How to trap unaligned memory access?

Question

I am working on a pet open-source project that implements some stream cipher algorithms and I am having trouble with a bug triggered only when I run it on an ARM processor.

Answer 1

Linux can do the fixup for you or warn about the access.

You can enable the behavior in /proc/cpu/alignment, see http://www.mjmwired.net/kernel/Documentation/arm/mem_alignment for an explanation of the different values.

0 - Do nothing (default behavior)
1 - Warning in kernel-log with PC and Memory-Address printed.
2 - Fixup error
3 - Warn and Fixup
4 - Send a SIGBUS to the process
5 - Send SIGBUS and output Warning

Answer 2

ARM Linux maintains a list of alignment handler exceptions,

$ cat /proc/cpu/alignment 
User:           0
System:         0
Skipped:        0
Half:           0
Word:           0
DWord:          0
Multi:          0
User faults:    0 (ignored)

It is only active with procfs, but it is hard to imagine a system without procfs. The specific code handling this is in alignment.c. You can use echo 3 > /proc/cpu/alignment to have Linux fixup the instruction and provide some dmesg output. Generally, handling un-aligned accesses through emulation is very in-efficient. It is better to correct the code. The signal option with a debugger attached should give some clue as to the source of the exception.

Read the manual. ;-)