trace: reenable preemption if we modify the ip

Things got moved around between the original bpf_override_return patches
and the final version, and now the ftrace kprobe dispatcher assumes if
you modified the ip that you also enabled preemption.  Make a comment of
this and enable preemption, this fixes the lockdep splat that happened
when using this feature.

Fixes: 9802d86585db ("bpf: add a bpf_override_function helper")
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
index 5db8498..91f4b57 100644 (file)
--- a/kernel/trace/trace_kprobe.c
+++ b/kernel/trace/trace_kprobe.c
@@ -1322,8 +1322,15 @@ static int kprobe_dispatcher(struct kprobe *kp, struct pt_regs *regs)
        if (tk->tp.flags & TP_FLAG_TRACE)
                kprobe_trace_func(tk, regs);
 #ifdef CONFIG_PERF_EVENTS
-       if (tk->tp.flags & TP_FLAG_PROFILE)
+       if (tk->tp.flags & TP_FLAG_PROFILE) {
                ret = kprobe_perf_func(tk, regs);
+               /*
+                * The ftrace kprobe handler leaves it up to us to re-enable
+                * preemption here before returning if we've modified the ip.
+                */
+               if (ret)
+                       preempt_enable_no_resched();
+       }
 #endif
        return ret;
 }