tracing: Prevent hist_field_var_ref() from accessing NULL tracing_map_elts
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 case. Link: http://lkml.kernel.org/r/64ec8dc15c14d305295b64cdfcc6b2b9dd14753f.1555597045.git.tom.zanussi@linux.intel.comReported-by: NVincent Bernat <vincent@bernat.ch> Signed-off-by: NTom Zanussi <tom.zanussi@linux.intel.com> Signed-off-by: NSteven Rostedt (VMware) <rostedt@goodmis.org>
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