linux-kernel - Re: [PATCH v3 04/22] x86/doublefault: Make memmove() notrace/NOKPROBE

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Date:   Wed, 19 Feb 2020 17:04:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        mingo@...nel.org, joel@...lfernandes.org,
        gregkh@...uxfoundation.org, gustavo@...eddedor.com,
        tglx@...utronix.de, paulmck@...nel.org, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        luto@...nel.org, tony.luck@...el.com, frederic@...nel.org,
        dan.carpenter@...cle.com, mhiramat@...nel.org
Subject: Re: [PATCH v3 04/22] x86/doublefault: Make memmove() notrace/NOKPROBE

On Wed, Feb 19, 2020 at 04:57:15PM +0100, Peter Zijlstra wrote:
> Something like so, I suppose...
> 
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 6ef00eb6fbb9..543de932dc7c 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -350,14 +350,20 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code, unsign
>  		regs->ip == (unsigned long)native_irq_return_iret)
>  	{
>  		struct pt_regs *gpregs = (struct pt_regs *)this_cpu_read(cpu_tss_rw.x86_tss.sp0) - 1;
> +		unsigned long *dst = &gpregs->ip;
> +		unsigned long *src = (void *)regs->dp;
> +		int i, count = 5;
>  
>  		/*
>  		 * regs->sp points to the failing IRET frame on the
>  		 * ESPFIX64 stack.  Copy it to the entry stack.  This fills
>  		 * in gpregs->ss through gpregs->ip.
> -		 *
>  		 */
> -		memmove(&gpregs->ip, (void *)regs->sp, 5*8);
> +		for (i = 0; i < count; i++) {
> +			int idx = (dst <= src) ? i : count - i;

That's an off-by-one for going backward; 'count - 1 - i' should work
better, or I should just stop typing for today ;-)

> +			dst[idx] = src[idx];
> +		}
> +
>  		gpregs->orig_ax = 0;  /* Missing (lost) #GP error code */
>  
>  		/*