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Subject: Re: [PATCH v2 5/5] of/fdt: only store the device node basename in
 full_name
From:   Pantelis Antoniou <pantelis.antoniou@konsulko.com>
In-Reply-To: <59E7CBAF.9080501@gmail.com>
Date:   Thu, 19 Oct 2017 11:41:16 +0300
Cc:     Rob Herring <robh@kernel.org>, Alan Tull <atull@kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        David Laight <David.Laight@aculab.com>,
        linux-fpga@vger.kernel.org
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To:     Frank Rowand <frowand.list@gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
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Hi Frank,

> On Oct 19, 2017, at 00:46 , Frank Rowand <frowand.list@gmail.com> =
wrote:
>=20
> On 10/18/17 11:30, Rob Herring wrote:
>> On Wed, Oct 18, 2017 at 10:53 AM, Pantelis Antoniou
>> <pantelis.antoniou@konsulko.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@kernel.org> =
wrote:
>>>>> On Tue, Oct 17, 2017 at 6:51 PM, Frank Rowand =
<frowand.list@gmail.com> wrote:
>>>>>> On 10/17/17 14:46, Rob Herring wrote:
>>>>>>> On Tue, Oct 17, 2017 at 4:32 PM, Alan Tull <atull@kernel.org> =
wrote:
>>>>>>>> On Mon, Aug 21, 2017 at 10:16 AM, Rob Herring <robh@kernel.org> =
wrote:
>>>>>>>>=20
>>>>>>>> Hi Rob,
>>>>>>>>=20
>>>>>>>>> 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.
>>>>>>>>>=20
>>>>>>>>> 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.
>>>>>>>>>=20
>>>>>>>>> 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.
>>>>>>>>>=20
>>>>>>>>> Signed-off-by: Rob Herring <robh@kernel.org>
>>>>>>>>> ---
>>>>>>>>> v2:
>>>>>>>>> - rebase to linux-next
>>>>>>>>>=20
>>>>>>>>> drivers/of/fdt.c | 69 =
+++++++++-----------------------------------------------
>>>>>>>>> 1 file changed, 11 insertions(+), 58 deletions(-)
>>>>>>>>=20
>>>>>>>> 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:
>>>>>>>>=20
>>>>>>>> [   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=3D-12)
>>>>>>>=20
>>>>>>> Frank's series with overlay updates should fix this.
>>>>>>=20
>>>>>> Yes, it does:
>>>>>>=20
>>>>>>  [PATCH v3 11/12] of: overlay: remove a dependency on device node =
full_name
>>>>>=20
>>>>> 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?
>>>>=20
>>>> With a drive-by posting once every few years, no.
>>>>=20
>>>=20
>>> I take offense to that. There's nothing changed in the patch for =
years.
>>> Reposting the same patch without changes would achieve nothing.
>>=20
>> 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).
>>=20
>> 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.
>>=20
>=20
>=20
>>>> 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.
>=20
> That paragraph is key to any discussion of accepting code to apply =
overlays.
> Solving that issue has been stated to be a gating factor for such code =
from
> the beginning of overlay development.
>=20

Overlays are not only used only for cases where you have external =
expansion boards, or
FPGAs where every change is contained under a few designated nodes, so =
that=E2=80=99s why I=E2=80=99m
pushing for a in-kernel validator that=E2=80=99s more flexible than a =
single whitelist.

An eBPF validator would handle a whitelist trivially easy, and would be =
flexible enough
for any other more complicated use case.

>=20
>>>> One example of what you could do is you can only add
>>>> sub-trees to whitelisted nodes. That's probably acceptable for your
>>>> usecase.
>>>>=20
>>>=20
>>> Defining what can and what cannot be changed is not as trivial as a
>>> list of white-listed nodes.
>>=20
>> 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.
>>=20
>>> In some cases there is a whole node hierarchy being inserted (like =
in
>>> a FPGA).
>>=20
>> 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.
>>=20
>>> In others, it's merely changing a status property to "okay" and
>>> a few device parameters.
>>=20
>> 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?
>>=20
>> What about changing a node from enabled to disabled? The kernel would
>> need to handle that or not allow it.
>>=20
>>> 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.
>>>=20
>>> 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.
>>=20
>> Or have some security hole or a mechanism for userspace to crash the =
system.
>>=20
>>> 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.
>>=20
>> It is if the kernel will break doing so.
>>=20
>> Rob
>>=20
>=20


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