tree 72c111c03db9bc722fb88cd0f046b59952e5922b
parent c8faaa4c594f55ddf903d61180029a3ea1da5286
author Tom Zanussi <tom.zanussi@linux.intel.com> 1541537353 -0600
committer Tom Zanussi <tom.zanussi@linux.intel.com> 1555441403 -0500

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.

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Reported-by: Vincent Bernat <vincent@bernat.ch>