Re: [Qemu-devel] [PATCH v16 02/24] qapi: Guarantee NULL obj on input visitor callback error

[Markus Armbruster]

Eric Blake <address@hidden> writes:

> Our existing input visitors were not very consistent on errors
> in a function taking 'TYPE **obj' (that is, start_struct(),
> start_alternate(), type_str(), and type_any(). next_list() is
> similar, except that since commit 08f9541, it can't fail).

Multiple sentences in a parenthesis, ugh :)

    Our existing input visitors were not very consistent on errors in a
    function taking 'TYPE **obj'.  These are start_struct(),
    start_alternate(), type_str(), and type_any().  next_list() is
    similar, but can't fail (see commit since 08f9541).

Can touch up on commit.

> While all of them set '*obj' to allocated storage on success,
> it was not obvious whether '*obj' was guaranteed safe on failure,
> or whether it was left uninitialized.  But a future patch wants
> to guarantee that visit_type_FOO() does not leak a partially-
> constructed obj back to the caller; it is easier to implement
> this if we can reliably state that input visitors assign '*obj'
> regardless of success or failure, and that on failure *obj is
> NULL.  Add assertions to enforce consistency in the final
> setting of err vs. *obj.
>
> The opts-visitor start_struct() doesn't set an error, but it
> also was doing a weird check for 0 size; all callers pass in
> non-zero size if obj is non-NULL.
>
> The testsuite has at least one spot where we no longer need
> to pre-initialize a variable prior to a visit; valgrind confirms
> that the test is still fine with the cleanup.
>
> A later patch will document the design constraint implemented
> here.
>
> Signed-off-by: Eric Blake <address@hidden>
>
> ---
> v16: tighten assertions, another commit message tweak
> v15: enhance commit message, hoist assertions from later in series
> v14: no change
> v13: no change
> v12: new patch
> ---
>  qapi/qapi-visit-core.c        | 34 ++++++++++++++++++++++++++++++----
>  qapi/opts-visitor.c           |  3 ++-
>  qapi/qmp-input-visitor.c      |  4 ++++
>  qapi/string-input-visitor.c   |  1 +
>  tests/test-qmp-input-strict.c |  2 +-
>  5 files changed, 38 insertions(+), 6 deletions(-)
>
> diff --git a/qapi/qapi-visit-core.c b/qapi/qapi-visit-core.c
> index 3cd7edc..7ad5ff4 100644
> --- a/qapi/qapi-visit-core.c
> +++ b/qapi/qapi-visit-core.c
> @@ -23,7 +23,13 @@
>  void visit_start_struct(Visitor *v, const char *name, void **obj,
>                          size_t size, Error **errp)
>  {
> -    v->start_struct(v, name, obj, size, errp);
> +    Error *err = NULL;
> +
> +    v->start_struct(v, name, obj, size, &err);
> +    if (obj && v->type == VISITOR_INPUT) {
> +        assert(!err != !*obj);
> +    }
> +    error_propagate(errp, err);
>  }
>
>  void visit_end_struct(Visitor *v, Error **errp)
> @@ -51,10 +57,16 @@ void visit_start_alternate(Visitor *v, const char *name,
>                             GenericAlternate **obj, size_t size,
>                             bool promote_int, Error **errp)
>  {
> +    Error *err = NULL;
> +
>      assert(obj && size >= sizeof(GenericAlternate));
>      if (v->start_alternate) {
> -        v->start_alternate(v, name, obj, size, promote_int, errp);
> +        v->start_alternate(v, name, obj, size, promote_int, &err);
>      }
> +    if (v->type == VISITOR_INPUT) {
> +        assert(!err != !*obj);

Could assert(v->start_alternate && !err != !*obj), to preempt "what if
!v->start_alternate" worries.  If you like that, I can do it on commit.

