Re: [PATCH 1/1] perf_event: add check for group_list if the parent isn't counted

[Wang Liming <liming.wang@windriver.com>]

Peter Zijlstra wrote:
> On Wed, 2009-12-30 at 22:36 +0800, Wang Liming wrote:
>>> Best I can make of it is that there is a race where the parent gets his
>>> context instantiated and we manage to get the mutex before the other
>>> thread manages to add the first event.
>>>
>>> Then we observe parent_event_ctx but have an empty list.
>>>
>>> Is that it?
>> I didn't find this case.
>> In my case, if I perf record a existing process with "--pid" and finish record,
>> and if later the recorded process forks a process, the condition will occur.
> 
> Ah, right, that will lead to the same state, since closing the last
> counter will not remove the context.
> 
> Does the below also fix your issue?
Yes, it's OK to me.
Thanks a lot!

Liming Wang
> 
> ---
> Subject: perf: Fix NULL deref in inheritance code
> From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Date: Wed Dec 30 16:00:35 CET 2009
> 
> Liming found a NULL deref when a task has a perf context but no counters 
> when it forks.
> 
> This can occur in two cases, a race during construction where the fork hits
> after installing the context but before the first counter gets inserted, or
> more reproducably, a fork after the last counter is closed (which leaves the
> context around).
> 
> CC: stable@kernel.org
> Reported-by: Wang Liming <liming.wang@windriver.com>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
>  kernel/perf_event.c |    5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> Index: linux-2.6/kernel/perf_event.c
> ===================================================================
> --- linux-2.6.orig/kernel/perf_event.c
> +++ linux-2.6/kernel/perf_event.c
> @@ -5149,7 +5149,7 @@ int perf_event_init_task(struct task_str
>  					    GFP_KERNEL);
>  			if (!child_ctx) {
>  				ret = -ENOMEM;
> -				goto exit;
> +				break;
>  			}
>  
>  			__perf_event_init_context(child_ctx, child);
> @@ -5165,7 +5165,7 @@ int perf_event_init_task(struct task_str
>  		}
>  	}
>  
> -	if (inherited_all) {
> +	if (child_ctx && inherited_all) {
>  		/*
>  		 * Mark the child context as a clone of the parent
>  		 * context, or of whatever the parent is a clone of.
> @@ -5185,7 +5185,6 @@ int perf_event_init_task(struct task_str
>  		get_ctx(child_ctx->parent_ctx);
>  	}
>  
> -exit:
>  	mutex_unlock(&parent_ctx->mutex);
>  
>  	perf_unpin_context(parent_ctx);
> 
> 
>