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Clear caml_backtrace_last_exn before registering as root #205

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commented Jun 23, 2015

Switching on backtrace recording registers caml_backtrace_last_exn as a global root, switching it off removes it.
But the value is never cleared, so that an arbitrary pointer to the ocaml heap can end up there and corrupts the GC.

Testcase (tested with 4.02.1 and 4.02.2):

(* $ ocamlopt -g
   $ ./a.out 
   Segmentation fault
let test arg =
  Gc.minor ();
  Printexc.record_backtrace true;
  begin try raise (Invalid_argument (Bytes.copy "lalala"))
  with _ -> ()
  end; (* caml_backtrace_last_exn is now pointing to the exception *)
  Printexc.record_backtrace false; (* no longer a root *)
  Gc.minor (); (* minor heap clean, but caml_backtrace_last_exn still pointing to value *)
  ignore [arg; arg; arg]; (* allocate random stuff *)
  Printexc.record_backtrace true; (* again a root, pointing somewhere in minor heap *)
  Gc.minor () (* hopefully fails *)

let () = while true do test (); done

@let-def let-def force-pushed the let-def:fix-backtrace branch from f866b87 to 3c364b8 Jun 23, 2015


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commented Jun 23, 2015

Well spotted. OK for applying the proposed fix.


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commented Jun 23, 2015

Fix applied to trunk, rev 16187.

However, I don't manage to add a test in the suite (that finishes in finite time).


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commented Jul 1, 2015

@alainfrisch Of course, once fixed the test never finishes. I managed to have an easy to reproduce one with 4.02.1, but I had to introduce the loop for 4.02.2 (without satisfying reason, just to get to some wrong GC internal state). I think we can skip the test, I'll close the pull request :).

@let-def let-def closed this Jul 1, 2015

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