,

On 10/11/19 2:23 AM, Dmitry Safonov wrote:
> From: Andrei Vagin <avagin@xxxxxxxxxx>
> 
> Time Namespace isolates clock values.
> 
> The kernel provides access to several clocks CLOCK_REALTIME,
> CLOCK_MONOTONIC, CLOCK_BOOTTIME, etc.
> 
> CLOCK_REALTIME
>       System-wide clock that measures real (i.e., wall-clock) time.
> 
> CLOCK_MONOTONIC
>       Clock that cannot be set and represents monotonic time since
>       some unspecified starting point.
> 
> CLOCK_BOOTTIME
>       Identical to CLOCK_MONOTONIC, except it also includes any time
>       that the system is suspended.
> 
> For many users, the time namespace means the ability to changes date and
> time in a container (CLOCK_REALTIME).
> 
> But in a context of the checkpoint/restore functionality, monotonic and
> bootime clocks become interesting. Both clocks are monotonic with
> unspecified staring points. These clocks are widely used to measure time
> slices and set timers. After restoring or migrating processes, we have to
> guarantee that they never go backward. In an ideal case, the behavior of
> these clocks should be the same as for a case when a whole system is
> suspended. All this means that we need to be able to set CLOCK_MONOTONIC
> and CLOCK_BOOTTIME clocks, what can be done by adding per-namespace
> offsets for clocks.
> 
> A time namespace is similar to a pid namespace in a way how it is
> created: unshare(CLONE_NEWTIME) system call creates a new time namespace,
> but doesn't set it to the current process. Then all children of
> the process will be born in the new time namespace, or a process can
> use the setns() system call to join a namespace.
> 
> This scheme allows setting clock offsets for a namespace, before any
> processes appear in it.
> 
> All available clone flags have been used, so CLONE_NEWTIME uses the
> highest bit of CSIGNAL. It means that we can use it with the unshare()
> system call only. Rith now, this works for us, because time namespace
> offsets can be set only when a new time namespace is not populated. In a
> future, we will have the clone3() system call [1] which will allow to use
> the CSIGNAL mask for clone flags.
> 
> [1]: httmps://lkml.kernel.org/r/20190604160944.4058-1-christian@xxxxxxxxxx
> 
> Link: https://criu.org/Time_namespace
> Link: https://lists.openvz.org/pipermail/criu/2018-June/041504.html
> Signed-off-by: Andrei Vagin <avagin@xxxxxxxxx>
> Co-developed-by: Dmitry Safonov <dima@xxxxxxxxxx>
> Signed-off-by: Dmitry Safonov <dima@xxxxxxxxxx>
> ---
>  MAINTAINERS                    |   2 +
>  fs/proc/namespaces.c           |   4 +
>  include/linux/nsproxy.h        |   2 +
>  include/linux/proc_ns.h        |   3 +
>  include/linux/time_namespace.h |  66 ++++++++++
>  include/linux/user_namespace.h |   1 +
>  include/uapi/linux/sched.h     |   6 +
>  init/Kconfig                   |   7 ++
>  kernel/fork.c                  |  16 ++-
>  kernel/nsproxy.c               |  41 +++++--
>  kernel/time/Makefile           |   1 +
>  kernel/time/namespace.c        | 217 +++++++++++++++++++++++++++++++++
>  12 files changed, 356 insertions(+), 10 deletions(-)
>  create mode 100644 include/linux/time_namespace.h
>  create mode 100644 kernel/time/namespace.c