> +    }
> +    error_propagate(errp, err);
>  }
>
>  void visit_end_alternate(Visitor *v)
> @@ -188,7 +200,14 @@ void visit_type_bool(Visitor *v, const char *name, bool 
> *obj, Error **errp)
>
>  void visit_type_str(Visitor *v, const char *name, char **obj, Error **errp)
>  {
> -    v->type_str(v, name, obj, errp);
> +    Error *err = NULL;
> +
> +    assert(obj);
> +    v->type_str(v, name, obj, &err);
> +    if (v->type == VISITOR_INPUT) {
> +        assert(!err != !*obj);
> +    }
> +    error_propagate(errp, err);
>  }
>
>  void visit_type_number(Visitor *v, const char *name, double *obj,
> @@ -199,7 +218,14 @@ void visit_type_number(Visitor *v, const char *name, 
> double *obj,
>
>  void visit_type_any(Visitor *v, const char *name, QObject **obj, Error 
> **errp)
>  {
> -    v->type_any(v, name, obj, errp);
> +    Error *err = NULL;
> +
> +    assert(obj);
> +    v->type_any(v, name, obj, &err);
> +    if (v->type == VISITOR_INPUT) {
> +        assert(!err != !*obj);
> +    }
> +    error_propagate(errp, err);
>  }
>
>  static void output_type_enum(Visitor *v, const char *name, int *obj,
> diff --git a/qapi/opts-visitor.c b/qapi/opts-visitor.c
> index 66aeaed..4cb6436 100644
> --- a/qapi/opts-visitor.c
> +++ b/qapi/opts-visitor.c
> @@ -133,7 +133,7 @@ opts_start_struct(Visitor *v, const char *name, void 
> **obj,
>      const QemuOpt *opt;
>
>      if (obj) {
> -        *obj = g_malloc0(size > 0 ? size : 1);
> +        *obj = g_malloc0(size);
>      }
>      if (ov->depth++ > 0) {
>          return;
> @@ -314,6 +314,7 @@ opts_type_str(Visitor *v, const char *name, char **obj, 
> Error **errp)
>
>      opt = lookup_scalar(ov, name, errp);
>      if (!opt) {
> +        *obj = NULL;
>          return;
>      }
>      *obj = g_strdup(opt->str ? opt->str : "");
> diff --git a/qapi/qmp-input-visitor.c b/qapi/qmp-input-visitor.c
> index 02d4233..77cce8b 100644
> --- a/qapi/qmp-input-visitor.c
> +++ b/qapi/qmp-input-visitor.c
> @@ -120,6 +120,9 @@ static void qmp_input_start_struct(Visitor *v, const char 
> *name, void **obj,
>      QObject *qobj = qmp_input_get_object(qiv, name, true);
>      Error *err = NULL;
>
> +    if (obj) {
> +        *obj = NULL;
> +    }
>      if (!qobj || qobject_type(qobj) != QTYPE_QDICT) {
>          error_setg(errp, QERR_INVALID_PARAMETER_TYPE, name ? name : "null",
>                     "QDict");
> @@ -267,6 +270,7 @@ static void qmp_input_type_str(Visitor *v, const char 
> *name, char **obj,
>      QString *qstr = qobject_to_qstring(qmp_input_get_object(qiv, name, 
> true));
>
>      if (!qstr) {
> +        *obj = NULL;
>          error_setg(errp, QERR_INVALID_PARAMETER_TYPE, name ? name : "null",
>                     "string");
>          return;
> diff --git a/qapi/string-input-visitor.c b/qapi/string-input-visitor.c
> index d604575..797973a 100644
> --- a/qapi/string-input-visitor.c
> +++ b/qapi/string-input-visitor.c
> @@ -293,6 +293,7 @@ static void parse_type_str(Visitor *v, const char *name, 
> char **obj,
>      if (siv->string) {
>          *obj = g_strdup(siv->string);
>      } else {
> +        *obj = NULL;
>          error_setg(errp, QERR_INVALID_PARAMETER_TYPE, name ? name : "null",
>                     "string");
>      }
> diff --git a/tests/test-qmp-input-strict.c b/tests/test-qmp-input-strict.c
> index d71727e..d5f80ec 100644
> --- a/tests/test-qmp-input-strict.c
> +++ b/tests/test-qmp-input-strict.c
> @@ -263,7 +263,7 @@ static void 
> test_validate_fail_union_flat_no_discrim(TestInputVisitorData *data,
>  static void test_validate_fail_alternate(TestInputVisitorData *data,
>                                           const void *unused)
>  {
> -    UserDefAlternate *tmp = NULL;
> +    UserDefAlternate *tmp;
>      Visitor *v;
>      Error *err = NULL;