Re: [PATCH v5] pid_ns: Introduce ioctl to set vector of ns_last_pid's on ns hierarhy

[Kirill Tkhai]

Eric,

could you give comments about this?

On 02.05.2017 12:48, Kirill Tkhai wrote:
> On implementing of nested pid namespaces support in CRIU
> (checkpoint-restore in userspace tool) we run into
> the situation, that it's impossible to create a task with
> specific NSpid effectively. After commit 49f4d8b93ccf
> "pidns: Capture the user namespace and filter ns_last_pid"
> it is impossible to set ns_last_pid on any pid namespace,
> except task's active pid_ns (before the commit it was possible
> to write to pid_ns_for_children). Thus, if a restored task
> in a container has more than one pid_ns levels, the restorer
> code must have a task helper for every pid namespace
> of the task's pid_ns hierarhy.
> 
> This is a big problem, because of communication with
> a helper for every pid_ns in the hierarchy is not cheap.
> It's not performance-good as it implies many helpers wakeups
> to create a single task (independently, how you communicate
> with the helpers). This patch tries to decide the problem.
> 
> It introduces a new pid_ns ioctl(NS_SET_LAST_PID_VEC),
> which allows to write a vector of last pids on pid_ns hierarchy.
> The vector is passed as array of pids in struct ns_ioc_pid_vec,
> written in reverse order. The first number corresponds to
> the opened namespace ns_last_pid, the second is to its parent, etc.
> So, if you have the pid namespaces hierarchy like:
> 
> pid_ns1 (grand father)
>   |
>   v
> pid_ns2 (father)
>   |
>   v
> pid_ns3 (child)
> 
> and the pid_ns3 is open, then the corresponding vector will be
> {last_ns_pid3, last_ns_pid2, last_ns_pid1}. This vector may be
> short and it may contain less levels. For example,
> {last_ns_pid3, last_ns_pid2} or even {last_ns_pid3}, in dependence
> of which levels you want to populate.
> 
> v5: Move pid_max up for !CONFIG_CHECKPOINT_RESTORE builds
> (found by kbuild test robot).
> 
> v4: Declare struct ns_ioc_pid_vec directly instead of include uapi file.
> Make the interface independent of CONFIG_CHECKPOINT_RESTORE.
> Include linux/types.h in nsfs.h for pid_t.
> Get all vectors at once.
> Make checks atomical.
> 
> v3: Use __u32 in uapi instead of unsigned int.
> 
> v2: Kill pid_ns->child_reaper check as it's impossible to have
> such a pid namespace file open.
> Use generic namespaces ioctl() number.
> Pass pids as array, not as a string.
> 
> Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> ---
>  fs/nsfs.c                     |    5 +++++
>  include/linux/pid_namespace.h |    9 ++++++++-
>  include/uapi/linux/nsfs.h     |    8 ++++++++
>  kernel/pid_namespace.c        |   38 +++++++++++++++++++++++++++++++++++++-
>  4 files changed, 58 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/nsfs.c b/fs/nsfs.c
> index 323f492e0822..f669a1552003 100644
> --- a/fs/nsfs.c
> +++ b/fs/nsfs.c
> @@ -6,6 +6,7 @@
>  #include <linux/ktime.h>
>  #include <linux/seq_file.h>
>  #include <linux/user_namespace.h>
> +#include <linux/pid_namespace.h>
>  #include <linux/nsfs.h>
>  #include <linux/uaccess.h>
>  
> @@ -186,6 +187,10 @@ static long ns_ioctl(struct file *filp, unsigned int ioctl,
>  		argp = (uid_t __user *) arg;
>  		uid = from_kuid_munged(current_user_ns(), user_ns->owner);
>  		return put_user(uid, argp);
> +	case NS_SET_LAST_PID_VEC:
> +		if (ns->ops->type != CLONE_NEWPID)
> +			return -EINVAL;
> +		return pidns_set_last_pid_vec(ns, (void *)arg);
>  	default:
>  		return -ENOTTY;
>  	}
> diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
> index c2a989dee876..661fad08bf8c 100644
> --- a/include/linux/pid_namespace.h
> +++ b/include/linux/pid_namespace.h
> @@ -54,6 +54,7 @@ struct pid_namespace {