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index d44d6732510d..cabe7bddbf69 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -13009,6 +13009,8 @@ T:	git git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git timers/core
>  S:	Maintained
>  F:	fs/timerfd.c
>  F:	include/linux/timer*
> +F:	include/linux/time_namespace.h
> +F:	kernel/time_namespace.c
>  F:	kernel/time/*timer*
>  
>  POWER MANAGEMENT CORE
> diff --git a/fs/proc/namespaces.c b/fs/proc/namespaces.c
> index dd2b35f78b09..8b5c720fe5d7 100644
> --- a/fs/proc/namespaces.c
> +++ b/fs/proc/namespaces.c
> @@ -33,6 +33,10 @@ static const struct proc_ns_operations *ns_entries[] = {
>  #ifdef CONFIG_CGROUPS
>  	&cgroupns_operations,
>  #endif
> +#ifdef CONFIG_TIME_NS
> +	&timens_operations,
> +	&timens_for_children_operations,
> +#endif
>  };
>  
>  static const char *proc_ns_get_link(struct dentry *dentry,
> diff --git a/include/linux/nsproxy.h b/include/linux/nsproxy.h
> index 2ae1b1a4d84d..074f395b9ad2 100644
> --- a/include/linux/nsproxy.h
> +++ b/include/linux/nsproxy.h
> @@ -35,6 +35,8 @@ struct nsproxy {
>  	struct mnt_namespace *mnt_ns;
>  	struct pid_namespace *pid_ns_for_children;
>  	struct net 	     *net_ns;
> +	struct time_namespace *time_ns;
> +	struct time_namespace *time_ns_for_children;
>  	struct cgroup_namespace *cgroup_ns;
>  };
>  extern struct nsproxy init_nsproxy;
> diff --git a/include/linux/proc_ns.h b/include/linux/proc_ns.h
> index d31cb6215905..d312e6281e69 100644
> --- a/include/linux/proc_ns.h
> +++ b/include/linux/proc_ns.h
> @@ -32,6 +32,8 @@ extern const struct proc_ns_operations pidns_for_children_operations;
>  extern const struct proc_ns_operations userns_operations;
>  extern const struct proc_ns_operations mntns_operations;
>  extern const struct proc_ns_operations cgroupns_operations;
> +extern const struct proc_ns_operations timens_operations;
> +extern const struct proc_ns_operations timens_for_children_operations;
>  
>  /*
>   * We always define these enumerators
> @@ -43,6 +45,7 @@ enum {
>  	PROC_USER_INIT_INO	= 0xEFFFFFFDU,
>  	PROC_PID_INIT_INO	= 0xEFFFFFFCU,
>  	PROC_CGROUP_INIT_INO	= 0xEFFFFFFBU,
> +	PROC_TIME_INIT_INO	= 0xEFFFFFFAU,
>  };
>  
>  #ifdef CONFIG_PROC_FS
> diff --git a/include/linux/time_namespace.h b/include/linux/time_namespace.h
> new file mode 100644
> index 000000000000..873b908c9ba8
> --- /dev/null
> +++ b/include/linux/time_namespace.h
> @@ -0,0 +1,66 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_TIMENS_H
> +#define _LINUX_TIMENS_H
> +
> +
> +#include <linux/sched.h>
> +#include <linux/kref.h>
> +#include <linux/nsproxy.h>
> +#include <linux/ns_common.h>
> +#include <linux/err.h>
> +
> +struct user_namespace;
> +extern struct user_namespace init_user_ns;
> +
> +struct time_namespace {
> +	struct kref kref;
> +	struct user_namespace *user_ns;
> +	struct ucounts *ucounts;
> +	struct ns_common ns;
> +} __randomize_layout;
> +extern struct time_namespace init_time_ns;
> +
> +#ifdef CONFIG_TIME_NS
> +static inline struct time_namespace *get_time_ns(struct time_namespace *ns)
> +{
> +	kref_get(&ns->kref);
> +	return ns;
> +}
> +
> +extern struct time_namespace *copy_time_ns(unsigned long flags,
> +	struct user_namespace *user_ns, struct time_namespace *old_ns);
> +extern void free_time_ns(struct kref *kref);
> +extern int timens_on_fork(struct nsproxy *nsproxy, struct task_struct *tsk);
> +
> +static inline void put_time_ns(struct time_namespace *ns)
