  VeriFast is a powerful tool for verification of various correctness
properties of C programs using symbolic execution. However, VeriFast itself has
not been verified. We present a proof-of-concept extension which generates a
correctness certificate for each successful verification run individually. This
certificate takes the form of a Coq script containing two proofs which, when
successfully checked by Coq, together remove the need for trusting in the
correctness of VeriFast itself.
  The first proves a lemma expressing the correctness of the program with
respect to a big step operational semantics developed by ourselves, intended to
reflect VeriFast's interpretation of C. We have formalized this semantics in
Coq as cbsem. This lemma is proven by symbolic execution in Coq, which in turn
is implemented by transforming the exported AST of the program into a Coq
proposition representing the symbolic execution performed by VeriFast itself.
  The second proves the correctness of the same C program with respect to
CompCert's Clight big step semantics. This proof simply applies our proof of
the soundness of cbsem with respect to CompCert Clight to the first proof.
