There is one more same question
How to Identify all the resources who have Report Run rights over which Reports at instance level
Rights can be assigned in many ways (instance, OBS, group, global), so the answer is not as simple as you might think.
Theres an old thread ; Clarity rights Query that discusses all the various ways to get the answer via SQL, but you are going to have to read and understand that thread (and then understand which bits of the SQL you need - the full answer is about 9 different UNION'ed expressions - and which rights you are looking for). Your security model might be a lot simpler than the "full answer" too though.
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