ARMv7 unaligned access

[Denis Ovsienko via tcpdump-workers <tcpdump-workers () lists tcpdump org>]

> --- Begin Message ---
>
>
> From: Denis Ovsienko <denis () ovsienko info>
>
> Date: Sun, 21 Aug 2022 11:10:53 +0100
>
> Hello list.
>
> Based on some prior online research, I used to think that ARMv7 is a
> hardware that allows (by default or after simple tweaking) to terminate
> a userspace process when it tries to perform an unaligned memory
> access.  Which is exactly the behaviour wanted for the CI purposes.
>
> Not long ago I moved the linux-armv7l Buildbot worker to a true ARMv7
> board (RPI2B v1.1).  Consistent with the documentation [1], the kernel
> seems to have the right knob in place, which a boot-time script sets to
> the desired position:
>
> $ cat /proc/cpu/alignment
> User:           0
> System:         0 (0x0)
> Skipped:        0
> Half:           0
> Word:           0
> DWord:          0
> Multi:          0
> User faults:    4 (signal)
>
> However, this does not seem to have any effect, in that this short C
> program, which does trigger a SIGBUS/SIGILL on other OSes and
> architectures that do not like unaligned access, just completes
> successfully on linux-armv7l:
>
> ---------------------------------------------------------------------
> #include <stdint.h>
> #include <stddef.h>
> /*
> #include <sys/sysmips.h>
> */
>
> int main (void)
> {
> /*
>         sysmips (MIPS_FIXADE, 0);
> */
>         uint32_t *p;
>         union
>         {
>                 uint32_t u32[2];
>                 uint8_t u8[8];
>         } u;
>         u.u32[0] = 0;
>         u.u32[1] = 1;
>         p = (uint32_t *)(u.u8 + 0);
>         *p = 3;
>         p = (uint32_t *)(u.u8 + 1);
>         *p = 3;
>         p = (uint32_t *)(u.u8 + 2);
>         *p = 3;
>         p = (uint32_t *)(u.u8 + 3);
>         *p = 3;
>
>         uint8_t u8[20];
>         for (size_t i = 0; i < 10; i++)
>         {
>                 p = (uint32_t *)(u8 + i);
>                 *p = 4;
>         }
>
>         return 0;
> }
> ---------------------------------------------------------------------
>
> Is there anything obvious I am missing in these interpretations?
>
> 1: https://docs.kernel.org/arm/mem_alignment.html
>
> -- 
>     Denis Ovsienko
>
> --- End Message ---
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