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 19:33:46 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Oleg Nesterov <oleg@...hat.com>
CC:     <serge@...lyn.com>, <ebiederm@...ssion.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 19:11, 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.