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To: Anton Blanchard <anton@samba.org>, benh@kernel.crashing.org,
 paulus@samba.org
From: Michael Ellerman <mpe@ellerman.id.au>
Subject: Re: powerpc: Print instruction when logging unhandled exceptions
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Date: Thu,  2 Oct 2014 17:14:46 +1000 (EST)
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On Thu, 2014-25-09 at 05:05:45 UTC, Anton Blanchard wrote:
> It is often useful to see the instruction that caused an unhandled
> exception.
> 
> diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
> index 0dc43f9..27e30c8 100644
> --- a/arch/powerpc/kernel/traps.c
> +++ b/arch/powerpc/kernel/traps.c
> @@ -252,9 +252,18 @@ void _exception(int signr, struct pt_regs *regs, int code, unsigned long addr)
>  	}
>  
>  	if (show_unhandled_signals && unhandled_signal(current, signr)) {
> +		u32 __user *nia = (u32 __user *)regs->nip;
> +		u32 insn = 0;
> +
> +		pagefault_disable();
> +		if (!access_ok(VERIFY_READ, nia, sizeof(*nia)) ||
> +		    __get_user_inatomic(insn, nia))
> +			insn = 0xffffffffUL;
> +		pagefault_enable();

Can you add a comment explaining that interesting construct?

Looks like we do something similar in the perf callchain code, though without
the access_ok() check?

cheers