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

From: Alan Tull
Date: Tue Apr 24 2018 - 12:09:07 EST

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.

>> 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