[Qemu-devel] [PATCH v7 27/31] qapi: Rework deallocation of partial struct

[Eric Blake <eblake@redhat.com>]

Commit cee2dedb noticed that if you have a partial flat union
(such as if an input parse failed due to a missing
discriminator), calling the dealloc visitor could result in
trying to dereference the NULL pointer. But the fix it proposed
requires the use of a 'data' member in the union, which may or
may not be the same size as other branches of the union
(consider a 32-bit platform where one of the branches is an
int64), so it feels fairly dirty.  A better fix is to tweak all
of the generated visit_type_implicit_FOO() functions to avoid
dereferencing NULL in the first place, by not visiting the
fields if the struct pointer itself is not present, at which
point we no longer even need visit_start_union().  And no one
was implementing visit_end_union() callbacks.

While rewriting the code, use patterns that are closer to what
is used elsewhere in the generated visitors, by using 'goto'
to cleanup labels rather than putting followup code under 'if'
conditions.  The change keeps the contract that any successful
use of visit_start_implicit_struct() will be paired with a
matching visit_end_implicit_struct(), even if intermediate
processing is skipped.  We are safe in checking *obj alone, as
as the contract of visit_start_implicit_struct() requires a
non-NULL obj.

As an example of the changes to generated code:
|@@ -1331,10 +1331,16 @@ static void visit_type_implicit_Blockdev
|     Error *err = NULL;
|
|     visit_start_implicit_struct(v, (void **)obj, sizeof(BlockdevOptionsArchipelago), &err);
|-    if (!err) {
|-        visit_type_BlockdevOptionsArchipelago_fields(v, obj, errp);
|-        visit_end_implicit_struct(v);
|+    if (err) {
|+        goto out;
|+    }
|+    if (!*obj) {
|+        goto out_obj;
|     }
|+    visit_type_BlockdevOptionsArchipelago_fields(v, obj, &err);
|+out_obj:
|+    visit_end_implicit_struct(v);
|+out:
|     error_propagate(errp, err);
| }
...
|@@ -1479,9 +1539,6 @@ void visit_type_BlockdevOptions(Visitor
|     if (err) {
|         goto out_obj;
|     }
|-    if (!visit_start_union(v, !!(*obj)->u.data, &err) || err) {
|-        goto out_obj;
|-    }
|     switch ((*obj)->driver) {
|     case BLOCKDEV_DRIVER_ARCHIPELAGO:
|         visit_type_implicit_BlockdevOptionsArchipelago(v, &(*obj)->u.archipelago, &err);
|@@ -1570,11 +1627,6 @@ void visit_type_BlockdevOptions(Visitor
| out_obj:
|     error_propagate(errp, err);
|     err = NULL;
|-    if (*obj) {
|-        visit_end_union(v, !!(*obj)->u.data, &err);
|-    }
|-    error_propagate(errp, err);
|-    err = NULL;
|     visit_end_struct(v, &err);

Signed-off-by: Eric Blake <eblake@redhat.com>

---
v7: rebase to earlier context changes, simplify 'obj && !*obj'
condition based on contract
v6: rebase due to deferring 7/46, and gen_err_check() improvements;
rewrite gen_visit_implicit_struct() more like other patterns
---
 include/qapi/visitor-impl.h |  5 -----
 include/qapi/visitor.h      | 12 ------------
 qapi/qapi-dealloc-visitor.c | 26 --------------------------
 qapi/qapi-visit-core.c      | 15 ---------------
 scripts/qapi-visit.py       | 25 +++++++++----------------
 5 files changed, 9 insertions(+), 74 deletions(-)

diff --git a/include/qapi/visitor-impl.h b/include/qapi/visitor-impl.h
index 628cab7..3b67dc7 100644
--- a/include/qapi/visitor-impl.h
+++ b/include/qapi/visitor-impl.h
@@ -81,11 +81,6 @@ struct Visitor

     /* May be NULL; most useful for input visitors. */
     void (*optional)(Visitor *v, bool *present, const char *name);
-
-    /* FIXME - needs to be removed */
-    bool (*start_union)(Visitor *v, bool data_present, Error **errp);
-    /* FIXME - needs to be removed */
-    void (*end_union)(Visitor *v, bool data_present, Error **errp);
 };

 /**
diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
index 2592dd1..b8146d7 100644
--- a/include/qapi/visitor.h
+++ b/include/qapi/visitor.h
@@ -274,16 +274,4 @@ void visit_type_any(Visitor *v, QObject **obj, const char *name, Error **errp);
  */
 void visit_type_null(Visitor *v, const char *name, Error **errp);

