[PATCH v2] arm64: add alignment fault hanling

[will.deacon@arm.com (Will Deacon)]

On Tue, Feb 16, 2016 at 04:44:35AM +0000, EunTaik Lee wrote:
> Userspace memory is mapped as below:
> F2A7F000--F2A7FFFF Normal Memory
> F2A80000--F2A80FFF Device nGnRnE
> 
> And that userspace application makes a system call
> as below:
> 
> -009 |do_strncpy_from_user(inline)
> -009 |strncpy_from_user()
> -010 |getname_flags()
> -011 |user_path_at_empty()
> -012 |user_path_at()
> -013 |SYSC_faccessat(inline)
> -013 |sys_faccessat()
> -014 |__sys_trace(asm)
>  --> |exception
> 
> The string spans from 0xF2A7FFC1 to 0xF2A7FFFB.
> 
> When do_strncpy_from_user() reads the last (unsigned long)
> value, the alignement fault is triggered. The 8 byte
> from 0xF2A7FFC1 spans to the next page that is mapped as
> Device nGnRnE, which does not allow an unaligned access,
> causes the abort.
> 
> The instruction which caused the alignment fault is registered
> in the fixup table but the exception handler does not reach there.
> 
> This patch registers a alignment fault handler and fixes up the
> pc if appropriate.

As discussed with Catalin previously, we should solve this by adding a
guard page rather than handling the fault.

Will