> +{
> +	kref_put(&ns->kref, free_time_ns);
> +}
> +
> +#else
> +static inline struct time_namespace *get_time_ns(struct time_namespace *ns)
> +{
> +	return NULL;
> +}
> +
> +static inline void put_time_ns(struct time_namespace *ns)
> +{
> +}
> +
> +static inline struct time_namespace *copy_time_ns(unsigned long flags,
> +	struct user_namespace *user_ns, struct time_namespace *old_ns)
> +{
> +	if (flags & CLONE_NEWTIME)
> +		return ERR_PTR(-EINVAL);
> +
> +	return old_ns;
> +}
> +
> +static inline int timens_on_fork(struct nsproxy *nsproxy, struct task_struct *tsk)
> +{
> +	return 0;
> +}
> +
> +#endif
> +
> +#endif /* _LINUX_TIMENS_H */
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index fb9f4f799554..6ef1c7109fc4 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -45,6 +45,7 @@ enum ucount_type {
>  	UCOUNT_NET_NAMESPACES,
>  	UCOUNT_MNT_NAMESPACES,
>  	UCOUNT_CGROUP_NAMESPACES,
> +	UCOUNT_TIME_NAMESPACES,
>  #ifdef CONFIG_INOTIFY_USER
>  	UCOUNT_INOTIFY_INSTANCES,
>  	UCOUNT_INOTIFY_WATCHES,
> diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> index 99335e1f4a27..5b7511ac2005 100644
> --- a/include/uapi/linux/sched.h
> +++ b/include/uapi/linux/sched.h
> @@ -33,6 +33,12 @@
>  #define CLONE_NEWNET		0x40000000	/* New network namespace */
>  #define CLONE_IO		0x80000000	/* Clone io context */
>  
> +/*
> + * cloning flags intersect with CSIGNAL so can be used with unshare and clone3
> + * syscalls only:
> + */
> +#define CLONE_NEWTIME	0x00000080	/* New time namespace */
> +
>  #ifndef __ASSEMBLY__
>  /**
>   * struct clone_args - arguments for the clone3 syscall
> diff --git a/init/Kconfig b/init/Kconfig
> index b4daad2bac23..bc2ee93408ad 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1096,6 +1096,13 @@ config UTS_NS
>  	  In this namespace tasks see different info provided with the
>  	  uname() system call
>  
> +config TIME_NS
> +	bool "TIME namespace"
> +	default y

Having CONFIG_TIME_NS "default y" makes so that the option is selected even on
the architectures that have no support for time namespaces.
The direct consequence is that the fallbacks defined in this patch are never
selected and this ends up in kernel compilation errors due to missing symbols.

The error below shows what happens on arm64 (similar behavior on other
architectures):

aarch64-linux-gnu-ld: kernel/time/namespace.o: in function `timens_on_fork':
kernel/time/namespace.c:321: undefined reference to `vdso_join_timens'

aarch64-linux-gnu-ld: kernel/time/namespace.o: in function
`timens_set_vvar_page': kernel/time/namespace.c:218: undefined reference to
`arch_get_vdso_
data'

aarch64-linux-gnu-ld: kernel/time/namespace.o: in function `timens_install':
/data1/Projects/LinuxKernel/linux/kernel/time/namespace.c:295: undefined
reference to `vdso_join_time
ns'
/data1/Projects/LinuxKernel/linux/Makefile:1074: recipe for target 'vmlinux'
failed
make[1]: *** [vmlinux] Error 1
make[1]: Leaving directory '/data1/Projects/LinuxKernel/linux-out'
                    Makefile:179: recipe for target 'sub-make' failed
make: *** [sub-make] Error 2

My proposal is to keep TIME_NS "default n" (just remove "default y"), let the
architectures that enable time namespaces select it and make CONFIG_TIME_NS
select GENERIC_VDSO_TIME_NS if arch has HAVE_GENERIC_VDSO.

