Re: [PATCH v7 2/5] of: change overlay apply input data from unflattened to FDT

From: Alan Tull
Date: Tue Apr 24 2018 - 14:27:42 EST

On Tue, Apr 24, 2018 at 11:08 AM, Alan Tull <atull@xxxxxxxxxx> wrote:
> On Tue, Apr 24, 2018 at 12:29 AM, Jan Kiszka <jan.kiszka@xxxxxx> wrote:
>> On 2018-04-24 00:38, Frank Rowand wrote:
>>> Hi Jan,
>>> + Alan Tull for fpga perspective
>>> On 04/22/18 03:30, Jan Kiszka wrote:
>>>> On 2018-04-11 07:42, Jan Kiszka wrote:
>>>>> On 2018-04-05 23:12, Rob Herring wrote:
>>>>>> On Thu, Apr 5, 2018 at 2:28 PM, Frank Rowand <frowand.list@xxxxxxxxx> wrote:
>>>>>>> On 04/05/18 12:13, Jan Kiszka wrote:
>>>>>>>> On 2018-04-05 20:59, Frank Rowand wrote:
>>>>>>>>> Hi Jan,
>>>>>>>>> On 04/04/18 15:35, Jan Kiszka wrote:
>>>>>>>>>> Hi Frank,
>>>>>>>>>> On 2018-03-04 01:17, frowand.list@xxxxxxxxx wrote:
>>>>>>>>>>> From: Frank Rowand <frank.rowand@xxxxxxxx>
>>>>>>>>>>> Move duplicating and unflattening of an overlay flattened devicetree
>>>>>>>>>>> (FDT) into the overlay application code. To accomplish this,
>>>>>>>>>>> of_overlay_apply() is replaced by of_overlay_fdt_apply().
>>>>>>>>>>> The copy of the FDT (aka "duplicate FDT") now belongs to devicetree
>>>>>>>>>>> code, which is thus responsible for freeing the duplicate FDT. The
>>>>>>>>>>> caller of of_overlay_fdt_apply() remains responsible for freeing the
>>>>>>>>>>> original FDT.
>>>>>>>>>>> The unflattened devicetree now belongs to devicetree code, which is
>>>>>>>>>>> thus responsible for freeing the unflattened devicetree.
>>>>>>>>>>> These ownership changes prevent early freeing of the duplicated FDT
>>>>>>>>>>> or the unflattened devicetree, which could result in use after free
>>>>>>>>>>> errors.
>>>>>>>>>>> of_overlay_fdt_apply() is a private function for the anticipated
>>>>>>>>>>> overlay loader.
>>>>>>>>>> We are using of_fdt_unflatten_tree + of_overlay_apply in the
>>>>>>>>>> (out-of-tree) Jailhouse loader driver in order to register a virtual
>>>>>>>>>> device during hypervisor activation with Linux. The DT overlay is
>>>>>>>>>> created from a a template but modified prior to application to account
>>>>>>>>>> for runtime-specific parameters. See [1] for the current implementation.
>>>>>>>>>> I'm now wondering how to model that scenario best with the new API.
>>>>>>>>>> Given that the loader lost ownership of the unflattened tree but the
>>>>>>>>>> modification API exist only for the that DT state, I'm not yet seeing a
>>>>>>>>>> clear solution. Should we apply the template in disabled form (status =
>>>>>>>>>> "disabled"), modify it, and then activate it while it is already applied?
>>>>>>>>> Thank you for the pointer to the driver - that makes it much easier to
>>>>>>>>> understand the use case and consider solutions.
>>>>>>>>> If you can make the changes directly on the FDT instead of on the
>>>>>>>>> expanded devicetree, then you could move to the new API.
>>>>>>>> Are there some examples/references on how to edit FDTs in-place in the
>>>>>>>> kernel? I'd like to avoid writing the n-th FDT parser/generator.
>>>>>>> I don't know of any existing in-kernel edits of the FDT (but they might
>>>>>>> exist). The functions to access an FDT are in libfdt, which is in
>>>>>>> scripts/dtc/libfdt/.
>>>>>> Let's please not go down that route of doing FDT modifications. There
>>>>>> is little reason to other than for early boot changes. And it is much
>>>>>> easier to work on unflattened trees.
>>>>> I just briefly looked into libfdt, and it would have meant building it
>>>>> into the module as there are no library functions exported by the kernel
>>>>> either. Another reason to drop that.
>>>>> What's apparently working now is the pattern I initially suggested:
>>>>> Register template with status = "disabled" as overlay, then prepare and
>>>>> apply changeset that contains all needed modifications and sets the
>>>>> status to "ok". I might be leaking additional resources, but to find
>>>>> that out, I will now finally have to resolve clean unbinding of the
>>>>> generic PCI host controller [1] first.
>>>> static void free_overlay_changeset(struct overlay_changeset *ovcs)
>>>> {
>>>> [...]
>>>> /*
>>>> * TODO
>>>> *
>>>> * would like to: kfree(ovcs->overlay_tree);
>>>> * but can not since drivers may have pointers into this data
>>>> *
>>>> * would like to: kfree(ovcs->fdt);
>>>> * but can not since drivers may have pointers into this data
>>>> */
>>>> kfree(ovcs);
>>>> }
>>>> What's this? I have kmemleak now jumping at me over this. Who is suppose
>>>> to plug these leaks? The caller of of_overlay_fdt_apply has no pointers
>>>> to those objects. I would say that's a regression of the new API.
>>> The problem already existed but it was hidden. We have never been able to
>>> kfree() these object because we do not know if there are any pointers into
>>> these objects. The new API makes the problem visible to kmemleak.
