Subject: [PATCH 0/2] arm: alignment trap tweaks

From: [email-masked] (Robin Murphy)
Date: Wed, 28 May 2014 16:21:42 +0100

On 07/05/14 10:51, Robin Murphy wrote:
> Hi all,
>
> Revisiting the alignment trap again thanks to an obscure corner case
> with NEON alignment hints - patch 1 is a repost from a while back as a
> ping, because modern userspaces really should just get a resounding
> "Don't do that!" if they manage to step outside the architecture.
> Patch 2 is the fix for cases where fixup does need to be turned back on
> for whatever reason.
>

Ping. Any comment on these? Admittedly it's a very hard-to-hit bug, but 
it's a bug all the same.

Thanks,
Robin.

> The following testcase compiled with -marm illustrates the problem:
> with fixup enabled, when the VLD1 instruction with the alignment hint
> set faults by performing a misaligned access, it fails to load the NEON
> registers as expected and eventually writes back nonsense to the base
> register, resulting in a misleading segfault on the next iteration.
>
> --->8---
>
> #include <stdint.h>
> #include <stdio.h>
>
> static uint8_t buffer[256], got[256];
>
> int main() {
> 	uint8_t *base = (uint8_t *)((intptr_t)buffer + 0x7 & ~0x7);
> 	void *p = base, *q = got;
> 	int i;
>
> 	for (i=0; i<64; i++)
> 		base[i] = i;
> 	for (i=0; i<4; i++) {
> 		asm volatile ("vld1.8 {d0,d1}, [%0]\n vst1.8 {d0,d1}, [%1]\n"
> 				: "=r"(p),"=r"(q) : "0"(p),"1"(q) : "d0","d1");
> 		printf("unaligned: %p [%d,%d,%d,...]\n", p++, got[0], got[1], got[2]);
> 	}
> 	p = base;
> 	for (i=0; i<4; i++) {
> 		asm volatile ("vld1.8 {d0,d1}, [%0:64]\n vst1.8 {d0,d1}, [%1]\n"
> 				: "=r"(p),"=r"(q) : "0"(p),"1"(q) : "d0","d1");
> 		printf("misaligned: %p [%d,%d,%d,...]\n", p++, got[0], got[1], got[2]);
> 	}
> 	return 0;
> }
>
> --->8---
>
> Robin Murphy (2):
>    arm: SIGBUS on unsupported ARMv6 unaligned accesses
>    arm: don't break misaligned NEON load/store
>
>   arch/arm/mm/alignment.c |    9 ++++++---
>   1 file changed, 6 insertions(+), 3 deletions(-)
>
> --
> 1.7.9.5
>
>
>
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