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

From: Frank Rowand
Date: Wed Apr 25 2018 - 20:10:21 EST

On 04/25/18 17:07, Frank Rowand wrote:
> Hi Alan,
> On 04/25/18 11:19, Alan Tull wrote:
>> On Wed, Apr 25, 2018 at 12:41 PM, Frank Rowand <frowand.list@xxxxxxxxx> wrote:
>>> On 04/25/18 07:59, Alan Tull wrote:
>>>> On Tue, Apr 24, 2018 at 3:56 PM, Frank Rowand <frowand.list@xxxxxxxxx> wrote:
>>>>> Hi Alan,
>>>>> On 04/23/18 15:38, 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.
>>>>>> 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
>>>>>> 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).
>>>>> Question added below this paragraph.
>>>>>> 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
>>>>>> 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.
>>>>> Can the fpga notifiers be changed to have the changeset as an input
>>>>> instead of having the overlay devicetree fragment and target as an
>>>>> input?
>>>> I'll look into it. Just to be clear, are you suggesting passing
>>>> struct overlay_changeset instead in the notifier?
>>> Ah, poor phrasing on my part. I meant a "struct of_changeset", as is
>>> passed into __of_changeset_apply_entries(), which is called from
>>> of_overlay_apply(). This means that the call to overlay_notify()
>>> would have to move down a few lines to just after calling
>>> build_changeset().
>> Ah yes, I thought it was looking too easy. :) I had it working with
>> notify data passing the struct overlay_changeset, I was about to send
>> you the patch.
>> The FPGA code really wants the data as fragments, so it will know
>> first of all what the target is. Passing of_changeset would mean that
>> the code receiving the notification would be essentially be tasked
>> reassembling the changeset into fragments. Perhaps it could be done,
>> but it could easily be broken by changes to overlay.c and it would be
>> ugly. That breaks the exact thing that I added overlay notifications
>> for. I really need to see for each fragment what the target is, and
>> all the properties together.

I inadvertently chopped off part of my reply. I think is also meant to
say something like:

As Jan pointed out in another email, the

> approach I proposed here is not solving the underlying problem, but just
> moving it to another place. So I am not going to pursue this approach.
> -Frank
>>>> struct overlay_changeset and struct fragment would have to be moved to a header.
>>>>> The changeset lists nodes and properties to be added, but does not
>>>>> expose any pointers to the overlay fdt or the overlay unflattened
>>>>> devicetree. This guarantees no leakage of pointers into the overlay
>>>>> fdt or the overlay unflattened devicetree. The changeset contains
>>>>> pointers to copies of data, but those copies are never freed (and
>>>>> thus they are yet another existing memory leak).
>>>>> -Frank
>>>>>> 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.
>>>>>> -Frank