Re: [PATCH 3/3] perf, x86, lbr: Demand proper privileges for PERF_SAMPLE_BRANCH_KERNEL

[Peter Zijlstra <peterz@infradead.org>]

On Thu, May 16, 2013 at 11:09:16AM +0200, Peter Zijlstra wrote:
> How about something like the below? It also adds the branch flags Mikey wanted
> for PowerPC.

The asymmetry is unfortunate, but I think its more useful to have the from
kernel branch target than it is not to have it. This way you at least know
there was a kernel entry/exit and where you've continued.

Without either branch to kernel and branch from kernel entries you'd be
wondering WTF happend to your control flow.

> ---
>  arch/x86/kernel/cpu/perf_event_intel_lbr.c | 12 +++++++++---
>  include/linux/perf_event.h                 | 10 +++++++---
>  2 files changed, 16 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event_intel_lbr.c b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> index d978353..f44d635 100644
> --- a/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> @@ -585,17 +585,23 @@ intel_pmu_lbr_filter(struct cpu_hw_events *cpuc)
>  
>  		/* if type does not correspond, then discard */
>  		if (type == X86_BR_NONE || (br_sel & type) != type) {
> -			cpuc->lbr_entries[i].from = 0;
> +			cpuc->lbr_entries[i].__delete = 1;
>  			compress = true;
>  		}
> +
> +		/* hide kernel addresses if we're not privileged  */
> +		if (!(br_sel & X86_BR_KERNEL) && kernel_ip(from)) {
> +			cpuc->lbr_entries[i].from = -1L;
> +			cpuc->lbr_entries[i].invalid_from = 1;
> +		}
>  	}
>  
>  	if (!compress)
>  		return;
>  
> -	/* remove all entries with from=0 */
> +	/* remove all entries with __delete */
>  	for (i = 0; i < cpuc->lbr_stack.nr; ) {
> -		if (!cpuc->lbr_entries[i].from) {
> +		if (cpuc->lbr_entries[i].__delete) {
>  			j = i;
>  			while (++j < cpuc->lbr_stack.nr)
>  				cpuc->lbr_entries[j-1] = cpuc->lbr_entries[j];
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index f463a46..7acf1c9 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -77,9 +77,13 @@ struct perf_raw_record {
>  struct perf_branch_entry {
>  	__u64	from;
>  	__u64	to;
> -	__u64	mispred:1,  /* target mispredicted */
> -		predicted:1,/* target predicted */
> -		reserved:62;
> +	__u64	mispred:1,	/* target mispredicted		*/
> +		predicted:1,	/* target predicted		*/
> +		invalid_to:1,	/* @to isn't to be trusted	*/
> +		invalid_from:1, /* @from isn't to be trusted	*/
> +		reserved:59,
> +		__delete:1;	/* Implementation; userspace should
> +				   always see a 0 		*/
>  };
>  
>  /*
>