Re: [PATCH topic/core-for-CI] perf: Fix perf_event_validate_size() lockdep splat

[Rodrigo Vivi <rodrigo.vivi@intel.com>]

On Fri, Dec 15, 2023 at 08:22:17AM -0800, Lucas De Marchi wrote:
> From: Mark Rutland <mark.rutland@arm.com>
> 
> When lockdep is enabled, the for_each_sibling_event(sibling, event)
> macro checks that event->ctx->mutex is held. When creating a new group
> leader event, we call perf_event_validate_size() on a partially
> initialized event where event->ctx is NULL, and so when
> for_each_sibling_event() attempts to check event->ctx->mutex, we get a
> splat, as reported by Lucas De Marchi:
> 
>   WARNING: CPU: 8 PID: 1471 at kernel/events/core.c:1950 __do_sys_perf_event_open+0xf37/0x1080
> 
> This only happens for a new event which is its own group_leader, and in
> this case there cannot be any sibling events. Thus it's safe to skip the
> check for siblings, which avoids having to make invasive and ugly
> changes to for_each_sibling_event().
> 
> Avoid the splat by bailing out early when the new event is its own
> group_leader.
> 
> Fixes: 382c27f4ed28f803 ("perf: Fix perf_event_validate_size()")
> Closes: https://lore.kernel.org/lkml/20231214000620.3081018-1-lucas.demarchi@intel.com/
> Closes: https://lore.kernel.org/lkml/ZXpm6gQ%2Fd59jGsuW@xpf.sh.intel.com/
> Reported-by: Lucas De Marchi <lucas.demarchi@intel.com>
> Reported-by: Pengfei Xu <pengfei.xu@intel.com>
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Link: https://lkml.kernel.org/r/20231215112450.3972309-1-mark.rutland@arm.com
> [ cherry pick from tip/urgent heading to 6.7-rc6 ]
> Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>

Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

> ---
>  kernel/events/core.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index acfc5a569818..a64165af45c1 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -1947,6 +1947,16 @@ static bool perf_event_validate_size(struct perf_event *event)
>  				   group_leader->nr_siblings + 1) > 16*1024)
>  		return false;
>  
> +	/*
> +	 * When creating a new group leader, group_leader->ctx is initialized
> +	 * after the size has been validated, but we cannot safely use
> +	 * for_each_sibling_event() until group_leader->ctx is set. A new group
> +	 * leader cannot have any siblings yet, so we can safely skip checking
> +	 * the non-existent siblings.
> +	 */
> +	if (event == group_leader)
> +		return true;
> +
>  	for_each_sibling_event(sibling, group_leader) {
>  		if (__perf_event_read_size(sibling->attr.read_format,
>  					   group_leader->nr_siblings + 1) > 16*1024)
> -- 
> 2.40.1
>