linux-kernel - Re: [PATCH] arm64: traps: add dump instr before BUG in kernel

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Message-ID: <20210930084247.GC23389@willie-the-truck>
Date:   Thu, 30 Sep 2021 09:42:47 +0100
From:   Will Deacon <will@...nel.org>
To:     Chen Lin <chen45464546@....com>
Cc:     catalin.marinas@....com, mark.rutland@....com, joey.gouly@....com,
        maz@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Chen Lin <chen.lin5@....com.cn>
Subject: Re: [PATCH] arm64: traps: add dump instr before BUG in kernel

On Wed, Sep 29, 2021 at 09:29:46PM +0800, Chen Lin wrote:
> From: Chen Lin <chen.lin5@....com.cn>
> 
> we should dump the real instructions before BUG in kernel mode, and
> compare this to the instructions from objdump.
> 
> Signed-off-by: Chen Lin <chen.lin5@....com.cn>
> ---
>  arch/arm64/kernel/traps.c |    7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> index b03e383..621a9dd 100644
> --- a/arch/arm64/kernel/traps.c
> +++ b/arch/arm64/kernel/traps.c
> @@ -495,7 +495,12 @@ void do_undefinstr(struct pt_regs *regs)
>  	if (call_undef_hook(regs) == 0)
>  		return;
>  
> -	BUG_ON(!user_mode(regs));
> +	if (!user_mode(regs)) {
> +		pr_emerg("Undef instruction in kernel, dump instr:");
> +		dump_kernel_instr(KERN_EMERG, regs);
> +		BUG();
> +	}

Hmm, I'm not completely convinced about this as the instruction in the
i-cache could be completely different. I think the PC value (for addr2line)
is a lot more useful, and we should be printing that already.

Maybe you can elaborate on a situation where this information was helpful?

Thanks,

Will