[Olsr-dev] alignment faults on ARM processors

[Peter Bigot]

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