Re: [PATCH 2/2] x86, perf: Optimize stack walk user accesses

[Peter Zijlstra]

On Mon, Oct 19, 2015 at 03:54:29PM -0700, Andi Kleen wrote:
> @@ -2307,7 +2312,13 @@ perf_callchain_user(struct perf_callchain_entry *entry, struct pt_regs *regs)
>  		frame.next_frame	     = NULL;
>  		frame.return_address = 0;
>  
> -		bytes = copy_from_user_nmi(&frame, fp, sizeof(frame));
> +		if (!access_ok(VERIFY_READ, fp, 16))
> +			break;
> +
> +		bytes = __copy_from_user_nmi(&frame.next_frame, fp, 8);
> +		if (bytes != 0)
> +			break;
> +		bytes = __copy_from_user_nmi(&frame.return_address, fp+8, 8);
>  		if (bytes != 0)
>  			break;
>  

The previous patch that introduces this function states that any caller
must have pagefault_disable() or be from interrupt context.

Perf can call this function from !interrupt context (imagine a
tracepoint or other software event), should we therefore not add a
pagefault_disable()/enable() pair around the entire while() loop?
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