>  	struct ns_common ns;
>  };
>  
> +struct ns_ioc_pid_vec;
>  extern struct pid_namespace init_pid_ns;
>  
>  #define PIDNS_HASH_ADDING (1U << 31)
> @@ -71,7 +72,8 @@ extern struct pid_namespace *copy_pid_ns(unsigned long flags,
>  extern void zap_pid_ns_processes(struct pid_namespace *pid_ns);
>  extern int reboot_pid_ns(struct pid_namespace *pid_ns, int cmd);
>  extern void put_pid_ns(struct pid_namespace *ns);
> -
> +extern long pidns_set_last_pid_vec(struct ns_common *ns,
> +				   struct ns_ioc_pid_vec __user *vec);
>  #else /* !CONFIG_PID_NS */
>  #include <linux/err.h>
>  
> @@ -101,6 +103,11 @@ static inline int reboot_pid_ns(struct pid_namespace *pid_ns, int cmd)
>  {
>  	return 0;
>  }
> +static inline long pidns_set_last_pid_vec(struct ns_common *ns,
> +					  struct ns_ioc_pid_vec __user *vec)
> +{
> +	return -ENOTTY;
> +}
>  #endif /* CONFIG_PID_NS */
>  
>  extern struct pid_namespace *task_active_pid_ns(struct task_struct *tsk);
> diff --git a/include/uapi/linux/nsfs.h b/include/uapi/linux/nsfs.h
> index 1a3ca79f466b..9d320276eafe 100644
> --- a/include/uapi/linux/nsfs.h
> +++ b/include/uapi/linux/nsfs.h
> @@ -2,6 +2,7 @@
>  #define __LINUX_NSFS_H
>  
>  #include <linux/ioctl.h>
> +#include <linux/types.h>
>  
>  #define NSIO	0xb7
>  
> @@ -14,5 +15,12 @@
>  #define NS_GET_NSTYPE		_IO(NSIO, 0x3)
>  /* Get owner UID (in the caller's user namespace) for a user namespace */
>  #define NS_GET_OWNER_UID	_IO(NSIO, 0x4)
> +/* Set a vector of ns_last_pid for a pid namespace stack */
> +#define NS_SET_LAST_PID_VEC	_IO(NSIO, 0x5)
> +
> +struct ns_ioc_pid_vec {
> +	__u32	nr;
> +	pid_t	pid[0];
> +};
>  
>  #endif /* __LINUX_NSFS_H */
> diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
> index de461aa0bf9a..d675b4d22dfe 100644
> --- a/kernel/pid_namespace.c
> +++ b/kernel/pid_namespace.c
> @@ -21,6 +21,7 @@
>  #include <linux/export.h>
>  #include <linux/sched/task.h>
>  #include <linux/sched/signal.h>
> +#include <uapi/linux/nsfs.h>
>  
>  struct pid_cache {
>  	int nr_ids;
> @@ -32,6 +33,7 @@ struct pid_cache {
>  static LIST_HEAD(pid_caches_lh);
>  static DEFINE_MUTEX(pid_caches_mutex);
>  static struct kmem_cache *pid_ns_cachep;
> +extern int pid_max;
>  
>  /*
>   * creates the kmem cache to allocate pids from.
> @@ -311,7 +313,6 @@ static int pid_ns_ctl_handler(struct ctl_table *table, int write,
>  	return proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
>  }
>  
> -extern int pid_max;
>  static int zero = 0;
>  static struct ctl_table pid_ns_ctl_table[] = {
>  	{
> @@ -428,6 +429,41 @@ static struct ns_common *pidns_get_parent(struct ns_common *ns)
>  	return &get_pid_ns(pid_ns)->ns;
>  }
>  
> +long pidns_set_last_pid_vec(struct ns_common *ns,
> +			    struct ns_ioc_pid_vec __user *vec)
> +{
> +	struct pid_namespace *pid_ns = to_pid_ns(ns), *top;
> +	pid_t pid[MAX_PID_NS_LEVEL];
> +	u32 i, nr;
> +
> +	BUILD_BUG_ON(sizeof(pid_t) * MAX_PID_NS_LEVEL > 128);
> +	if (get_user(nr, &vec->nr))
> +		return -EFAULT;
> +	if (nr > MAX_PID_NS_LEVEL || nr < 1)
> +		return -EINVAL;
> +	if (copy_from_user(pid, &vec->pid[0], nr * sizeof(pid_t)) != 0)
> +		return -EFAULT;
> +
> +	top = pid_ns;
> +	for (i = 0; i < nr-1; i++) {
> +		top = top->parent;
> +		if (!top || pid[i] < 0 || pid[i] > pid_max)
> +			return -EINVAL;
> +	}
> +	if (!ns_capable(top->user_ns, CAP_SYS_ADMIN))
> +		return -EPERM;
> +	if (pid[nr-1] < 0 || pid[nr-1] > pid_max)
> +		return -EINVAL;
> +
> +	for (i = 0; i < nr; i++) {
> +		/* Write directly: see the comment in pid_ns_ctl_handler() */
> +		pid_ns->last_pid = pid[i];
> +		pid_ns = pid_ns->parent;
> +	}
> +
> +	return 0;
> +}
> +
>  static struct user_namespace *pidns_owner(struct ns_common *ns)
>  {
>  	return to_pid_ns(ns)->user_ns;
>