[Olsr-dev] alignment faults on ARM processors

Henning Rogge (spam-protected)
Mon Aug 13 13:55:45 CEST 2012

On 08/13/2012 10:21 AM, Kees-Jan Hermans wrote:
> I've had this problem on Marvell as well (with another application) - I
> resorted to create macroes for such casts that are casts on most
> platforms, and memcpy()s on ARM.

Could it be that the Marvell CPU wants to have an alignment of more than 
4 bytes?

Henning Rogge

> KJ
> On Thu, 2012-07-26 at 09:11 -0500, Peter Bigot wrote:
>> I was recently asked to validate OLSRD on a custom embedded platform
>> involving an AT91 ARM processor and Linux 2.6.39 under OpenEmbedded
>> Classic.  While the 0.5.5 release worked, 0.5.6-r8 and 0.6.3 produced
>> a cascade of errors attempting to route to invalid IP addresses.
>> A kernel error message indicated unaligned accesses.  I tracked this
>> down to several uses of the following idiom in src/lq_packet.h:
>>    *var = ntohl(**((const uint32_t **)p));
>> where p is a pointer to a pointer to const uint8_t.  It seems that in
>> recent Linux kernels CONFIG_ALIGNMENT_TRAP is enabled by default on
>> these ARM chips, and the version of Sourcery_G++_Lite required on this
>> platform does not support -mno-alignment-traps.  In practice, the
>> pointer at this point did not satisfy the alignment requirements of
>> the processor, and the kernel did not provide a recovery.
>> I worked around this by replacing the assignment with an intermediate
>> copy from the unaligned buffer into a 4-byte region in a local union
>> with a uint32_t; similarly for the int16_t accesses.  I regret that I
>> can't provide the patch (which is both trivial and a hack anyway), but
>> thought a heads-up might be helpful for others trying to make OLSRD
>> work in a similar environment.
>> Peter

Diplom-Informatiker Henning Rogge , Fraunhofer-Institut für
Kommunikation, Informationsverarbeitung und Ergonomie FKIE
Kommunikationssysteme (KOM)
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