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[PATCH 1/2] tracing: Prevent hist_field_var_ref() from accessing NULL tracing_map_elts

From: Tom Zanussi <tom.zanussi@xxxxxxxxxxxxxxx>

hist_field_var_ref() is an implementation of hist_field_fn_t(), which
can be called with a null tracing_map_elt elt param when assembling a
key in event_hist_trigger().

In the case of hist_field_var_ref() this doesn't make sense, because a
variable can only be resolved by looking it up using an already
assembled key i.e. a variable can't be used to assemble a key since
the key is required in order to access the variable.

Upper layers should prevent the user from constructing a key using a
variable in the first place, but in case one slips through, it
shouldn't cause a NULL pointer dereference.  Also if one does slip
through, we want to know about it, so emit a one-time warning in that

Signed-off-by: Tom Zanussi <tom.zanussi@xxxxxxxxxxxxxxx>
Reported-by: Vincent Bernat <vincent@xxxxxxxxx>
 kernel/trace/trace_events_hist.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index eb908ef2ecec..3795207a48a0 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -1632,6 +1632,9 @@ static u64 hist_field_var_ref(struct hist_field *hist_field,
 	struct hist_elt_data *elt_data;
 	u64 var_val = 0;
+	if (WARN_ON_ONCE(!elt))
+		return var_val;
 	elt_data = elt->private_data;
 	var_val = elt_data->var_ref_vals[hist_field->var_ref_idx];