linux-kernel - Re: [PATCH 8/9] [RFC] Example multi-bindable subsystem: a per-cgroup notes field

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Date:	Mon, 13 Jul 2009 17:49:56 -0700
From:	Paul Menage <menage@...gle.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	balbir@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, containers@...ts.linux-foundation.org,
	kamezawa.hiroyu@...fujitsu.com
Subject: Re: [PATCH 8/9] [RFC] Example multi-bindable subsystem: a per-cgroup 
	notes field

On Fri, Jul 3, 2009 at 1:58 AM, Li Zefan<lizf@...fujitsu.com> wrote:
>
> If it's not per cgroup, it can be a global value.

No it can't, since the notes subsystem can be mounted on multiple
hierarchies simultaneously.

> But why not make it per cgroup?

Setting notes.size to a large value allows userspace to create large
kmalloc'd objects; so if it's per-cgroup, then it's not safe to
delegate control of part of a cgroups hierarchy to untrusted users.
(E.g. we want to be able to give an untrusted user process the power
to create sub-cgroups in the CPU scheduler hierarchy, so that it can
give different CPU guarantees to each of its threadpools).

I guess that an alternative would be to have a per-cgroup size field,
and use the min of the cgroup and all its ancestors when doing length
checking.

>> +     if (nbytes > cg_info(cgrp->top_cgroup)->max_len)
>> +             return -E2BIG;
>> +     if (nbytes) {
>> +             notes = kmalloc(nbytes, GFP_USER);
>> +             if (!notes)
>> +                     return -ENOMEM;
>> +             if (copy_from_user(notes, userbuf, nbytes))
>
> missing kfree(notes)
>

Good catch, thanks. Fixed (and all the newlines added).

Paul
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