bpf: avoid rcu_dereference inside bpf_event_mutex lock region

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 <yhs@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 136aa6bb04223795b7ea091706df21e74bacbd87..506efe6e8ed97290ee95c7d3297d0ded7756b55d 100644 (file)
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -769,20 +769,19 @@ 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;
 }
@@ -796,11 +795,9 @@ 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);
@@ -812,6 +809,6 @@ 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);
 }