linux-kernel - Re: [PATCH v2] arm64: add alignment fault hanling

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Message-ID: <20160216103104.GC14509@arm.com>
Date:	Tue, 16 Feb 2016 10:31:04 +0000
From:	Will Deacon <will.deacon@....com>
To:	EunTaik Lee <eun.taik.lee@...sung.com>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	"vladimir.murzin@....com" <vladimir.murzin@....com>,
	"suzuki.poulose@....com" <suzuki.poulose@....com>,
	"riandrews@...roid.com" <riandrews@...roid.com>,
	"james.morse@....com" <james.morse@....com>,
	"salyzyn@...roid.com" <salyzyn@...roid.com>,
	"Dave.Martin@....com" <Dave.Martin@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] arm64: add alignment fault hanling

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