Re: [tip:x86/urgent] x86, trace: Fix CR2 corruption when tracing page faults

[Peter Zijlstra]

On Tue, Mar 04, 2014 at 04:03:25PM -0800, tip-bot for Jiri Olsa wrote:

FWIW I also prefer this patch.

> @@ -1252,9 +1249,11 @@ dotraplinkage void __kprobes
>  do_page_fault(struct pt_regs *regs, unsigned long error_code)
>  {
>  	enum ctx_state prev_state;
> +	/* Get the faulting address: */
> +	unsigned long address = read_cr2();
>  
>  	prev_state = exception_enter();
> -	__do_page_fault(regs, error_code);
> +	__do_page_fault(regs, error_code, address);
>  	exception_exit(prev_state);
>  }
>  
> @@ -1271,9 +1270,16 @@ dotraplinkage void __kprobes
>  trace_do_page_fault(struct pt_regs *regs, unsigned long error_code)
>  {
>  	enum ctx_state prev_state;
> +	/*
> +	 * The exception_enter and tracepoint processing could
> +	 * trigger another page faults (user space callchain
> +	 * reading) and destroy the original cr2 value, so read
> +	 * the faulting address now.
> +	 */
> +	unsigned long address = read_cr2();
>  
>  	prev_state = exception_enter();
>  	trace_page_fault_entries(regs, error_code);
> -	__do_page_fault(regs, error_code);
> +	__do_page_fault(regs, error_code, address);
>  	exception_exit(prev_state);
>  }

How about also marking these two functions as notrace? That would also
avoid getting __mcount calls from before we read CR2.
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