Re: [PATCH 02/13] x86/debug: Allow a single level of #DB recursion

[Josh Poimboeuf]

On Wed, Sep 02, 2020 at 03:25:51PM +0200, Peter Zijlstra wrote:
> @@ -863,6 +849,18 @@ static void handle_debug(struct pt_regs
>  static __always_inline void exc_debug_kernel(struct pt_regs *regs,
>  					     unsigned long dr6)
>  {
> +	/*
> +	 * Disable breakpoints during exception handling; recursive exceptions
> +	 * are exceedingly 'fun'.
> +	 *
> +	 * Since this function is NOKPROBE, and that also applies to
> +	 * HW_BREAKPOINT_X, we can't hit a breakpoint before this (XXX except a
> +	 * HW_BREAKPOINT_W on our stack)
> +	 *
> +	 * Entry text is excluded for HW_BP_X and cpu_entry_area, which
> +	 * includes the entry stack is excluded for everything.
> +	 */

I know this comment was copy/pasted, but I had to stare at the last
paragraph like one of those 3D paintings they used to have at the mall.

Recommended rewording:

	 * HW_BREAKPOINT_X is disallowed for entry text; all breakpoints
	 * are disallowed for cpu_entry_area (which includes the entry
	 * stack).

-- 
Josh