linux-kernel - Re: [PATCH] zram: restrict add/remove attributes to root only

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Message-ID: <20161204120834.GA4520@tigerII.localdomain>
Date:   Sun, 4 Dec 2016 21:08:34 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Minchan Kim <minchan@...nel.org>,
        Steven Allen <steven@...balien.com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH] zram: restrict add/remove attributes to root only

On (12/04/16 12:55), Greg KH wrote:
[..]
> That's fine, the issue is that reading a file should not cause the
> system state to change.  That's just not a logical thing to have happen,
> no other sysfs files do that. Why is zram "special" in this way?

yeah, zram is not really special, we just didn't come up with
anything better than that.

> > 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
> 
> you should have stuck with the "write a value to the sysfs file" api,
> for some reason that didn't stick...

yes, we had this 'echo ID > /sys/..../zram_add' at some point, but it
didn't fly.

> > I believe we have a documentation
> > 
> > 	Documentation/ABI/testing/sysfs-class-zram
> > and
> > 	Documentation/blockdev/zram.txt
> > 
> > both explain this attr.
> 
> Yes, but that's not in the code itself.  You are doing something VERY
> different here than any other sysfs file.  The code better explain it
> very well so that I don't go and change this back sometime in the future
> when I sweep the kernel for "odd sysfs mode values" like I do every few
> years.
> 
> So comment this please, why would you object to that?

oh, I'm not objecting. I just gave as much info as possible.
v2 [with a comment]  will be out soon.


will this comment suffice?

====

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index ee03464..3a0576f 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -1413,6 +1413,13 @@ static ssize_t hot_remove_store(struct class *class,
        return ret ? ret : count;
 }
 
+/*
+ * NOTE: hot_add attribute is not the usual read-only sysfs
+ * attribute. In a sence that reading from this file does alter
+ * the state of your system -- it creates a new un-initialized
+ * zram device and returns back this device's device_id (or an
+ * error code if it fails to create a new device).
+ */
 static struct class_attribute zram_control_class_attrs[] = {
        __ATTR(hot_add, 0400, hot_add_show, NULL),
        __ATTR(hot_remove, 0200, NULL, hot_remove_store),

--

	-ss