What do you think?

Thanks,
Vincenzo

> +	help
> +	  In this namespace boottime and monotonic clocks can be set.
> +	  The time will keep going with the same pace.
> +
>  config IPC_NS
>  	bool "IPC namespace"
>  	depends on (SYSVIPC || POSIX_MQUEUE)
> diff --git a/kernel/fork.c b/kernel/fork.c
> index bcdf53125210..aa65333dbc45 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1772,6 +1772,7 @@ static __latent_entropy struct task_struct *copy_process(
>  	struct multiprocess_signals delayed;
>  	struct file *pidfile = NULL;
>  	u64 clone_flags = args->flags;
> +	struct nsproxy *nsp = current->nsproxy;
>  
>  	/*
>  	 * Don't allow sharing the root directory with processes in a different
> @@ -1814,8 +1815,16 @@ static __latent_entropy struct task_struct *copy_process(
>  	 */
>  	if (clone_flags & CLONE_THREAD) {
>  		if ((clone_flags & (CLONE_NEWUSER | CLONE_NEWPID)) ||
> -		    (task_active_pid_ns(current) !=
> -				current->nsproxy->pid_ns_for_children))
> +		    (task_active_pid_ns(current) != nsp->pid_ns_for_children))
> +			return ERR_PTR(-EINVAL);
> +	}
> +
> +	/*
> +	 * If the new process will be in a different time namespace
> +	 * do not allow it to share VM or a thread group with the forking task.
> +	 */
> +	if (clone_flags & (CLONE_THREAD | CLONE_VM)) {
> +		if (nsp->time_ns != nsp->time_ns_for_children)
>  			return ERR_PTR(-EINVAL);
>  	}
>  
> @@ -2703,7 +2712,8 @@ static int check_unshare_flags(unsigned long unshare_flags)
>  	if (unshare_flags & ~(CLONE_THREAD|CLONE_FS|CLONE_NEWNS|CLONE_SIGHAND|
>  				CLONE_VM|CLONE_FILES|CLONE_SYSVSEM|
>  				CLONE_NEWUTS|CLONE_NEWIPC|CLONE_NEWNET|
> -				CLONE_NEWUSER|CLONE_NEWPID|CLONE_NEWCGROUP))
> +				CLONE_NEWUSER|CLONE_NEWPID|CLONE_NEWCGROUP|
> +				CLONE_NEWTIME))
>  		return -EINVAL;
>  	/*
>  	 * Not implemented, but pretend it works if there is nothing
> diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
> index c815f58e6bc0..ed9882108cd2 100644
> --- a/kernel/nsproxy.c
> +++ b/kernel/nsproxy.c
> @@ -18,6 +18,7 @@
>  #include <linux/pid_namespace.h>
>  #include <net/net_namespace.h>
>  #include <linux/ipc_namespace.h>
> +#include <linux/time_namespace.h>
>  #include <linux/proc_ns.h>
>  #include <linux/file.h>
>  #include <linux/syscalls.h>
> @@ -40,6 +41,10 @@ struct nsproxy init_nsproxy = {
>  #ifdef CONFIG_CGROUPS
>  	.cgroup_ns		= &init_cgroup_ns,
>  #endif
> +#ifdef CONFIG_TIME_NS
> +	.time_ns		= &init_time_ns,
> +	.time_ns_for_children	= &init_time_ns,
> +#endif
>  };
>  
>  static inline struct nsproxy *create_nsproxy(void)
