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

[Peter Zijlstra <peterz@infradead.org>]

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?

---
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);