RE: [Caml-list] Exception values may crash GC when interfacing C and Caml

[David Allsopp <dra-news@metastack.com>]

Alexey Rodriguez wrote:
> Dear all,
> 
> We are experiencing crashes in Caml-calling C code. This happens if
> garbage collection runs after Caml code has raised an exception. We now
> understand why this happens but we are puzzled as to why the "Interfacing
> C with Ocaml" chapter of the Ocaml manual doesn't warn about this
> situation.
> 
> Suppose you have C code that calls Caml code as follows:
> 
> ...
> CAMLparam2(v1,v2);
> CAMLlocal2(...,res);
> res = callback2_exn(...,v1,v2);
> foobar();
> ...
> 
> We have found that this code will crash with "Fatal error: out of memory."
> if the following two things happen:
> * the function called by [callback2_exn] raises an exception, and
> * [foobar] triggers a garbage collection through the allocation of values
> in the Caml heap. (just calling [caml_gc_full_major] is enough to cause
> the crash).
> 
> The reason for this crash is that [res] will contain an invalid pointer if
> an exception is thrown. The GC follows this bogus pointer ([res] is
> registered as a root by [CAMLlocal2]) which ultimately causes a crash in
> the GC code. Why does [res] contain a bogus pointer?
> It's not really a bogus pointer, but the lower bits are tagged in order to
> denote a thrown exception. These bits are usually tested/cleared by
> [Is_exception_result] and [Extract_exception].

This is already in the manual, but I agree that the requirement to do so could be stated more clearly. Section 18.7.1[1], last paragraph states "The return v of the caml_callback*_exn function **must** be tested with the macro Is_exception_result(v)". It also clearly indicates that v is only a valid [value] if Is_exception_result(v) returns false so storing the return of caml_callback*_exn in a local root and allowing the Gc to run before you update that root with the result of Extract_exception is "obviously" a Gc violation.


David

[1] http://caml.inria.fr/pub/docs/manual-ocaml/manual032.html#htoc245