Re: [PATCH v1 2/2] perf/core: Fake regs for leaked kernel samples

[peterz]

On Fri, Jul 31, 2020 at 10:56:17AM +0800, Jin Yao wrote:
> @@ -6973,7 +6973,8 @@ static struct perf_callchain_entry __empty_callchain = { .nr = 0, };
>  struct perf_callchain_entry *
>  perf_callchain(struct perf_event *event, struct pt_regs *regs)
>  {
> -	bool kernel = !event->attr.exclude_callchain_kernel;
> +	bool kernel = !event->attr.exclude_callchain_kernel &&
> +		      !event->attr.exclude_kernel;

This seems weird; how can we get there. Also it seems to me that if you
have !exclude_callchain_kernel you already have permission for kernel
bits, so who cares.

>  	bool user   = !event->attr.exclude_callchain_user;
>  	/* Disallow cross-task user callchains. */
>  	bool crosstask = event->ctx->task && event->ctx->task != current;
> @@ -6988,12 +6989,39 @@ perf_callchain(struct perf_event *event, struct pt_regs *regs)
>  	return callchain ?: &__empty_callchain;
>  }
>  
> +static struct pt_regs *leak_check(struct perf_event_header *header,
> +				  struct perf_event *event,
> +				  struct pt_regs *regs)
> +{
> +	struct pt_regs *regs_fake = NULL;
> +
> +	if (event->attr.exclude_kernel && !user_mode(regs)) {
> +		if (!(current->flags & PF_KTHREAD)) {
> +			regs_fake = task_pt_regs(current);
> +			if (!user_mode(regs_fake)) {
> +				regs_fake = NULL;
> +				instruction_pointer_set(regs, -1L);
> +			}
> +		} else
> +			instruction_pointer_set(regs, -1L);

That violates coding style, also:

		if (!(current->flags & PF_KTHREAD)) {
			regs_fake = task_pt_regs(current);
			if (!user_mode(regs_fake)) /* is this not a BUG?  */
				regs_fake = NULL;
		}

		if (!regs_fake)
			instruction_pointer_set(regs, -1L);

Seems simpler to me.

> +		if ((header->misc & PERF_RECORD_MISC_CPUMODE_MASK) ==
> +		     PERF_RECORD_MISC_KERNEL) {
> +			header->misc &= ~PERF_RECORD_MISC_CPUMODE_MASK;
> +			header->misc |= PERF_RECORD_MISC_USER;
> +		}

Why the conditional? At this point it had better be unconditionally
user, no?

		headers->misc &= ~PERF_RECORD_MISC_CPUMODE_MASK;
		headers->misc |= PERF_RECORD_MISC_USER;

> +	}
> +
> +	return regs_fake;
> +}