Re: [PATCH 1/3] x86/debug: Fix BTF handling

[Peter Zijlstra]

On Tue, Oct 27, 2020 at 10:15:05AM +0100, Peter Zijlstra wrote:

> @@ -873,6 +866,20 @@ static __always_inline void exc_debug_ke
>  	 */
>  	WARN_ON_ONCE(user_mode(regs));
>  
> +	if (test_thread_flag(TIF_BLOCKSTEP)) {
> +		/*
> +		 * The SDM says "The processor clears the BTF flag when it
> +		 * generates a debug exception." but PTRACE_BLOCKSTEP requested
> +		 * it for userspace, but we just took a kernel #DB, so re-set
> +		 * BTF.
> +		 */
> +		unsigned long debugctl;
> +
> +		rdmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
> +		debugctl |= DEBUGCTLMSR_BTF;
> +		wrmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
> +	}
> +
>  	/*
>  	 * Catch SYSENTER with TF set and clear DR_STEP. If this hit a
>  	 * watchpoint at the same time then that will still be handled.

Masami, how does BTF interact with !optimized kprobes that single-step?

The best answer I can come up with is 'poorly' :/