Re: [OSS-Tools] [PATCH] libdt: fix of_get_devicepath looking up sibling if device unavailable

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hello Uwe,

On 07.06.23 10:55, Uwe Kleine-König wrote:
> Hello Ahmad,
> 
> On Wed, Jun 07, 2023 at 10:08:18AM +0200, Ahmad Fatoum wrote:
>> of_get_devicepath code flow is split into two:
>>
>>   A) Either the device tree node in question has a direct udev_device
>>      associated with it
>>
>>   B) Or we assume it's a partition and lookup udev_device for the parent
>>      first, before finding a child udev_device or setting a partition
>>      offset within the parent udev_device.
>>
>> Since v2017.03.0, we have had a fallthrough from case A into case B:
>> If we have a udev_device, but it's neither a EEPROMs, MTDs or block
>> device, we just consider it a partition. This is problematic, because
>> this may result in us pointing at a very different device:
>>
>>   - backend points at a SD-Card host. Host is enabled, but SD-Card
>>     is not inserted, so no block device
>>
>>   - case A fails, so it's assumed it's a partition and case B
>>     uses parent SoC bus to lookup appropriate device
>>
>>   - We fall through into the second device_find_block_device, which
>>     will take the first matching block device across the SoC. So
>>     we could end up with the eMMC: a completely different device
>>     than what was pointed at.
> 
> So another surprise is that device_find_block_device() recurses to find
> a device when starting on /soc, isn't it? Is this worth addressing?

I don't know what I'd break if I limit iteration depth and I don't know
what else I could do to curtail this..

> 
>> Fixes: 929ed64cb42f ("of_get_devicepath: make partition finding more robust")
>> Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
>> ---
>>  src/libdt.c | 6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/src/libdt.c b/src/libdt.c
>> index e54d7fb5649d..7b99efe5b2de 100644
>> --- a/src/libdt.c
>> +++ b/src/libdt.c
>> @@ -2492,9 +2492,11 @@ int of_get_devicepath(struct device_node *partition_node, char **devpath, off_t
>>  		}
>>  
>>  		/*
>> -		 * If we found a device but couldn't classify it above, we fall
>> -		 * through.
>> +		 * If we find a udev_device but couldn't classify it above,
>> +		 * it's an error. Falling through would mean to handle it as a
>> +		 * partition and could lead us to return an arbitrary sibling device
>>  		 */
>> +		return -ENODEV;
> 
> I don't remember the details of 929ed64cb42f any more, but probably I
> didn't have a specific case that were fixed by that commit. Your
> rationale makes sense.
> 
> Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

Thanks,
Ahmad

> 
> Best regards
> Uwe
> 

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