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SubjectRe: [PATCH] zram: restrict add/remove attributes to root only
On (12/04/16 12:28), Greg KH wrote:
> Date: Sun, 4 Dec 2016 12:28:20 +0100
> From: Greg KH <gregkh@linuxfoundation.org>
> To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>, Minchan Kim
> <minchan@kernel.org>, Steven Allen <steven@stebalien.com>,
> linux-kernel@vger.kernel.org, stable@vger.kernel.org, Sergey Senozhatsky
> <sergey.senozhatsky.work@gmail.com>
> Subject: Re: [PATCH] zram: restrict add/remove attributes to root only
> User-Agent: Mutt/1.7.1 (2016-10-04)
>
> On Sun, Dec 04, 2016 at 07:52:08PM +0900, Sergey Senozhatsky wrote:
> > On (12/04/16 11:28), Greg KH wrote:
> > > On Sun, Dec 04, 2016 at 11:35:15AM +0900, Sergey Senozhatsky wrote:
> > [..]
> >
> > > Why can't a normal user read the attribute? Does a read actually modify
> > > something?
> >
> > yes, it does.

to clarify a bit more:

we allocate a new device ID using idr_alloc(). so the IDs are limited
and, thus, the number of devices is limited as well - signed int. each
new device has NO:
-- zspoll (zsmalloc pool in zram case)
-- compression per-CPU backends (working-mem/scratch buffers, etc.)
-- meta table

so no big memory allocations. (a 'normal' user can't init the device,
he/she can just create it. which is the problem here: we don't want a
'normal' user be able to do this).

every device has:
-- blk queue
-- sysfs attrs
-- gendisk
-- zram structure allocated.

so each new device consumes some memory, but not insane amounts of it.


> Oh that's totally and completely broken then.
>
> Reading from a sysfs file should NEVER cause side affects to the system.
> Please fix up this api.

some history. we started with a 'loop device'-like scheme, but
ended up with a sysfs approach

[1] https://marc.info/?l=linux-kernel&m=142495984002611
[2] https://marc.info/?l=linux-kernel&m=142507747808572
[3] https://marc.info/?l=linux-kernel&m=142530591720172
[4] https://marc.info/?l=linux-kernel&m=142509446812318
[5] https://marc.info/?l=linux-kernel&m=142509782112819


> > reading from a hot_add file creates a new zram device and returns a new
> > device's device_id. not initialized device (so it does not eat the memory
> > for handle table, etc.), but with its own set of sysfs attrs, etc. which
> > consumes memory after all. so a 'normal' user, doing a simple read from a
> > hot_add file in a loop just for fun, can create a lot of devices and,
> > quite likely, cause some troubles (as reported by Steven Allen).
>
> Please switch this to be a char device node if you wish to "write and
> get a device handle back". I don't know how I missed that in the
> original api review, sorry about that.
>
> For now, you need to document the heck out of this in the attribute
> declaration that this is what is going on. Otherwise someone like me
> will come along and "fix up" the file to use ATTR_RO again in the
> future and you will have the same problem again.


I believe we have a documentation

Documentation/ABI/testing/sysfs-class-zram
and
Documentation/blockdev/zram.txt

both explain this attr.

-ss

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