Unaligned accesses are not detected by Raspberry PI version 1

Question

I'm performing a set of activities to make sure Redis runs well in a set of embedded systems, including the Raspberry PI. In order to fix certain code paths of Redis where unaligned memory accesses are performed (due to a change introduced in Redis 3.2) I'm trying to force the PI to either log a message on unaligned memory accesses or send a signal to the process when this happens. In this way I can both make sure that Redis will run well where unaligned accesses are a violation, and that it will run faster in platforms where instead such accesses can be performed but are slower. ARM v6, the one used in the PI v1, is apparently able to deal with unaligned memory accesses, so if I use following command to configure Linux in order to sent a signal to the process performing the unaligned access:

echo 4 > /proc/cpu/alignment

And then run the following program:

#include <stdio.h>
#include <stdint.h>

int main(int argc, char **argv) {
    char *buf = "foobareklsjdfklsjdfslkjfskdljfskdfjdslkjfdslkjfsd";
    uint32_t *l = (uint32_t*) (buf+1);
    printf("%p\n", l);
    printf("%d\n", (int)*l);
    return 0;
}

I can't see any signal received by the process, or the counters at /proc/cpu/alignment incrementing.

My guess is that this is due to ARM v6 ability to deal with unaligned addresses automatically, if a given CPU configuration flag is set. My question is, is my hypothesis correct? And if so, how to force a PI version 1 to actually raise an exception in case of unaligned accesses so that the Linux kernel can trap it and send a signal, log the access, and so forth, according to /proc/cpu/alignment settings?

EDIT: It is worth to note that not all the instructions can perform unaligned accesses even in ARM v6. For instance STMDB, STMFD, LDMDB, LDMEA and similar multiple words instructions will indeed raise an exception and will be trapped by the Linux kernel.

antirez antirez · Accepted Answer · 2017-02-17T14:09:38

I think I eventually found my answers:

Yes I'm correct, up to the word size ARM v6 (or greater) can silently handle the unaligned accesses so no trap is generated and is completely transparent for the Linux kernel. Nothing will be logged, nor the traps counter in /proc/cpu/alignment will be incremented.
AFAIK there is no way I can force the kernel to trap word-sized unaligned accesses, since to do that apparently the CPU should be configured in order to trap the unaligned addresses in every case, but the Linux kernel does not do that AFAIK, probably because there is alignment unsafe code inside the kernel itself. Checking the Linux kernel source code indeed one can see:
```
if (cpu_is_v6_unaligned()) {
         set_cr(__clear_cr(CR_A));
         ai_usermode = safe_usermode(ai_usermode, false);
}
```
What this means is that the SCTLR.A bit is always cleared, so no trap will be generated for unaligned accesses ARM v6 can handle.
There are a great deal of instructions that will still generate traps when used with unaligned addresses, for example multi store/load instructions, loading and storing of double values.
However, there are instructions that GCC (the version shipped in the default Raspberry Linux distribution) will happily produced that are not handled by the Linux kernel correctly, that will result in a SIGBUS generated even when /proc/cpu/alignment is set to fix the access.

So point number 4 basically means that, it is not a good idea to fix programs to run in ARM v6 just letting the Linux kernel handle unaligned addresses for us, even when the performance implications of unaligned addresses are not a problem: the program can still crash since not all the instructions are handled.

How to reliably find all the unaligned accesses in a program remains an open question AFAIK, since unfortunately, the otherwise wonderful valgrind program, never implemented this feature. In the past I had to use QEMU emulating Sparc, however this is a very slow process. Valgrind would be the trivial way to do that.

Unaligned accesses are not detected by Raspberry PI version 1

1 Answers