[Bug analyzer/107582] - -Wanalyzer-use-of-uninitialized-value false positive with while loop in pthread_cleanup_push

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107582

--- Comment #11 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by David Malcolm
<dmalcolm@gcc.gnu.org>:

https://gcc.gnu.org/g:e7f7483d50069fede8450091449714d122c58fca

commit r12-9358-ge7f7483d50069fede8450091449714d122c58fca
Author: David Malcolm <dmalcolm@redhat.com>
Date:   Wed Mar 29 14:16:47 2023 -0400

    analyzer: fix feasibility false +ve on jumps through function ptrs
[PR107582]

    PR analyzer/107582 reports a false +ve from
    -Wanalyzer-use-of-uninitialized-value where
    the analyzer's feasibility checker erroneously decides
    that point (B) in the code below is reachable, with "x" being
    uninitialized there:

        pthread_cleanup_push(func, NULL);

        while (ret != ETIMEDOUT)
            ret = rand() % 1000;

        /* (A): after the while loop  */

        if (ret != ETIMEDOUT)
          x = &z;

        pthread_cleanup_pop(1);

        if (ret == ETIMEDOUT)
          return 0;

        /* (B): after not bailing out  */

    due to these contradictionary conditions somehow both holding:
      * (ret == ETIMEDOUT), at (A) (skipping the initialization of x), and
      * (ret != ETIMEDOUT), at (B)

    The root cause is that after the while loop, state merger puts ret in
    the exploded graph in an UNKNOWN state, and saves the diagnostic at (B).

    Later, as we explore the feasibilty of reaching the enode for (B),
    dynamic_call_info_t::update_model is called to push/pop the
    frames for handling the call to "func" in pthread_cleanup_pop.
    The "ret" at these nodes in the feasible_graph has a conjured_svalue for
    "ret", and a constraint on it being either == *or* != ETIMEDOUT.

    However dynamic_call_info_t::update_model blithely clobbers the
    model with a copy from the exploded_graph, in which "ret" is UNKNOWN.

    This patch fixes dynamic_call_info_t::update_model so that it
    simulates pushing/popping a frame on the model we're working with,
    preserving knowledge of the constraint on "ret", and enabling the
    analyzer to "know" that the bail-out must happen.

    Doing so fixes the false positive.

    Cherrypicked from r13-4158-ga7aef0a5a2b7e2.

    gcc/analyzer/ChangeLog:
            PR analyzer/107582
            * engine.cc (dynamic_call_info_t::update_model): Update the model
            by pushing or pop a frame, rather than by clobbering it with the
            model from the exploded_node's state.

    gcc/testsuite/ChangeLog:
            PR analyzer/107582
            * gcc.dg/analyzer/feasibility-4.c: New test.
            * gcc.dg/analyzer/feasibility-pr107582-1.c: New test.
            * gcc.dg/analyzer/feasibility-pr107582-2.c: New test.

    Signed-off-by: David Malcolm <dmalcolm@redhat.com>