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Re: [libvirt] [PATCH 4/9] util: Add magic_marker for all virObjects



On Tue, May 30, 2017 at 07:38:16AM -0400, John Ferlan wrote:
> The virObject logic "assumes" that whatever is passed to its API's
> would be some sort of virObjectPtr; however, if it is not then some
> really bad things can happen.
> 
> So far there's been only virObject{Ref|Unref}, virObject{Lock|Unlock},
> and virObjectIsClass and the virObject and virObjectLockable class
> consumers have been well behaved and code well tested. Soon there will
> be more consumers and one such consumer tripped over this during testing
> by passing a virHashTablePtr to virObjectIsClass which ends up calling
> virClassIsDerivedFrom using "obj->klass", which wasn't really a klass
> object causing one of those bad things to happen.
> 
> To avoid the future possibility that a non virObject class memory was
> passed to some virObject* API, let's add a "magic_marker" to the base
> virObject class that contains a value "0xFEEDBEEF", then any place in
> the code which would accept or process the base opaque virObjectPtr,
> compare the magic_marker against 0xFEEDBEEF to make sure this is an
> object allocated by this code.
> 
> It is still left up to the caller to handle the failed API calls just
> as it would be if it passed a NULL opaque pointer anyobj.
> 
> Signed-off-by: John Ferlan <jferlan redhat com>
> ---
>  src/util/virobject.c | 12 ++++++++----
>  src/util/virobject.h |  1 +
>  2 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/src/util/virobject.c b/src/util/virobject.c
> index 9f5f187..a1934941 100644
> --- a/src/util/virobject.c
> +++ b/src/util/virobject.c
> @@ -47,10 +47,12 @@ struct _virClass {
>      virObjectDisposeCallback dispose;
>  };
>  
> +#define VIR_OBJECT_NOTVALID(obj) (!obj || obj->magic_marker != 0xFEEDBEEF)
> +
>  #define VIR_OBJECT_USAGE_PRINT_WARNING(anyobj, objclass)                    \
>      do {                                                                    \
>          virObjectPtr obj = anyobj;                                          \
> -        if (!obj)                                                           \
> +        if (VIR_OBJECT_NOTVALID(obj))                                       \
>              VIR_WARN("Object %p is not a virObject class instance", anyobj);\
>          else                                                                \
>              VIR_WARN("Object %p (%s) is not a %s instance",                 \
> @@ -212,6 +214,7 @@ virObjectNew(virClassPtr klass)
>          return NULL;
>  
>      obj->u.s.magic = klass->magic;
> +    obj->magic_marker = 0xFEEDBEEF;
>      obj->klass = klass;
>      virAtomicIntSet(&obj->u.s.refs, 1);
>  
> @@ -272,7 +275,7 @@ virObjectUnref(void *anyobj)
>  {
>      virObjectPtr obj = anyobj;
>  
> -    if (!obj)
> +    if (VIR_OBJECT_NOTVALID(obj))
>          return false;
>  
>      bool lastRef = virAtomicIntDecAndTest(&obj->u.s.refs);
> @@ -289,6 +292,7 @@ virObjectUnref(void *anyobj)
>          /* Clear & poison object */
>          memset(obj, 0, obj->klass->objectSize);
>          obj->u.s.magic = 0xDEADBEEF;
> +        obj->magic_marker = 0xDEADBEEF;
>          obj->klass = (void*)0xDEADBEEF;
>          VIR_FREE(obj);
>      }
> @@ -311,7 +315,7 @@ virObjectRef(void *anyobj)
>  {
>      virObjectPtr obj = anyobj;
>  
> -    if (!obj)
> +    if (VIR_OBJECT_NOTVALID(obj))
>          return NULL;
>      virAtomicIntInc(&obj->u.s.refs);
>      PROBE(OBJECT_REF, "obj=%p", obj);
> @@ -389,7 +393,7 @@ virObjectIsClass(void *anyobj,
>                   virClassPtr klass)
>  {
>      virObjectPtr obj = anyobj;
> -    if (!obj)
> +    if (VIR_OBJECT_NOTVALID(obj))
>          return false;
>  
>      return virClassIsDerivedFrom(obj->klass, klass);
> diff --git a/src/util/virobject.h b/src/util/virobject.h
> index f4c292b..89f8050 100644
> --- a/src/util/virobject.h
> +++ b/src/util/virobject.h
> @@ -51,6 +51,7 @@ struct _virObject {
>              int refs;
>          } s;
>      } u;
> +    unsigned int magic_marker;
>      virClassPtr klass;
>  };

I'm wondering whether this will risk re-introducing the bug fixed in

  commit fca4f2334072d87f7faeb2948e6f83201309e1b9
  Author: Eric Blake <eblake redhat com>
  Date:   Thu Dec 12 16:01:15 2013 -0700

    object: require maximal alignment in base class

I'm also thinking we don't really need to have 2 magic fields in the
same struct - we already have a 'magic' field that is initialized from
the class object magic value.

Now, this existing magic is different for each object subclass - we allocate
class magic starting with

  static unsigned int magicCounter = 0xCAFE0000;

I'm thinking though, that we're never going to have > 65556 different
sub-classes (well at least not for a long time).

So instead of adding this new field you could just check

     ((object->u.s.magic & 0xCAFE0000) == 0xCAFE0000)


Regards,
Daniel
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