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qapi: Change visit_type_FOO() to no longer return partial objects

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Returning a partial object on error is an invitation for a careless
caller to leak memory.  We already fixed things in an earlier
patch to guarantee NULL if visit_start fails ("qapi: Guarantee
NULL obj on input visitor callback error"), but that does not
help the case where visit_start succeeds but some other failure
happens before visit_end, such that we leak a partially constructed
object outside visit_type_FOO(). As no one outside the testsuite
was actually relying on these semantics, it is cleaner to just
document and guarantee that ALL pointer-based visit_type_FOO()
functions always leave a safe value in *obj during an input visitor
(either the new object on success, or NULL if an error is
encountered), so callers can now unconditionally use
qapi_free_FOO() to clean up regardless of whether an error occurred.

The decision is done by adding visit_is_input(), then updating the
generated code to check if additional cleanup is needed based on
the type of visitor in use.

Note that we still leave *obj unchanged after a scalar-based
visit_type_FOO(); I did not feel like auditing all uses of
visit_type_Enum() to see if the callers would tolerate a specific
sentinel value (not to mention having to decide whether it would
be better to use 0 or ENUM__MAX as that sentinel).

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1461879932-9020-25-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
This commit is contained in:
Eric Blake 2016-04-28 15:45:32 -06:00 committed by Markus Armbruster
parent d9f62dde13
commit 68ab47e4b4
7 changed files with 57 additions and 43 deletions
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8
docs/qapi-code-gen.txt
View File

@ -905,6 +905,10 @@ Example:
visit_check_struct(v, &err);
out_obj:
visit_end_struct(v);
if (err && visit_is_input(v)) {
qapi_free_UserDefOne(*obj);
*obj = NULL;
}
out:
error_propagate(errp, err);
}
@ -929,6 +933,10 @@ Example:
}
visit_end_list(v);
if (err && visit_is_input(v)) {
qapi_free_UserDefOneList(*obj);
*obj = NULL;
}
out:
error_propagate(errp, err);
}

25
include/qapi/visitor.h
View File

@ -66,12 +66,14 @@
* member @name is not present, or is present but not the specified
* type).
*
* FIXME: At present, visit_type_FOO() is an awkward interface: input
* visitors may allocate an incomplete *@obj even when reporting an
* error, but using an output visitor with an incomplete object has
* undefined behavior. To avoid a memory leak, callers must use
* qapi_free_FOO() even on error (this uses the dealloc visitor, and
* safely handles an incomplete object).
* If an error is detected during visit_type_FOO() with an input
* visitor, then *@obj will be NULL for pointer types, and left
* unchanged for scalar types. Using an output visitor with an
* incomplete object has undefined behavior (other than a special case
* for visit_type_str() treating NULL like ""), while the dealloc
* visitor safely handles incomplete objects. Since input visitors
* never produce an incomplete object, such an object is possible only
* by manual construction.
*
* For the QAPI object types (structs, unions, and alternates), there
* is an additional generated function in qapi-visit.h compatible
@ -106,7 +108,6 @@
* v = ...obtain input visitor...
* visit_type_Foo(v, NULL, &f, &err);
* if (err) {
* qapi_free_Foo(f);
* ...handle error...
* } else {
* ...use f...
@ -124,7 +125,6 @@
* v = ...obtain input visitor...
* visit_type_FooList(v, NULL, &l, &err);
* if (err) {
* qapi_free_FooList(l);
* ...handle error...
* } else {
* for ( ; l; l = l->next) {
@ -154,7 +154,9 @@
* helpers that rely on in-tree information to control the walk:
* visit_optional() for the 'has_member' field associated with
* optional 'member' in the C struct; and visit_next_list() for
* advancing through a FooList linked list. Only the generated
* advancing through a FooList linked list. Similarly, the
* visit_is_input() helper makes it possible to write code that is
* visitor-agnostic everywhere except for cleanup. Only the generated
* visit_type functions need to use these helpers.
*
* It is also possible to use the visitors to do a virtual walk, where
@ -405,6 +407,11 @@ bool visit_optional(Visitor *v, const char *name, bool *present);
void visit_type_enum(Visitor *v, const char *name, int *obj,
const char *const strings[], Error **errp);
/*
* Check if visitor is an input visitor.
*/
bool visit_is_input(Visitor *v);
/*** Visiting built-in types ***/
/*

5
qapi/qapi-visit-core.c
View File

@ -104,6 +104,11 @@ bool visit_optional(Visitor *v, const char *name, bool *present)
return *present;
}
bool visit_is_input(Visitor *v)
{
return v->type == VISITOR_INPUT;
}
void visit_type_int(Visitor *v, const char *name, int64_t *obj, Error **errp)
{
assert(obj);

