Date:   Thu, 19 Oct 2017 11:51:40 +0300
From:   Pantelis Antoniou <pantelis.antoniou@...sulko.com>
To:     Rob Herring <robh@...nel.org>
Cc:     Alan Tull <atull@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        David Laight <David.Laight@...lab.com>,
        linux-fpga@...r.kernel.org
Subject: Re: [PATCH v2 5/5] of/fdt: only store the device node basename in
 full_name

Hi Rob,

> On Oct 18, 2017, at 21:30 , Rob Herring <robh@...nel.org> wrote:
> 
> On Wed, Oct 18, 2017 at 10:53 AM, Pantelis Antoniou
> <pantelis.antoniou@...sulko.com> wrote:
>> On Wed, 2017-10-18 at 10:44 -0500, Rob Herring wrote:
>>> On Wed, Oct 18, 2017 at 10:12 AM, Alan Tull <atull@...nel.org> wrote:
>>>> On Tue, Oct 17, 2017 at 6:51 PM, Frank Rowand <frowand.list@...il.com> wrote:
>>>>> On 10/17/17 14:46, Rob Herring wrote:
>>>>>> On Tue, Oct 17, 2017 at 4:32 PM, Alan Tull <atull@...nel.org> wrote:
>>>>>>> On Mon, Aug 21, 2017 at 10:16 AM, Rob Herring <robh@...nel.org> wrote:
>>>>>>> 
>>>>>>> Hi Rob,
>>>>>>> 
>>>>>>>> With dependencies on a statically allocated full path name converted to
>>>>>>>> use %pOF format specifier, we can store just the basename of node, and
>>>>>>>> the unflattening of the FDT can be simplified.
>>>>>>>> 
>>>>>>>> This commit will affect the remaining users of full_name. After
>>>>>>>> analyzing these users, the remaining cases should only change some print
>>>>>>>> messages. The main users of full_name are providing a name for struct
>>>>>>>> resource. The resource names shouldn't be important other than providing
>>>>>>>> /proc/iomem names.
>>>>>>>> 
>>>>>>>> We no longer distinguish between pre and post 0x10 dtb formats as either
>>>>>>>> a full path or basename will work. However, less than 0x10 formats have
>>>>>>>> been broken since the conversion to use libfdt (and no one has cared).
>>>>>>>> The conversion of the unflattening code to be non-recursive also broke
>>>>>>>> pre 0x10 formats as the populate_node function would return 0 in that
>>>>>>>> case.
>>>>>>>> 
>>>>>>>> Signed-off-by: Rob Herring <robh@...nel.org>
>>>>>>>> ---
>>>>>>>> v2:
>>>>>>>> - rebase to linux-next
>>>>>>>> 
>>>>>>>> drivers/of/fdt.c | 69 +++++++++-----------------------------------------------
>>>>>>>> 1 file changed, 11 insertions(+), 58 deletions(-)
>>>>>>> 
>>>>>>> I've just updated to the latest next branch and am finding problems
>>>>>>> applying overlays.   Reverting this commit alleviates things.  The
>>>>>>> errors I get are:
>>>>>>> 
>>>>>>> [   88.498704] OF: overlay: Failed to apply prop @/__symbols__/clk_0
>>>>>>> [   88.513447] OF: overlay: apply failed '/__symbols__'
>>>>>>> [   88.518423] create_overlay: Failed to create overlay (err=-12)
>>>>>> 
>>>>>> Frank's series with overlay updates should fix this.
>>>>> 
>>>>> Yes, it does:
>>>>> 
>>>>>  [PATCH v3 11/12] of: overlay: remove a dependency on device node full_name
>>>> 
>>>> Thanks for the fast response.  I fetched the dt/next branch to test
>>>> this but there are sufficient changes that Pantelis' "OF: DT-Overlay
>>>> configfs interface (v7)" is broken now.  I've been adding that
>>>> downstream since 4.4.  We're using it as an interface for applying
>>>> overlays to program FPGAs.  If we fix it again, is there any chance
>>>> that can go upstream now?
>>> 
>>> With a drive-by posting once every few years, no.
>>> 
>> 
>> I take offense to that. There's nothing changed in the patch for years.
>> Reposting the same patch without changes would achieve nothing.
> 
> Are you still expecting review comments on it or something?
> Furthermore, If something is posted infrequently, then I'm not
> inclined to comment or care if the next posting is going to be after I
> forget what I previously said (which is not very long).
> 
> I'm just saying, don't expect to forward port, post and it will be
> accepted. Below is minimally one of the issues that needs to be
> addressed.
> 
>>> The issue remains that the kernel is not really setup to deal with any
>>> random property or node to be changed at any point in run-time. I
>>> think there needs to be some restrictions around what the overlays can
>>> touch. We can't have it be wide open and then lock things down later
>>> and break users. One example of what you could do is you can only add
>>> sub-trees to whitelisted nodes. That's probably acceptable for your
>>> usecase.
>>> 
>> 
>> Defining what can and what cannot be changed is not as trivial as a
>> list of white-listed nodes.
> 
> No, but we have to start somewhere and we are not starting with any
> change allowed anywhere at anytime. If that is what people want, then
> they are going to get to maintain that out of tree.
> 

I am still not sold on this ‘dangerous’ idea. No-one is crazy enough to
suggest overlays to be loadable by an unprivileged user. It’s exactly the
same ‘danger’ as loading a kernel module, while is sure capable of much
greater mischief.

>> In some cases there is a whole node hierarchy being inserted (like in
>> a FPGA).
> 
> Yes, so you'd have a target fpga region. That sounds fine to me. Maybe
> its not a static whitelist, but drivers have to register target
> nodes/paths.
> 
>> In others, it's merely changing a status property to "okay" and
>> a few device parameters.
> 
> That seems fine too. Disabled nodes could be allowed. But what if you
> add/change properties on a node that is not disabled? Once a node is
> enabled, who is responsible for registering the device?
> 
> What about changing a node from enabled to disabled? The kernel would
> need to handle that or not allow it.
> 

So it seems a simple whitelist won’t cut it. We’re already talking about
special casing for this or that property.

My argument is that this kind of validation is not part of the core-device tree,
but instead is a policy decision different for each board.
 
>> The real issue is that the kernel has no way to verify that a given
>> device tree, either at boot time or at overlay application time, is
>> correct.
>> 
>> When the tree is wrong at boot-time you'll hang (if you're lucky).
>> If the tree is wrong at run-time you'll get some into some unidentified
>> funky state.
> 
> Or have some security hole or a mechanism for userspace to crash the system.
> 

User-space as in regular users should never have enough privileges to apply an
overlay, same as in loading a kernel module.

>> Finally what is, and what is not 'correct' is not for the kernel to
>> decide arbitrarily, it's a matter of policy, different for each
>> use-case.
> 
> It is if the kernel will break doing so.
> 

I still haven’t seen a real example of the kernel breaking.

I have seen a lot of cases where the kernel is crashing due to the device
removal path being broken, but those are kernel bugs to fix, not something
to use to hold back functionality that people want to use.

> Rob

Regards

— Pantelis

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