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SubjectRe: [PATCH v3] of: overlay: user space synchronization
On 10/18/18 12:32, Rob Herring wrote:
> On Tue, Oct 16, 2018 at 05:34:26PM -0700, wrote:
>> From: Frank Rowand <>
>> When an overlay is applied or removed, the live devicetree visible in
>> /proc/device-tree/, aka /sys/firmware/devicetree/base/, reflects the
>> changes. There is no method for user space to determine whether the
>> live devicetree was modified by overlay actions.
> Because userspace has no way to modify the DT and the ways the kernel
> can do modifications is limited.
> Do you have an actually need for this outside of testing/development?

I do not know if anyone uses /proc/device-tree for anything outside of
testing/development. If we believe that there is no other use of
/proc/device-tree we can simply document that there is no expectation
that accessors will see a consistent, unchanging /proc/device-tree.

That would be a much smaller patch.

>> Provide a sysfs file, /sys/firmware/devicetree/tree_version, to allow
>> user space to determine if the live devicetree has remained unchanged
>> while a series of one or more accesses of /proc/device-tree/ occur.
>> The use of both (1) dynamic devicetree modifications and (2) overlay
>> apply and removal are not supported during the same boot cycle. Thus
>> non-overlay dynamic modifications are not reflected in the value of
>> tree_version.
> I'd prefer to see some sort of information on overlays exported and user
> space can check if that changed. IIRC, Pantelis had a series to do that
> along with a kill switch to prevent further modifications. At least some
> of that series only had minor issues to fix.

The kill switch addresses a different concern, which was from the security
community. The kill switch is on my todo list.

I don't remember exactly what info the overlay information export patch
provided. I'll have to go find it and re-read it.

> Also, shouldn't we get uevents if the tree changes? Maybe that's not

Yes (off the top of my head). But a shell script accessing /proc/device-tree
is not going to get uevents.

> guaranteed, but I'd bet we can't handle cases where we don't get events.
> A property added to an existing node comes to mind.>
> Rob

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