22
scripts/qapi-visit.py
View File

@ -108,10 +108,6 @@ out:
def gen_visit_list(name, element_type):
# FIXME: if *obj is NULL on entry, and the first visit_next_list()
# assigns to *obj, while a later one fails, we should clean up *obj
# rather than leaving it non-NULL. As currently written, the caller must
# call qapi_free_FOOList() to avoid a memory leak of the partial FOOList.
return mcgen('''
void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, Error **errp)
@ -134,6 +130,10 @@ void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, Error
}
visit_end_list(v);
if (err && visit_is_input(v)) {
qapi_free_%(c_name)s(*obj);
*obj = NULL;
}
out:
error_propagate(errp, err);
}
@ -211,20 +211,20 @@ void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, Error
"%(name)s");
}
visit_end_alternate(v);
if (err && visit_is_input(v)) {
qapi_free_%(c_name)s(*obj);
*obj = NULL;
}
out:
error_propagate(errp, err);
}
''',
name=name)
name=name, c_name=c_name(name))
return ret
def gen_visit_object(name, base, members, variants):
# FIXME: if *obj is NULL on entry, and visit_start_struct() assigns to
# *obj, but then visit_type_FOO_members() fails, we should clean up *obj
# rather than leaving it non-NULL. As currently written, the caller must
# call qapi_free_FOO() to avoid a memory leak of the partial FOO.
return mcgen('''
void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, Error **errp)
@ -245,6 +245,10 @@ void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, Error
visit_check_struct(v, &err);
out_obj:
visit_end_struct(v);
if (err && visit_is_input(v)) {
qapi_free_%(c_name)s(*obj);
*obj = NULL;
}
out:
error_propagate(errp, err);
}

13
tests/test-qmp-commands.c
View File

@ -228,14 +228,13 @@ static void test_dealloc_partial(void)
QDECREF(ud2_dict);
}
/* verify partial success */
assert(ud2 != NULL);
assert(ud2->string0 != NULL);
assert(strcmp(ud2->string0, text) == 0);
assert(ud2->dict1 == NULL);
/* confirm & release construction error */
/* verify that visit_type_XXX() cleans up properly on error */
error_free_or_abort(&err);
assert(!ud2);
/* Manually create a partial object, leaving ud2->dict1 at NULL */
ud2 = g_new0(UserDefTwo, 1);
ud2->string0 = g_strdup(text);
/* tear down partial object */
qapi_free_UserDefTwo(ud2);

17
tests/test-qmp-input-strict.c
View File

@ -182,10 +182,7 @@ static void test_validate_fail_struct(TestInputVisitorData *data,
visit_type_TestStruct(v, NULL, &p, &err);
error_free_or_abort(&err);
if (p) {
g_free(p->string);
}
g_free(p);
g_assert(!p);
}
static void test_validate_fail_struct_nested(TestInputVisitorData *data,
@ -199,7 +196,7 @@ static void test_validate_fail_struct_nested(TestInputVisitorData *data,
visit_type_UserDefTwo(v, NULL, &udp, &err);
error_free_or_abort(&err);
qapi_free_UserDefTwo(udp);
g_assert(!udp);
}
static void test_validate_fail_list(TestInputVisitorData *data,
@ -213,7 +210,7 @@ static void test_validate_fail_list(TestInputVisitorData *data,
visit_type_UserDefOneList(v, NULL, &head, &err);
error_free_or_abort(&err);
qapi_free_UserDefOneList(head);
g_assert(!head);
}
static void test_validate_fail_union_native_list(TestInputVisitorData *data,
@ -228,7 +225,7 @@ static void test_validate_fail_union_native_list(TestInputVisitorData *data,
visit_type_UserDefNativeListUnion(v, NULL, &tmp, &err);
error_free_or_abort(&err);
qapi_free_UserDefNativeListUnion(tmp);
g_assert(!tmp);
}
static void test_validate_fail_union_flat(TestInputVisitorData *data,
@ -242,7 +239,7 @@ static void test_validate_fail_union_flat(TestInputVisitorData *data,
visit_type_UserDefFlatUnion(v, NULL, &tmp, &err);
error_free_or_abort(&err);
qapi_free_UserDefFlatUnion(tmp);
g_assert(!tmp);
}
static void test_validate_fail_union_flat_no_discrim(TestInputVisitorData *data,
@ -257,7 +254,7 @@ static void test_validate_fail_union_flat_no_discrim(TestInputVisitorData *data,
visit_type_UserDefFlatUnion2(v, NULL, &tmp, &err);
error_free_or_abort(&err);
qapi_free_UserDefFlatUnion2(tmp);
g_assert(!tmp);
}
static void test_validate_fail_alternate(TestInputVisitorData *data,
@ -271,7 +268,7 @@ static void test_validate_fail_alternate(TestInputVisitorData *data,
visit_type_UserDefAlternate(v, NULL, &tmp, &err);
error_free_or_abort(&err);
qapi_free_UserDefAlternate(tmp);
g_assert(!tmp);
}
static void do_test_validate_qmp_introspect(TestInputVisitorData *data,

10
tests/test-qmp-input-visitor.c
View File

@ -773,18 +773,12 @@ static void test_visitor_in_errors(TestInputVisitorData *data,
visit_type_TestStruct(v, NULL, &p, &err);
error_free_or_abort(&err);
/* FIXME - a failed parse should not leave a partially-allocated p
* for us to clean up; this could cause callers to leak memory. */
g_assert(p->string == NULL);
g_free(p->string);
g_free(p);
g_assert(!p);
v = visitor_input_test_init(data, "[ '1', '2', false, '3' ]");
visit_type_strList(v, NULL, &q, &err);
error_free_or_abort(&err);
assert(q);
qapi_free_strList(q);
assert(!q);
}
static void test_visitor_in_wrong_type(TestInputVisitorData *data,