linux-kernel - Re: [PATCH 2/2] pid_ns: Introduce ioctl to set vector of ns_last

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Date:   Wed, 26 Apr 2017 19:43:33 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
CC:     Oleg Nesterov <oleg@...hat.com>, <serge@...lyn.com>,
        <agruenba@...hat.com>, <linux-api@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <paul@...l-moore.com>,
        <viro@...iv.linux.org.uk>, <avagin@...nvz.org>,
        <linux-fsdevel@...r.kernel.org>, <mtk.manpages@...il.com>,
        <akpm@...ux-foundation.org>, <luto@...capital.net>,
        <gorcunov@...nvz.org>, <mingo@...nel.org>, <keescook@...omium.org>
Subject: Re: [PATCH 2/2] pid_ns: Introduce ioctl to set vector of
 ns_last_pid's on ns hierarhy

On 26.04.2017 19:32, Eric W. Biederman wrote:
> Kirill Tkhai <ktkhai@...tuozzo.com> writes:
> 
>> On 26.04.2017 19:11, Kirill Tkhai wrote:
>>> On 26.04.2017 18:53, Oleg Nesterov wrote:
>>>> On 04/17, Kirill Tkhai wrote:
>>>>>
>>>>> +struct pidns_ioc_req {
>>>>> +/* Set vector of last pids in namespace hierarchy */
>>>>> +#define PIDNS_REQ_SET_LAST_PID_VEC	0x1
>>>>> +	unsigned int req;
>>>>> +	void __user *data;
>>>>> +	unsigned int data_size;
>>>>> +	char std_fields[0];
>>>>> +};
>>>>
>>>> see below,
>>>>
>>>>> +static long set_last_pid_vec(struct pid_namespace *pid_ns,
>>>>> +			     struct pidns_ioc_req *req)
>>>>> +{
>>>>> +	char *str, *p;
>>>>> +	int ret = 0;
>>>>> +	pid_t pid;
>>>>> +
>>>>> +	read_lock(&tasklist_lock);
>>>>> +	if (!pid_ns->child_reaper)
>>>>> +		ret = -EINVAL;
>>>>> +	read_unlock(&tasklist_lock);
>>>>> +	if (ret)
>>>>> +		return ret;
>>>>
>>>> why do you need to check ->child_reaper under tasklist_lock? this looks pointless.
>>>>
>>>> In fact I do not understand how it is possible to hit pid_ns->child_reaper == NULL,
>>>> there must be at least one task in this namespace, otherwise you can't open a file
>>>> which has f_op == ns_file_operations, no?
>>>
>>> Sure, it's impossible to pick a pid_ns, if there is no the pid_ns's tasks. I added
>>> it under impression of
>>> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=dfda351c729733a401981e8738ce497eaffcaa00
>>> but here it's completely wrong. It will be removed in v2.
>>>  
>>>>> +	if (req->data_size >= PAGE_SIZE)
>>>>> +		return -EINVAL;
>>>>> +	str = vmalloc(req->data_size + 1);
>>>>
>>>> then I don't understand why it makes sense to use vmalloc()
>>>>
>>>>> +	if (!str)
>>>>> +		return -ENOMEM;
>>>>> +	if (copy_from_user(str, req->data, req->data_size)) {
>>>>> +		ret = -EFAULT;
>>>>> +		goto out_vfree;
>>>>> +	}
>>>>> +	str[req->data_size] = '\0';
>>>>> +
>>>>> +	p = str;
>>>>> +	while (p && *p != '\0') {
>>>>> +		if (!ns_capable(pid_ns->user_ns, CAP_SYS_ADMIN)) {
>>>>> +			ret = -EPERM;
>>>>> +			goto out_vfree;
>>>>> +		}
>>>>> +
>>>>> +		if (sscanf(p, "%d", &pid) != 1 || pid < 0 || pid > pid_max) {
>>>>> +			ret = -EINVAL;
>>>>> +			goto out_vfree;
>>>>> +		}
>>>>
>>>> Well, this is ioctl(), do we really want to parse the strings?
>>>>
>>>> Can't we make
>>>>
>>>> 	struct pidns_ioc_req {
>>>> 		...
>>>> 		int nr_pids;
>>>> 		pid_t  pids[0];
>>>> 	}
>>>>
>>>> and just use get_user() in a loop? This way we can avoid vmalloc() or anything
>>>> else altogether.
>>>
>>> Since it's a generic structure for different types of the requests, it may be extended
>>> in the future. We won't be able to add new fields, if we compose the structure the way
>>> you suggested, will we?
>>
>> Though, we may go this way if just do the fields generic:
>>
>> struct pidns_ioc_req {
>>         unsigned int req;
>>         unsigned int data_size;
>>         union {
>> 	        pid_t pid[0];
>> 	};
>> };
>>
>> Ok, I'll rework the patchset in this way.
> 
> You don't need that.  That is what new ioctl numbers are for.
> 
> Interfaces to the kernel don't need to become multiplexors to prepare
> for the future when there is already an appropriate multiplexing
> interface in place.

That is, do you suggest to not introduce NS_SPECIFIC_IO from the first patch,
and add PIDNS_REQ_SET_LAST_PID_VEC to the list of generic ns ioctls?

...
#define NS_GET_OWNER_UID		_IO(NSIO, 0x4)
#define PIDNS_REQ_SET_LAST_PID_VEC	_IO(NSIO, 0x5)