Re: [PATCH v4] pidns: introduce syscall translate_pid

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

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