> @@ -106,8 +111,18 @@ static struct nsproxy *create_new_namespaces(unsigned long flags,
>  		goto out_net;
>  	}
>  
> +	new_nsp->time_ns_for_children = copy_time_ns(flags, user_ns,
> +					tsk->nsproxy->time_ns_for_children);
> +	if (IS_ERR(new_nsp->time_ns_for_children)) {
> +		err = PTR_ERR(new_nsp->time_ns_for_children);
> +		goto out_time;
> +	}
> +	new_nsp->time_ns = get_time_ns(tsk->nsproxy->time_ns);
> +
>  	return new_nsp;
>  
> +out_time:
> +	put_net(new_nsp->net_ns);
>  out_net:
>  	put_cgroup_ns(new_nsp->cgroup_ns);
>  out_cgroup:
> @@ -136,15 +151,16 @@ int copy_namespaces(unsigned long flags, struct task_struct *tsk)
>  	struct nsproxy *old_ns = tsk->nsproxy;
>  	struct user_namespace *user_ns = task_cred_xxx(tsk, user_ns);
>  	struct nsproxy *new_ns;
> +	int ret;
>  
>  	if (likely(!(flags & (CLONE_NEWNS | CLONE_NEWUTS | CLONE_NEWIPC |
>  			      CLONE_NEWPID | CLONE_NEWNET |
> -			      CLONE_NEWCGROUP)))) {
> -		get_nsproxy(old_ns);
> -		return 0;
> -	}
> -
> -	if (!ns_capable(user_ns, CAP_SYS_ADMIN))
> +			      CLONE_NEWCGROUP | CLONE_NEWTIME)))) {
> +		if (likely(old_ns->time_ns_for_children == old_ns->time_ns)) {
> +			get_nsproxy(old_ns);
> +			return 0;
> +		}
> +	} else if (!ns_capable(user_ns, CAP_SYS_ADMIN))
>  		return -EPERM;
>  
>  	/*
> @@ -162,6 +178,12 @@ int copy_namespaces(unsigned long flags, struct task_struct *tsk)
>  	if (IS_ERR(new_ns))
>  		return  PTR_ERR(new_ns);
>  
> +	ret = timens_on_fork(new_ns, tsk);
> +	if (ret) {
> +		free_nsproxy(new_ns);
> +		return ret;
> +	}
> +
>  	tsk->nsproxy = new_ns;
>  	return 0;
>  }
> @@ -176,6 +198,10 @@ void free_nsproxy(struct nsproxy *ns)
>  		put_ipc_ns(ns->ipc_ns);
>  	if (ns->pid_ns_for_children)
>  		put_pid_ns(ns->pid_ns_for_children);
> +	if (ns->time_ns)
> +		put_time_ns(ns->time_ns);
> +	if (ns->time_ns_for_children)
> +		put_time_ns(ns->time_ns_for_children);
>  	put_cgroup_ns(ns->cgroup_ns);
>  	put_net(ns->net_ns);
>  	kmem_cache_free(nsproxy_cachep, ns);
> @@ -192,7 +218,8 @@ int unshare_nsproxy_namespaces(unsigned long unshare_flags,
>  	int err = 0;
>  
>  	if (!(unshare_flags & (CLONE_NEWNS | CLONE_NEWUTS | CLONE_NEWIPC |
> -			       CLONE_NEWNET | CLONE_NEWPID | CLONE_NEWCGROUP)))
> +			       CLONE_NEWNET | CLONE_NEWPID | CLONE_NEWCGROUP |
> +			       CLONE_NEWTIME)))
>  		return 0;
>  
>  	user_ns = new_cred ? new_cred->user_ns : current_user_ns();
> diff --git a/kernel/time/Makefile b/kernel/time/Makefile
> index 1867044800bb..c8f00168afe8 100644
> --- a/kernel/time/Makefile
> +++ b/kernel/time/Makefile