>> My old code didn't have the problem because there was no one steeling
>> pointers to my overlay, and I was able to safely release all the
>> resources that I or the core on my behalf allocated. In fact, I recently
>> even dropped the duplication the fdt prior to unflattening it because I
>> got its lifecycle under control (and both kmemleak as well as kasan
>> confirmed this). I still consider this intentional leak a regression of
>> the new API.
>>> The reason that we do not know if there are any pointers into these objects
>>> is that devicetree access APIs return pointers into the devicetree internal
>>> data structures (that is, into the overlay unflattened devicetree). If we
>>> want to be able to do the kfree()s, we could change the devicetree access
>>> APIs.
>>> The reason that pointers into the overlay flattened tree (ovcs->fdt) are
>>> also exposed is that the overlay unflattened devicetree property values
>>> are pointers into the overlay fdt.
>>> ** This paragraph becomes academic (and not needed) if the fix in the next
>>> paragraph can be implemented. **
>>> I _think_ that the fdt issue __for overlays__ can be fixed somewhat easily.
>>> (I would want to read through the code again to make sure I'm not missing
>>> any issues.) If the of_fdt_unflatten_tree() called by of_overlay_fdt_apply()
>>> was modified so that property values were copied into newly allocated memory
>>> and the live tree property pointers were set to the copy instead of to
>>> the value in the fdt, then I _think_ the fdt could be freed in
>>> of_overlay_fdt_apply() after calling of_overlay_apply(). The code that
>> I don't see yet how more duplicating of objects would help. Then we
>> would not leak the fdt or the unflattened tree on overlay destruction
>> but that duplicates, no?
>>> frees a devicetree would also have to be aware of this change -- I'm not
>>> sure if that leads to ugly complications or if it is easy. The other
>>> question to consider is whether to make the same change to
>>> of_fdt_unflatten_tree() when it is called in early boot to unflatten
>>> the base devicetree. Doing so would increase the memory usage of the
>>> live tree (we would not be able to free the base fdt after unflattening
>>> it because we make the fdt visible in /sys/firmware/fdt -- though
>>> _maybe_ that could be conditioned on CONFIG_KEXEC).
>>> But all of the complexity of that fix is _only_ because of_overlay_apply()
>>> and of_overlay_remove() call overlay_notify(), passing in the overlay
>>> unflattened devicetree (which has pointers into the overlay fdt). Pointers
>>> into the overlay unflattened devicetree are then passed to the notifiers.
>>> (Again, I may be missing some other place that the overlay unflattened
>>> devicetree is made visible to other code -- a more thorough reading of
>>> the code is needed.) If the notifiers could be modified to accept the
>>> changeset list instead of of pointers to the fragments in the overlay
>>> unflattened devicetree then there would be no possibility of the notifiers
>>> keeping a pointer into the overlay fdt. I do not know if this is a
>> But then again the convention has to be that those changeset pointers
>> must not be kept - because the changeset is history after of_overlay_remove.
>>> practical change for the notifiers -- there are no callers of
>>> of_overlay_notifier_register() in the mainline kernel source. My
>>> recollection is that the overlay notifiers were added for the fpga
>>> subsystem.
> That's right.
>> We have drivers/fpga/of-fpga-region.c in-tree, and that does not seem to
>> store any pointers to objects, rather consumes them in-place. And I
>> would consider it fair to impose such a limitation on the notifier
>> interface.
> The FPGA code was written assuming that overlays could be removed.

To be more specific, drivers/fpga/of-fpga-region.c currently saves a
pointer to the overlay and uses it only during the pre-apply

>>> Why is overlay_notify() the only issue related to unknown users having
>>> pointers into the overlay fdt? The answer is that the overlay code
>>> does not directly expose the overlay unflattened devicetree (and thus
>>> indirectly the overlay fdt) to the live devicetree -- when the
>>> overlay code creates the overlay changeset, it copies from the
>>> overlay unflattened devicetree and overlay fdt and only exposes
>>> pointers to the copies.
>>> And hopefully the issues with the overlay unflattened devicetree can
>>> be resolved in the same way as for the overlay fdt.
>> As noted above, I don't see there is a technical solution to this issue
>> but it's rather a matter of convention: no overlay notifier callback is
>> allowed to keep references to the passed tree content (unless it
>> reference-counts some tree nodes) beyond the execution of the callback.
>> With that in place, we can safely drop the backing memory IMHO.
>> Jan