Re: [PATCH 0/1] ns: introduce proc_get_ns_by_fd()

[Chen Fan <chen.fan.fnst@cn.fujitsu.com>]

On 09/29/2015 12:37 AM, Eric W. Biederman wrote:
> Oleg Nesterov <oleg@redhat.com> writes:
>
>> On 09/25, Konstantin Khlebnikov wrote:
>>> +struct ns_common *proc_ns_fdget(int fd, int nstype, struct fd *fd_ref)
>>>   {
>>> -	struct file *file;
>>> +	struct ns_common *ns;
>>> +	struct fd f;
>>>   
>>> -	file = fget(fd);
>>> -	if (!file)
>>> +	f = fdget(fd);
>>> +	if (!f.file)
>>>   		return ERR_PTR(-EBADF);
>>>   
>>> -	if (file->f_op != &ns_file_operations)
>>> +	if (f.file->f_op != &ns_file_operations)
>>> +		goto out_invalid;
>>> +
>>> +	ns = get_proc_ns(file_inode(f.file));
>>> +	if (nstype && (ns->ops->type != nstype))
>>>   		goto out_invalid;
>>>   
>>> -	return file;
>>> +	*fd_ref = f;
>>> +	return ns;
>>>   
>>>   out_invalid:
>>> -	fput(file);
>>> +	fdput(f);
>>>   	return ERR_PTR(-EINVAL);
>>>   }
>> Well yes, fdget() makes sense but this is minor.
>>
>> Honestly, I do not really like the new helper... I understand this
>> is subjective, so I won't insist. But how about 1/1? We do not need
>> fd/file at all. With this patch your sys_getvpid() can just use
>> proc_get_ns_by_fd(fd, CLONE_NEWPID) and put_pid_ns().
>>
>> Eric, what do you think?
> At some level I don't care this is not exposed to userspace.
>
> However since we are going several rounds with this.
>
> Of the existing uses several of them sleep, which unfortunately means we
> can not use rcu locking for everything.  The network namespace ones wind
> up taking a reference to struct net because the have the legacy pid case
> to deal with.  Which makes we can not use fdget for all callers either.
>
> For this translate_pid rcu locking is sufficient, rcu locking is easy
> and doing any more than rcu locking just seems silly.  So let me
> respectfully suggest.
>
> struct ns_common *ns_by_fd_rcu(int fd, int type)
> {
> 	struct files_struct *files = current->files;
> 	struct file *file;
> 	struct ns_common *ns;
> 	void *ret;
pointer files seems no more useful. we can
use fcheck(fd) instead.

Thanks,

>
> 	file = fcheck_files(files, fd);
>          if (!file)
> 		return ERR_PTR(-EBADF);
>
> 	if (file->f_mode & FMODE_PATH)
> 		return ERR_PTR(-EINVAL);
>
> 	if (file->f_op != &ns_file_operations)
>          	return ERR_PTR(-EINVAL);
>
> 	ns = get_proc_ns(file_inode(file));
> 	if (ns->ops->type != type)
> 		return ERR_PTR(-EINVAL);
>
> 	return ns;
> }
>
> struct pid_namespace *pidns_by_fd_rcu(int fd)
> {
> 	struct ns_common *ns = ns_by_fd_rcu(fd, CLONE_NEWPID);
>          if (IS_ERR(ns))
> 		return ERR_CAST(ns);
> 	return container_of(ns, struct pid_namespace, ns);
> }
>
> SYSCALL_DEFINE3(translate_pid, pid_t, pid_nr, int, sourcefd, int, targetfd)
> {
> 	struct pid_namespace *source, *target;
> 	struct pid *pid;
> 	pid_t result;
>
> 	rcu_read_lock();
>
> 	if (sourcefd >= 0)
>          	source = pidns_by_fd_rcu(sourcefd);
> 	else
> 		source = task_active_pid_ns(current);
>
> 	if (targetfd >= 0)
>          	target = pidns_by_fd_rcu(targetfd);
> 	else
> 		target = task_active_pid_ns(current);
>
> 	pid = find_pid_ns(pid_nr, source);
>          result = pid_nr_ns(pid, target);
>          if (result == 0)
> 		result = -ESRCH;
>
> 	rcu_read_unlock();
>
> 	return result;
> }
>
> Eric
> .
>