Re: [RFC PATCH] tmpfs: support user quotas

KOSAKI Motohiro
2011-11-07T22:15:19Z

(11/7/2011 6:30 AM), Lennart Poettering wrote:
> On Mon, 07.11.11 13:58, Alan Cox (alan@lxorguk.ukuu.org.uk) wrote:
>
>>
>>> Right, rlimit approach guarantees a simple way of dealing with users
>>> across all tmpfs instances.
>>
>> Which is almost certainly not what you want to happen. Think about direct
>> rendering.
>
> I don't see what direct rendering has to do with closing the security
> hole that /dev/shm currently is.
>
>> For simple stuff tmpfs already supports size/nr_blocks/nr_inodes mount
>> options so you can mount private resource constrained tmpfs objects
>> already without kernel changes. No rlimit hacks needed - and rlimit is
>> the wrong API anyway.
>
> Uh? I am pretty sure we don't want to mount a private tmpfs for each
> user in /dev/shm and /tmp. If you have 500 users you'd have 500 tmpfs on
> /tmp and on /dev/shm. Despite that without some ugly namespace hackery
> you couldn't make them all appear in /tmp as /dev/shm without
> subdirectories. Don't forget that /dev/shm and /tmp are an established
> userspace API.
>
> Resource limits are exactly the API that makes sense here, because:
>
> a) we only want one tmpfs on /tmp, and one tmpfs on /dev/shm, not 500 on
> each for each user

Ok, seems fair.

> b) we cannot move /dev/shm, /tmp around without breaking userspace
> massively

agreed.

>
> c) we want a global limit across all tmpfs file systems for each user

Why? Is there any benefit this?

> d) we don't want to have to upload the quota database into each tmpfs at
> mount time.
>
> And hence: a per user RLIMIT is exactly the minimal solution we want
> here.

If you want per-user limitation, RLIMIT is bad idea. RLIMIT is only inherited
by fork. So, The api semantics clearly mismatch your usecase.

Instead, I suggest to implement new sysfs knob.

Thank you.