-/**
- * Mark the start of visiting the branches of a union. Return true if
- * @data_present.
- * FIXME: Should not be needed
- */
-bool visit_start_union(Visitor *v, bool data_present, Error **errp);
-/**
- * Mark the end of union branches, after visit_start_union().
- * FIXME: Should not be needed
- */
-void visit_end_union(Visitor *v, bool data_present, Error **errp);
-
 #endif
diff --git a/qapi/qapi-dealloc-visitor.c b/qapi/qapi-dealloc-visitor.c
index e3a4493..92345aa 100644
--- a/qapi/qapi-dealloc-visitor.c
+++ b/qapi/qapi-dealloc-visitor.c
@@ -172,31 +172,6 @@ static void qapi_dealloc_type_enum(Visitor *v, int *obj,
 {
 }

-/* If there's no data present, the dealloc visitor has nothing to free.
- * Thus, indicate to visitor code that the subsequent union fields can
- * be skipped. This is not an error condition, since the cleanup of the
- * rest of an object can continue unhindered, so leave errp unset in
- * these cases.
- *
- * NOTE: In cases where we're attempting to deallocate an object that
- * may have missing fields, the field indicating the union type may
- * be missing. In such a case, it's possible we don't have enough
- * information to differentiate data_present == false from a case where
- * data *is* present but happens to be a scalar with a value of 0.
- * This is okay, since in the case of the dealloc visitor there's no
- * work that needs to done in either situation.
- *
- * The current inability in QAPI code to more thoroughly verify a union
- * type in such cases will likely need to be addressed if we wish to
- * implement this interface for other types of visitors in the future,
- * however.
- */
-static bool qapi_dealloc_start_union(Visitor *v, bool data_present,
-                                     Error **errp)
-{
-    return data_present;
-}
-
 Visitor *qapi_dealloc_get_visitor(QapiDeallocVisitor *v)
 {
     return &v->visitor;
@@ -228,7 +203,6 @@ QapiDeallocVisitor *qapi_dealloc_visitor_new(void)
     v->visitor.type_number = qapi_dealloc_type_number;
     v->visitor.type_any = qapi_dealloc_type_anything;
     v->visitor.type_null = qapi_dealloc_type_null;
-    v->visitor.start_union = qapi_dealloc_start_union;

     QTAILQ_INIT(&v->stack);

diff --git a/qapi/qapi-visit-core.c b/qapi/qapi-visit-core.c
index 36917f3..09cc5c9 100644
--- a/qapi/qapi-visit-core.c
+++ b/qapi/qapi-visit-core.c
@@ -76,21 +76,6 @@ void visit_end_list(Visitor *v)
     v->end_list(v);
 }

-bool visit_start_union(Visitor *v, bool data_present, Error **errp)
-{
-    if (v->start_union) {
-        return v->start_union(v, data_present, errp);
-    }
-    return true;
-}
-
-void visit_end_union(Visitor *v, bool data_present, Error **errp)
-{
-    if (v->end_union) {
-        v->end_union(v, data_present, errp);
-    }
-}
-
 bool visit_optional(Visitor *v, bool *present, const char *name)
 {
     if (v->optional) {
diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py
index e045017..5c8f1e6 100644
--- a/scripts/qapi-visit.py
+++ b/scripts/qapi-visit.py
@@ -60,10 +60,16 @@ static void visit_type_implicit_%(c_type)s(Visitor *v, %(c_type)s **obj, Error *
     Error *err = NULL;

     visit_start_implicit_struct(v, (void **)obj, sizeof(%(c_type)s), &err);
-    if (!err) {
-        visit_type_%(c_type)s_fields(v, obj, errp);
-        visit_end_implicit_struct(v);
+    if (err) {
+        goto out;
     }
+    if (!*obj) {
+        goto out_obj;
+    }
+    visit_type_%(c_type)s_fields(v, obj, &err);
+out_obj:
+    visit_end_implicit_struct(v);
+out:
     error_propagate(errp, err);
 }
 ''',
@@ -254,9 +260,6 @@ void visit_type_%(c_name)s(Visitor *v, %(c_name)s **obj, const char *name, Error

     if variants:
         ret += mcgen('''
-    if (!visit_start_union(v, !!(*obj)->u.data, &err) || err) {
-        goto out_obj;
-    }
     switch ((*obj)->%(c_name)s) {
 ''',
                      c_name=c_name(variants.tag_member.name))
@@ -293,16 +296,6 @@ void visit_type_%(c_name)s(Visitor *v, %(c_name)s **obj, const char *name, Error

     ret += mcgen('''
 out_obj:
-''')
-    if variants:
-        ret += mcgen('''
-    error_propagate(errp, err);
-    err = NULL;
-    if (*obj) {
-        visit_end_union(v, !!(*obj)->u.data, &err);
-    }
-''')
-    ret += mcgen('''
     error_propagate(errp, err);
     err = NULL;
     visit_end_struct(v, &err);
-- 
2.4.3