> @@ -19,3 +19,4 @@ obj-$(CONFIG_TICK_ONESHOT)			+= tick-oneshot.o tick-sched.o
>  obj-$(CONFIG_HAVE_GENERIC_VDSO)			+= vsyscall.o
>  obj-$(CONFIG_DEBUG_FS)				+= timekeeping_debug.o
>  obj-$(CONFIG_TEST_UDELAY)			+= test_udelay.o
> +obj-$(CONFIG_TIME_NS)				+= namespace.o
> diff --git a/kernel/time/namespace.c b/kernel/time/namespace.c
> new file mode 100644
> index 000000000000..2662a69e0382
> --- /dev/null
> +++ b/kernel/time/namespace.c
> @@ -0,0 +1,217 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Author: Andrei Vagin <avagin@xxxxxxxxxx>
> + * Author: Dmitry Safonov <dima@xxxxxxxxxx>
> + */
> +
> +#include <linux/time_namespace.h>
> +#include <linux/user_namespace.h>
> +#include <linux/sched/signal.h>
> +#include <linux/sched/task.h>
> +#include <linux/proc_ns.h>
> +#include <linux/export.h>
> +#include <linux/time.h>
> +#include <linux/slab.h>
> +#include <linux/cred.h>
> +#include <linux/err.h>
> +
> +static struct ucounts *inc_time_namespaces(struct user_namespace *ns)
> +{
> +	return inc_ucount(ns, current_euid(), UCOUNT_TIME_NAMESPACES);
> +}
> +
> +static void dec_time_namespaces(struct ucounts *ucounts)
> +{
> +	dec_ucount(ucounts, UCOUNT_TIME_NAMESPACES);
> +}
> +
> +/**
> + * clone_time_ns - Clone a time namespace
> + * @user_ns:	User namespace which owns a new namespace.
> + * @old_ns:	Namespace to clone
> + *
> + * Clone @old_ns and set the clone refcount to 1
> + *
> + * Return: The new namespace or ERR_PTR.
> + */
> +static struct time_namespace *clone_time_ns(struct user_namespace *user_ns,
> +					  struct time_namespace *old_ns)
> +{
> +	struct time_namespace *ns;
> +	struct ucounts *ucounts;
> +	int err;
> +
> +	err = -ENOSPC;
> +	ucounts = inc_time_namespaces(user_ns);
> +	if (!ucounts)
> +		goto fail;
> +
> +	err = -ENOMEM;
> +	ns = kmalloc(sizeof(*ns), GFP_KERNEL);
> +	if (!ns)
> +		goto fail_dec;
> +
> +	kref_init(&ns->kref);
> +
> +	err = ns_alloc_inum(&ns->ns);
> +	if (err)
> +		goto fail_free;
> +
> +	ns->ucounts = ucounts;
> +	ns->ns.ops = &timens_operations;
> +	ns->user_ns = get_user_ns(user_ns);
> +	return ns;
> +
> +fail_free:
> +	kfree(ns);
> +fail_dec:
> +	dec_time_namespaces(ucounts);
> +fail:
> +	return ERR_PTR(err);
> +}
> +
> +/**
> + * copy_time_ns - Create timens_for_children from @old_ns
> + * @flags:	Cloning flags
> + * @user_ns:	User namespace which owns a new namespace.
> + * @old_ns:	Namespace to clone
> + *
> + * If CLONE_NEWTIME specified in @flags, creates a new timens_for_children;
> + * adds a refcounter to @old_ns otherwise.
> + *
> + * Return: timens_for_children namespace or ERR_PTR.
