From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754732AbeDYPAV (ORCPT ); Wed, 25 Apr 2018 11:00:21 -0400 Received: from mail.kernel.org ([198.145.29.99]:36854 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753571AbeDYPAS (ORCPT ); Wed, 25 Apr 2018 11:00:18 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 548B221836 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=atull@kernel.org X-Google-Smtp-Source: AIpwx49hBHLefuDMzLlXg62JNfiZRxOgZaqN8rwPyRnjbYajTHrnfXUrU+dSDfG9PWjog4bndDb+KQar5N7FhVOUZXM= MIME-Version: 1.0 In-Reply-To: <4483492d-37d2-63ad-6739-2cb297fa5058@gmail.com> References: <1520122673-11003-1-git-send-email-frowand.list@gmail.com> <1520122673-11003-3-git-send-email-frowand.list@gmail.com> <09e3db63-cbf9-52a2-ee77-520979f17fea@web.de> <7bbf615b-3cdd-6bb4-6918-33e48de4225d@gmail.com> <7bbb9472-9c96-6012-68e6-4ec2773c7732@gmail.com> <4483492d-37d2-63ad-6739-2cb297fa5058@gmail.com> From: Alan Tull Date: Wed, 25 Apr 2018 09:59:35 -0500 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v7 2/5] of: change overlay apply input data from unflattened to FDT To: Frank Rowand Cc: Jan Kiszka , Rob Herring , Pantelis Antoniou , Pantelis Antoniou , "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" , "linux-kernel@vger.kernel.org" , Geert Uytterhoeven , Laurent Pinchart , Jailhouse Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 24, 2018 at 3:56 PM, Frank Rowand 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 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@gmail.com wrote: >>>>>>>>>> From: Frank Rowand >>>>>>>>>> >>>>>>>>>> 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 >> >> >> >