Re: [Qemu-devel] [PATCH v2] qapi: Fix regression with '-netdev ?'

[Markus Armbruster <armbru@redhat.com>]

Eric Blake <eblake@redhat.com> writes:

> Commit e36c714e causes 'qemu -netdev ?' to dump core, because the
> call to visit_end_union() is no longer conditional on whether
> *obj was allocated.
>
> Reported by Marc-André Lureau <marcandre.lureau@gmail.com>
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
> v2: don't depend on unreleased patches
>
>  scripts/qapi-visit.py | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py
> index 2a9fab8..d0759d7 100644
> --- a/scripts/qapi-visit.py
> +++ b/scripts/qapi-visit.py
> @@ -301,7 +301,9 @@ void visit_type_%(c_name)s(Visitor *v, %(c_name)s **obj, const char *name, Error
>  out_obj:
>      error_propagate(errp, err);
>      err = NULL;
> -    visit_end_union(v, !!(*obj)->data, &err);
> +    if (*obj) {
> +        visit_end_union(v, !!(*obj)->data, &err);
> +    }
>      error_propagate(errp, err);
>      err = NULL;
>      visit_end_struct(v, &err);

Let's see.

Before commit e36c714e, we generated

        visit_start_struct(v, (void **)obj, "%(name)s", name, sizeof(%(c_name)s), &err);
        if (err) {
            goto out;
        }
        if (*obj) {
    [...]
    out_obj:
            error_propagate(errp, err);
            err = NULL;
            visit_end_union(v, !!(*obj)->data, &err);
            error_propagate(errp, err);
            err = NULL;
        }
        visit_end_struct(v, &err);
    out:

Since then

        visit_start_struct(v, (void **)obj, "%(name)s", name, sizeof(%(c_name)s), &err);
        if (err) {
            goto out;
        }
        if (!*obj) {
            goto out_obj;       // goto out_end would've been faithful trafo
        }
        [...]
    out_obj:
        error_propagate(errp, err);                     //
        err = NULL;                                     // This code became
        visit_end_union(v, !!(*obj)->data, &err);       // accidentally
        error_propagate(errp, err);                     // unconditional
        err = NULL;                                     //
    // out_end:
        visit_end_struct(v, &err);
    out:
        error_propagate(errp, err);

We screwed up the if !*obj.  Instead of correcting the goto, you exploit
that err is null, and thus the accidentally unconditional code is a
no-op except for the visit_end_union(), so you protect that.  Okay.

In case anyone thinks correcting the goto would be nicer: the
visit_end_union() will go away soon.

I'll take this through my tree.  Expect a pull request today.  Thanks!