UBI: ignore/overwrite old data/PEBs after flashing

[Rafał Miłecki]

On 16 October 2014 13:27, Artem Bityutskiy <dedekind1 at gmail.com> wrote:
> On Thu, 2014-10-16 at 13:17 +0200, Rafał Miłecki wrote:
>> On 16 October 2014 12:40, Artem Bityutskiy <dedekind1 at gmail.com> wrote:
>> > On Thu, 2014-10-16 at 12:29 +0200, Rafał Miłecki wrote:
>> >> On 16 October 2014 12:09, Artem Bityutskiy <dedekind1 at gmail.com> wrote:
>> >> > Then the solution would be to pad your image with 0xFFs and write it.
>> >> > UBI will notice PEBs which do not contain UBI headers and will erase
>> >> > them in background.
>> >>
>> >> That would be trivial trick to implement, however it's far from being
>> >> user friendly. Most devices have 128 MiB flash and our firmware
>> >> (OpenWrt) is only ~4 MiB big. It would be a waste of
>> >> space/bandwidth/time to provide firmware images padded with over 120
>> >> MiB of 0xFF.
>> >
>> > Right, this is a work-around, and it is OK for a work-around to be
>> > suboptimal, generally speaking.
>> >
>> > If you want a more optimal work-around, it will be more intrusive. One
>> > idea would be to can create a small specialized kernel driver which will
>> > start before UBI and clean-up your flash: erase PEBs you specify it.
>> > Something like 'mtd/flashcleaner.ko'.
>>
>> I'm not sure if something like this is possible. My driver would need
>> to have a very good understanding of UBI to know which PEBs to leave
>> and which to clean. I would need to find out which PEBs were recently
>> flashed (are part of the just-installed firmware) and which are left
>> from the previous installation.
>
> Why? All you need is to erase all PEBs which contain those "100MiB of
> rubbish". They will be sequential.

The problem is all my 118 MiB UBI partition consist of PEBs with UBI
headers. This is because I was using firmware with UBI partition
before flashing the new firmware.

So there is 118 MiB of space filled with UBI-header-ed PEBs, but only
first few first MiBs of PEBs are coming from the recently flashed
firmware. The rest of PEBs are old data (but they still contain UBI
headers).


> And of course this would be a one-time driver. You'd need to run it only
> on the very first boot.
>
> On the other hand, may be you can add an "wipe_rubbish" kind of UBI
> module parameter, which would make it to just erase any PEB containing
> "rubbish", i.e., anything without an EC header. My worry is that this
> could be potentially dangerous - if you have a PEB corrupted, and it
> contains precious data, UBI will just destroy the precious data instead
> of refusing the flash. So a feature like this would need to be used
> carefully.

If I could modify UBI this would be much easier. I was thinking about
appending a one special block to the ubinized image. This could be PEB
with UBI header containing some flag START_ERASING_FROM_HERE. Such a
flag could be removed after erasing rubbish to minimize the risk of
situation you described.

-- 
Rafał