[PATCH net-next] bpf: avoid rcu_dereference inside bpf_event_mutex lock region

[Yonghong Song]

During perf event attaching/detaching bpf programs,
the tp_event->prog_array change is protected by the
bpf_event_mutex lock in both attaching and deteching
functions. Although tp_event->prog_array is a rcu
pointer, rcu_derefrence is not needed to access it
since mutex lock will guarantee ordering.

Verified through "make C=2" that sparse
locking check still happy with the new change.

Also change the label name in perf_event_{attach,detach}_bpf_prog
from "out" to "unlock" to reflect the code action after the label.

Signed-off-by: Yonghong Song <y...@fb.com>
Acked-by: Alexei Starovoitov <a...@kernel.org>
Acked-by: Martin KaFai Lau <ka...@fb.com>
---
 kernel/trace/bpf_trace.c | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

Peter,

Could you check whether the below change to remove rcu_dereference_protected
is what you wanted or not?

Thanks!

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index b65011d..e7685c5 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -767,20 +767,19 @@ int perf_event_attach_bpf_prog(struct perf_event *event,
        mutex_lock(&bpf_event_mutex);
 
        if (event->prog)
-               goto out;
+               goto unlock;
 
-       old_array = rcu_dereference_protected(event->tp_event->prog_array,
-                                             
lockdep_is_held(&bpf_event_mutex));
+       old_array = event->tp_event->prog_array;
        ret = bpf_prog_array_copy(old_array, NULL, prog, &new_array);
        if (ret < 0)
-               goto out;
+               goto unlock;
 
        /* set the new array to event->tp_event and set event->prog */
        event->prog = prog;
        rcu_assign_pointer(event->tp_event->prog_array, new_array);
        bpf_prog_array_free(old_array);
 
-out:
+unlock:
        mutex_unlock(&bpf_event_mutex);
        return ret;
 }
@@ -794,11 +793,9 @@ void perf_event_detach_bpf_prog(struct perf_event *event)
        mutex_lock(&bpf_event_mutex);
 
        if (!event->prog)
-               goto out;
-
-       old_array = rcu_dereference_protected(event->tp_event->prog_array,
-                                             
lockdep_is_held(&bpf_event_mutex));
+               goto unlock;
 
+       old_array = event->tp_event->prog_array;
        ret = bpf_prog_array_copy(old_array, event->prog, NULL, &new_array);
        if (ret < 0) {
                bpf_prog_array_delete_safe(old_array, event->prog);
@@ -810,6 +807,6 @@ void perf_event_detach_bpf_prog(struct perf_event *event)
        bpf_prog_put(event->prog);
        event->prog = NULL;
 
-out:
+unlock:
        mutex_unlock(&bpf_event_mutex);
 }
-- 
2.9.5