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Subject: Re: [PATCH] of: Don't allow __of_attached_node_sysfs() without
 CONFIG_SYSFS
To:     Rob Herring <robh@kernel.org>
Cc:     Marc Zyngier <maz@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        devicetree@vger.kernel.org,
        Android Kernel Team <kernel-team@android.com>
References: <20210820144722.169226-1-maz@kernel.org>
 <a67743f9-869b-28df-d714-db15da4ebe06@gmail.com>
 <YSPtI//SJh1CpHRP@robh.at.kernel.org>
 <bcb2e89f-9768-8435-35d9-d02140628b9a@gmail.com>
 <CAL_JsqJs2ayHFpOo7kS4K96Sy3xDkxeWYNMTDF9Ssd9D79LKng@mail.gmail.com>
From:   Frank Rowand <frowand.list@gmail.com>
Message-ID: <d7548302-7e70-9a93-750c-a4de359eac0b@gmail.com>
Date:   Mon, 23 Aug 2021 17:21:39 -0500
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On 8/23/21 5:14 PM, Rob Herring wrote:
> On Mon, Aug 23, 2021 at 4:27 PM Frank Rowand <frowand.list@gmail.com> wrote:
>>
>> On 8/23/21 1:46 PM, Rob Herring wrote:
>>> On Sun, Aug 22, 2021 at 11:01:15PM -0500, Frank Rowand wrote:
>>>> Hi Marc,
>>>>
>>>> On 8/20/21 9:47 AM, Marc Zyngier wrote:
>>>>> Trying to boot without SYSFS, but with OF_DYNAMIC quickly
>>>>> results in a crash:
>>>>>
>>>>> [    0.088460] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000070
>>>>> [...]
>>>>> [    0.103927] CPU: 1 PID: 1 Comm: swapper/0 Not tainted 5.14.0-rc3 #4179
>>>>> [    0.105810] Hardware name: linux,dummy-virt (DT)
>>>>> [  0.107147] pstate: 80000005 (Nzcv daif -PAN -UAO -TCO BTYPE=--)
>>>>> [    0.108876] pc : kernfs_find_and_get_ns+0x3c/0x7c
>>>>> [    0.110244] lr : kernfs_find_and_get_ns+0x3c/0x7c
>>>>> [...]
>>>>> [    0.134087] Call trace:
>>>>> [    0.134800]  kernfs_find_and_get_ns+0x3c/0x7c
>>>>> [    0.136054]  safe_name+0x4c/0xd0
>>>>> [    0.136994]  __of_attach_node_sysfs+0xf8/0x124
>>>>> [    0.138287]  of_core_init+0x90/0xfc
>>>>> [    0.139296]  driver_init+0x30/0x4c
>>>>> [    0.140283]  kernel_init_freeable+0x160/0x1b8
>>>>> [    0.141543]  kernel_init+0x30/0x140
>>>>> [    0.142561]  ret_from_fork+0x10/0x18
>>>>>
>>>>> While not having sysfs isn't a very common option these days,
>>>>> it is still expected that such configuration would work.
>>>>>
>>>>> Paper over it by bailing out from __of_attach_node_sysfs() if
>>>>> CONFIG_SYSFS isn't enabled.
>>>>
>>>> CONFIG_SYSFS should be automatically selected when CONFIG_OF_DYNAMIC
>>>> is enabled, and it should not be possible to disable CONFIG_SYSFS
>>>> in this case.
>>>
>>> That used to be true, but isn't now.
>>
>> OK.  I agree with you, but when I investigated the original patch
>> email I came to a different conclusion because of the way that I
>> used make menuconfig to debug the situation.
>>
>> It is true when I start with a .config created from 'make qcom_defconfig',
>> then select OF_UNITTEST, which is the only way I can see OF_DYNAMIC.  It
>> is the "if OF_UNITTEST" that means SYSFS can not be disabled.
> 
> Not really. Disabling SYSFS has nothing to do with the DT code. It's
> not super easy though. It required setting EXPERT and disabling
> CONFIGFS_FS and things selecting it (PCIE endpoint and USB gadget).
> 
>> If I start with the .config that Marc supplied, then make menuconfig
>> still does not show the OC_DYNAMIC option, but leaves it set since
>> it was already set.  In this case SYSFS remains disabled because
>> OF_UNITTEST is also disabled.
> 
> I don't see the relationship between SYSFS and OF_UNITTEST.

I don't either.  Other than the results of experimenting showing
that if I enable OF_UNITTEST then I can no long disable SYSFS.

> 
>> Using '/OF_DYNAMIC' from within make menuconfig, to get more info
>> about OF_DYNAMIC tells me that the prompt for OF_DYNAMIC is visible
>> if OF && OF_UNITTEST.  This is due to the "if OF_UNITTEST" in line 58
>> of drivers/of/Kconfig in the OF_DYNAMIC specification.
>>
>> Thus I can't figure out how to use make menuconfig to set OF_DYNAMIC
>> without setting OF_UNITTEST.  I tried setting OF_UNITTEST, then setting
>> OF_DYNAMIC, saving the changes, then another make menuconfig to
>> disable OF_UNITTEST, which then has the side effect of unsetting
>> OF_DYNAMIC.
> 
> Selecting OF_OVERLAY also.
> 
>> Would you accept a patch that removes the "if OF_UNITTEST" from
>> the Kconfig entry for OF_DYNAMIC?
> 
> I guess. The purpose of making it visible was for compile testing, so
> maybe make it 'if COMPILE_TEST' instead?

After thinking a bit, I didn't like my original suggestion.
I'll think some more, but I think that I will not follow up
on my original suggestion.

> 
> 
> Looking at occurrences of CONFIG_OF_DYNAMIC, there's more than I'd
> like. This for example is a common pattern:
> 
> drivers/spi/spi.c:      if (IS_ENABLED(CONFIG_OF_DYNAMIC))
> drivers/spi/spi.c-
> WARN_ON(of_reconfig_notifier_register(&spi_of_notifier));
> 
> Really, of_reconfig_notifier_register() should just return 0 if
> !IS_ENABLED(CONFIG_OF_DYNAMIC).

I'll add researching use of CONFIG_OF_DYNAMIC to my todo list.

-Frank

> 
> Rob
>