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[Qemu-devel] [PATCH v9 12/37] qapi: Don't cast Enum* to int*

From: Eric Blake
Subject: [Qemu-devel] [PATCH v9 12/37] qapi: Don't cast Enum* to int*
Date: Tue, 19 Jan 2016 09:10:20 -0700

C compilers are allowed to represent enums as a smaller type
than int, if all enum values fit in the smaller type.  There
are even compiler flags that force the use of this smaller
representation, and using them changes the ABI of a binary.
Therefore, our generated code for visit_type_ENUM() (for all
qapi enums) was wrong for casting Enum* to int* when calling

It appears that no one has been doing this for qemu, because
if they had, we are potentially dereferencing beyond bounds
or even risking a SIGBUS on platforms where unaligned pointer
dereferencing is fatal.  Better is to avoid the practice
entirely, and just use the correct types.

This matches the fix for alternate qapi types, done earlier in
commit 0426d53 "qapi: Simplify visiting of alternate types",
with generated code changing as:

| void visit_type_GuestDiskBusType(Visitor *v, GuestDiskBusType *obj, const 
char *name, Error **errp)
| {
|-    visit_type_enum(v, (int *)obj, GuestDiskBusType_lookup, 
"GuestDiskBusType", name, errp);
|+    int tmp = *obj;
|+    visit_type_enum(v, &tmp, GuestDiskBusType_lookup, "GuestDiskBusType", 
name, errp);
|+    *obj = tmp;
| }

Signed-off-by: Eric Blake <address@hidden>
Reviewed-by: Marc-André Lureau <address@hidden>

v9: mention earlier commit id, enhance commit message
v8: no change
v7: rebase on typo fix
v6: new patch
 scripts/qapi-visit.py | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py
index 4a4f67d..6bd188b 100644
--- a/scripts/qapi-visit.py
+++ b/scripts/qapi-visit.py
@@ -178,12 +178,13 @@ out:

 def gen_visit_enum(name):
-    # FIXME cast from enum *obj to int * invalidly assumes enum is int
     return mcgen('''

 void visit_type_%(c_name)s(Visitor *v, %(c_name)s *obj, const char *name, 
Error **errp)
-    visit_type_enum(v, (int *)obj, %(c_name)s_lookup, "%(name)s", name, errp);
+    int tmp = *obj;
+    visit_type_enum(v, &tmp, %(c_name)s_lookup, "%(name)s", name, errp);
+    *obj = tmp;
                  c_name=c_name(name), name=name)

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