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SubjectRe: [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

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