> + */
> +struct time_namespace *copy_time_ns(unsigned long flags,
> +	struct user_namespace *user_ns, struct time_namespace *old_ns)
> +{
> +	if (!(flags & CLONE_NEWTIME))
> +		return get_time_ns(old_ns);
> +
> +	return clone_time_ns(user_ns, old_ns);
> +}
> +
> +void free_time_ns(struct kref *kref)
> +{
> +	struct time_namespace *ns;
> +
> +	ns = container_of(kref, struct time_namespace, kref);
> +	dec_time_namespaces(ns->ucounts);
> +	put_user_ns(ns->user_ns);
> +	ns_free_inum(&ns->ns);
> +	kfree(ns);
> +}
> +
> +static struct time_namespace *to_time_ns(struct ns_common *ns)
> +{
> +	return container_of(ns, struct time_namespace, ns);
> +}
> +
> +static struct ns_common *timens_get(struct task_struct *task)
> +{
> +	struct time_namespace *ns = NULL;
> +	struct nsproxy *nsproxy;
> +
> +	task_lock(task);
> +	nsproxy = task->nsproxy;
> +	if (nsproxy) {
> +		ns = nsproxy->time_ns;
> +		get_time_ns(ns);
> +	}
> +	task_unlock(task);
> +
> +	return ns ? &ns->ns : NULL;
> +}
> +
> +static struct ns_common *timens_for_children_get(struct task_struct *task)
> +{
> +	struct time_namespace *ns = NULL;
> +	struct nsproxy *nsproxy;
> +
> +	task_lock(task);
> +	nsproxy = task->nsproxy;
> +	if (nsproxy) {
> +		ns = nsproxy->time_ns_for_children;
> +		get_time_ns(ns);
> +	}
> +	task_unlock(task);
> +
> +	return ns ? &ns->ns : NULL;
> +}
> +
> +static void timens_put(struct ns_common *ns)
> +{
> +	put_time_ns(to_time_ns(ns));
> +}
> +
> +static int timens_install(struct nsproxy *nsproxy, struct ns_common *new)
> +{
> +	struct time_namespace *ns = to_time_ns(new);
> +
> +	if (!current_is_single_threaded())
> +		return -EUSERS;
> +
> +	if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN) ||
> +	    !ns_capable(current_user_ns(), CAP_SYS_ADMIN))
> +		return -EPERM;
> +
> +	get_time_ns(ns);
> +	put_time_ns(nsproxy->time_ns);
> +	nsproxy->time_ns = ns;
> +
> +	get_time_ns(ns);
> +	put_time_ns(nsproxy->time_ns_for_children);
> +	nsproxy->time_ns_for_children = ns;
> +	return 0;
> +}
> +
> +int timens_on_fork(struct nsproxy *nsproxy, struct task_struct *tsk)
> +{
> +	struct ns_common *nsc = &nsproxy->time_ns_for_children->ns;
> +	struct time_namespace *ns = to_time_ns(nsc);
> +
> +	/* create_new_namespaces() already incremented the ref counter */
> +	if (nsproxy->time_ns == nsproxy->time_ns_for_children)
> +		return 0;
> +
> +	get_time_ns(ns);
> +	put_time_ns(nsproxy->time_ns);
> +	nsproxy->time_ns = ns;
> +
> +	return 0;
> +}
> +
> +static struct user_namespace *timens_owner(struct ns_common *ns)
> +{
> +	return to_time_ns(ns)->user_ns;
> +}
> +
> +const struct proc_ns_operations timens_operations = {
> +	.name		= "time",
> +	.type		= CLONE_NEWTIME,
> +	.get		= timens_get,
> +	.put		= timens_put,
> +	.install	= timens_install,
> +	.owner		= timens_owner,
> +};
> +
> +const struct proc_ns_operations timens_for_children_operations = {
> +	.name		= "time_for_children",
> +	.type		= CLONE_NEWTIME,
> +	.get		= timens_for_children_get,
> +	.put		= timens_put,
> +	.install	= timens_install,
> +	.owner		= timens_owner,
> +};
> +
> +struct time_namespace init_time_ns = {
> +	.kref		= KREF_INIT(3),
> +	.user_ns	= &init_user_ns,
> +	.ns.inum	= PROC_TIME_INIT_INO,
> +	.ns.ops		= &timens_operations,
> +};
> +
> +static int __init time_ns_init(void)
> +{
> +	return 0;
> +}
> +subsys_initcall(time_ns_init);
> 

-- 
Regards,
Vincenzo