[U-Boot] [PATCH] ARM: fixed relocation using proper alignment

[Manfred Schlaegl]

On 2017-05-18 16:59, Lothar Waßmann wrote:
> Manfred Schlaegl <manfred.schlaegl at ginzinger.com> wrote:
> 
>> On 2017-05-17 06:13, Lokesh Vutla wrote:
>>>
>>>
>>> On Tuesday 16 May 2017 07:59 PM, Manfred Schlaegl wrote:
>>>> On 2017-05-11 08:53, Lokesh Vutla wrote:
>>>>>
>>>>>
>>>>> On Wednesday 10 May 2017 07:11 PM, Manfred Schlaegl wrote:
>>>>>> Using u-boot-2017.05 on i.MX6UL we ran into following problem:
>>>>>> Initially U-Boot could be started normally.
>>>>>> If we added one random command in configuration, the newly generated
>>>>>> image hung at startup (last output was DRAM:  256 MiB).
>>>>>>
>>>>>> We tracked this down to a data abort within relocation (relocated_code).
>>>>>>
> [...]
>>>> In a good case (rel_dyn_start aligned to 8 byte), u-boot is starting up normally
>>>> rel_dyn_start is 0x8785FC28
>>>> rel_dyn_end is 0x87857BD0
>>>> A dump of this memory area shows no abnormality
>>>>
>>>> In a bad case (same source, but rel_dyn_start aligned to 4 byte), the data abort happens
>>>> rel_dyn_start is 0x8785FC24
>>>> rel_dyn_end is 0x87857BCC
>>>> So we have the same size of 32856 bytes but a memory dump showed exactly one difference, which is
>>>> very interesting:
>>>>
>>>> At offset 0x610 (relative to rel_dyn_start) we have following difference
>>>> -00000610  30 3e 80 87 17 00 00 00  34 3e 80 87 00 00 00 00  |0>......4>......|
>>>> +00000610  30 3e 80 87 17 00 00 00  00 00 00 00 17 00 00 00  |0>..............|
>>>
>>> Looks like someone is corrupting the data(assuming). Is it all 0's just
>>> at this location or continuously after this?
>>
>> No. Above diff is the only difference of the good and bad case in memory located between
>> rel_dyn_start and rel_dyn_end.
>>
>> To see if it might be a corruption I compared the the rel_dyn with the created u-boot.img and
>> found the same difference
>> -00000610  30 3e 80 87 17 00 00 00  34 3e 80 87 17 00 00 00  |0>......4>......|   <--- generated image
>> +00000610  30 3e 80 87 17 00 00 00  00 00 00 00 17 00 00 00  |0>..............|   <--- memory dump
>>
>> So it must be some kind of corruption.
>>
> This can be caused by a static variable, that is written to prior to
> relocation. Since the .rel section overlays the .bss section, the write
> to a variable in the BSS will corrupt the relocation data.
> 

Yes! That's it!

Using a watchpoint I tracked the corruption down to an early write to a static variable in our custom code.

So finally: 
The whole thing was a problem in a custom modification and was solved there. It has no implication on u-boot itself.

Thanks a lot for your help and time!

Best regards
Manfred