linux-kernel - Re: [PATCH] arm64: setup return path for el1

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Message-ID: <20141223155714.GC21816@e104818-lin.cambridge.arm.com>
Date:	Tue, 23 Dec 2014 15:57:14 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Min-Hua Chen <orca.chen@...il.com>
Cc:	Will Deacon <Will.Deacon@....com>,
	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	Kevin Hilman <khilman@...aro.org>,
	Larry Bassel <larry.bassel@...aro.org>,
	Laura Abbott <lauraa@...eaurora.org>,
	"zhangwm@...vell.com" <zhangwm@...vell.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] arm64: setup return path for el1_undef

On Tue, Dec 23, 2014 at 03:15:10PM +0000, Min-Hua Chen wrote:
> Setup return path for el1_undef since el1_undef may
> be handled by handlers.

Did you find a real issue or it was just code inspection.

> asmlinkage void __exception do_undefinstr(struct pt_regs *regs)
> {
>     siginfo_t info;
>     void __user *pc = (void __user *)instruction_pointer(regs);
> 
>     /* check for AArch32 breakpoint instructions */
>     if (!aarch32_break_handler(regs))
>         return;
> 
>     if (call_undef_hook(regs) == 0)
>         return;
> 
>     ...
> }
> Signed-off-by: Min-Hua Chen <orca.chen@...il.com>
> ---
>  arch/arm64/kernel/entry.S |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index fd4fa37..86ebec5 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -313,7 +313,8 @@ el1_undef:
>       */
>      enable_dbg
>      mov    x0, sp
> -    b    do_undefinstr
> +    bl    do_undefinstr
> +    kernel_exit 1
>  el1_dbg:
>      /*
>       * Debug exception handling

I don't think this is needed. The code is pretty convoluted but for an
EL1 undefined exception we should never return from do_undefinstr(). The
call_undef_hook() function returns 1 if !user_mode(regs) and this should
cause a kernel panic. Basically we do not allow any kind of undefined
instructions in the arm64 kernel.

-- 
Catalin
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