Re: [PATCH v4] pidns: introduce syscall translate_pid

[Konstantin Khlebnikov]

On 13.10.2017 19:05, Oleg Nesterov wrote:
> On 10/13, Konstantin Khlebnikov wrote:
> > pid_t translate_pid(pid_t pid, int source, int target);
> >
> > This syscall converts pid from source pid-ns into pid in target pid-ns.
> > If pid is unreachable from target pid-ns it returns zero.
> >
> > Pid-namespaces are referred file descriptors opened to proc files
> > /proc/[pid]/ns/pid or /proc/[pid]/ns/pid_for_children. Negative argument
> > refers to current pid namespace, same as file /proc/self/ns/pid.
> >
> > Kernel expose virtual pids in /proc/[pid]/status:NSpid, but backward
> > translation requires scanning all tasks. Also pids could be translated
> > by sending them through unix socket between namespaces, this method is
> > slow and insecure because other side is exposed inside pid namespace.
> >
> > Examples:
> > translate_pid(pid, ns, -1)      - get pid in our pid namespace
> > translate_pid(pid, -1, ns)      - get pid in other pid namespace
> > translate_pid(1, ns, -1)        - get pid of init task for namespace
> > translate_pid(pid, -1, ns) > 0  - is pid is reachable from ns?
> > translate_pid(1, ns1, ns2) > 0  - is ns1 inside ns2?
> > translate_pid(1, ns1, ns2) == 0 - is ns1 outside ns2?
> > translate_pid(1, ns1, ns2) == 1 - is ns1 equal ns2?
>
> Add Eugene, strace probably wants this too.
>
> I have a vague feeling we have already discussed this in the past, but
> I can't recall anything...

Yeah, v3 was two years ago.

> > +static struct pid_namespace *get_pid_ns_by_fd(int fd)
> > +{
> > +	struct pid_namespace *pidns;
> > +	struct ns_common *ns;
> > +	struct file *file;
> > +
> > +	file = proc_ns_fget(fd);
> > +	if (IS_ERR(file))
> > +		return ERR_CAST(file);
> > +
> > +	ns = get_proc_ns(file_inode(file));
> > +	if (ns->ops->type == CLONE_NEWPID)
> > +		pidns = get_pid_ns(to_pid_ns(ns));
> > +	else
> > +		pidns = ERR_PTR(-EINVAL);
> > +
> > +	fput(file);
> > +	return pidns;
> > +}
>
> I won't insist, but this suggests we should add a new helper,
> get_ns_by_fd_type(fd, type), and convert get_net_ns_by_fd() to use it
> as well.

That was in v3.

I'll prefer to this later, separately. And replace fget with fdget which
allows to do this without atomic operations if task is single-threaded.

> > +SYSCALL_DEFINE3(translate_pid, pid_t, pid, int, source, int, target)
> > +{
> > +	struct pid_namespace *source_ns, *target_ns;
> > +	struct pid *struct_pid;
> > +	pid_t result;
> > +
> > +	if (source >= 0) {
> > +		source_ns = get_pid_ns_by_fd(source);
> > +		result = PTR_ERR(source_ns);
> > +		if (IS_ERR(source_ns))
> > +			goto err_source;
> > +	} else
> > +		source_ns = task_active_pid_ns(current);
> > +
> > +	if (target >= 0) {
> > +		target_ns = get_pid_ns_by_fd(target);
> > +		result = PTR_ERR(target_ns);
> > +		if (IS_ERR(target_ns))
> > +			goto err_target;
> > +	} else
> > +		target_ns = task_active_pid_ns(current);
> > +
> > +	rcu_read_lock();
> > +	struct_pid = find_pid_ns(pid, source_ns);
> > +	result = struct_pid ? pid_nr_ns(struct_pid, target_ns) : -ESRCH;
> > +	rcu_read_unlock();
>
> Stupid question. Can't we make a simpler API which doesn't need /proc/ ?
> I mean,
>
> 	sys_translate_pid(pid_t pid, pid_t source_pid, pid_t target_pid)
> 	{
> 		struct pid_namespace *source_ns, *target_ns;
>
> 		source_ns = task_active_pid_ns(find_task_by_vpid(source_pid));
> 		target_ns = task_active_pid_ns(find_task_by_vpid(target_pid));
>
> 		...
> 	}
>  > Yes, this is more limited... Do you have a use-case when this is not enough?

That was in v1 but considered too racy.

>> v1: https://lkml.org/lkml/2015/9/15/411
>> v2: https://lkml.org/lkml/2015/9/24/278
>> v3: https://lkml.org/lkml/2015/9/28/3