linux-kernel - Re: [PACTH v1] mm, proc: Implement /proc/<pid>/totmaps

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Date:	Tue, 9 Aug 2016 16:17:40 -0400
From:	Robert Foss <robert.foss@...labora.com>
To:	Mateusz Guzik <mguzik@...hat.com>
Cc:	akpm@...ux-foundation.org, keescook@...omium.org,
	viro@...iv.linux.org.uk, gorcunov@...nvz.org,
	john.stultz@...aro.org, plaguedbypenguins@...il.com,
	sonnyrao@...omium.org, adobriyan@...il.com, jdanis@...gle.com,
	calvinowens@...com, jann@...jh.net, mhocko@...e.com,
	koct9i@...il.com, vbabka@...e.cz, n-horiguchi@...jp.nec.com,
	kirill.shutemov@...ux.intel.com, ldufour@...ux.vnet.ibm.com,
	hannes@...xchg.org, linux-kernel@...r.kernel.org,
	Ben Zhang <benzh@...omium.org>,
	Bryan Freed <bfreed@...omium.org>,
	Filipe Brandenburger <filbranden@...omium.org>
Subject: Re: [PACTH v1] mm, proc: Implement /proc/<pid>/totmaps



On 2016-08-09 12:29 PM, Mateusz Guzik wrote:
> On Tue, Aug 09, 2016 at 12:05:43PM -0400, robert.foss@...labora.com wrote:
>> From: Sonny Rao <sonnyrao@...omium.org>
>>
>> This is based on earlier work by Thiago Goncales. It implements a new
>> per process proc file which summarizes the contents of the smaps file
>> but doesn't display any addresses.  It gives more detailed information
>> than statm like the PSS (proprotional set size).  It differs from the
>> original implementation in that it doesn't use the full blown set of
>> seq operations, uses a different termination condition, and doesn't
>> displayed "Locked" as that was broken on the original implemenation.
>>
>> This new proc file provides information faster than parsing the potentially
>> huge smaps file.
>
> I have no idea about usefulness of this.
>
> The patch is definitely buggy with respect to how it implements actual
> access to mm.
>
>> +static int totmaps_proc_show(struct seq_file *m, void *data)
>> +{
>> +	struct proc_maps_private *priv = m->private;
>> +	struct mm_struct *mm;
>> +	struct vm_area_struct *vma;
>> +	struct mem_size_stats *mss_sum = priv->mss;
>> +
>> +	/* reference to priv->task already taken */
>> +	/* but need to get the mm here because */
>> +	/* task could be in the process of exiting */
>> +	mm = get_task_mm(priv->task);
>> +	if (!mm || IS_ERR(mm))
>> +		return -EINVAL;
>> +
>
> That's not how it's done in smaps.

Alright, I'll have to look into the difference between this approach and 
the smaps one.

>
>> +static int totmaps_open(struct inode *inode, struct file *file)
>> +{
>> +	struct proc_maps_private *priv;
>> +	int ret = -ENOMEM;
>> +	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
>> +	if (priv) {
>> +		priv->mss = kzalloc(sizeof(*priv->mss), GFP_KERNEL);
>> +		if (!priv->mss)
>> +			return -ENOMEM;
>
> Cases below explicitly kfree(priv). I can't remember whether the close
> routine gets called if this one fails. Either way, something is wrong
> here.

It looks fishy to me too, I'll have it reworked in v2.

>
>> +
>> +		/* we need to grab references to the task_struct */
>> +		/* at open time, because there's a potential information */
>> +		/* leak where the totmaps file is opened and held open */
>> +		/* while the underlying pid to task mapping changes */
>> +		/* underneath it */
>> +		priv->task = get_pid_task(proc_pid(inode), PIDTYPE_PID);
>
> This performs no permission checks that I would see. If you take a look
> at smaps you will see the user ends up in proc_maps_open which performs
> proc_mem_open(inode, PTRACE_MODE_READ) and gets a mm from there.

The proc_maps_open() function does seem to be doing everything I need 
it. I'll have a look at switching to using it.

Thanks for the heads up!


Rob.

>
>
>> +		if (!priv->task) {
>> +			kfree(priv->mss);
>> +			kfree(priv);
>> +			return -ESRCH;
>> +		}
>> +
>> +		ret = single_open(file, totmaps_proc_show, priv);
>> +		if (ret) {
>> +			put_task_struct(priv->task);
>> +			kfree(priv->mss);
>> +			kfree(priv);
>> +		}
>> +	}
>> +	return ret;
>> +}
>> +
>