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

[Alan Tull]

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?

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
>>
>>
>>
>