Re: kobject_init and the zeroed-out-memory requirement

[Jason Noakes]

> Is there any in-kernel code that does not properly zero out the memory
> before calling kobject_init()?

I'm not sure. I didn't find any, but I've seen it bite people writing
drivers more than once where I work, and the latest oops I just
debugged a few days ago prompted me to address the issue and at least
get the documentation updated.

>> Many other *_init kernel routines (cdev_init, kref_init, mutex_init,
>> spin_lock_init, etc) do not have the same requirement - they work on fully
>> uninitialized memory.
>
> They all do different things, you can't compare apples to oranges :)

Well, they are all named "_init", which to me implies that they
initialize the object/memory passed in, which is true in all of the
other cases I mentioned, but not for kobject_init. Perhaps it should
have been called kobject_setup or something. But I digress...

>> Documentation/kobject.txt does not mention the requirement that the memory be
>> zero-initialized before it is passed to kobject_init.
>
> Add a line of text to the kerneldoc for kobject_init to mention this?
>
> Or (a) is fine as well, if it makes you feel better, but if you do so,
> just say that all memory for kobjects must be created with kzalloc() and
> don't mention memset as that will cause people to try to reuse kobjects,
> like they have in the past, and bad things will happen then.

Here is a patch updating the documentation (kobject.txt and
kobject_init's usage documentation) so that the zeroed-out-memory
requirement is explicit.

Signed-off-by: Jason J. Noakes <jjnoakes@xxxxxxxxx>

diff --git a/Documentation/kobject.txt b/Documentation/kobject.txt
index f87241d..1b38727 100644
--- a/Documentation/kobject.txt
+++ b/Documentation/kobject.txt
@@ -113,8 +113,14 @@ question.  That macro is subsequently invoked with:

 Initialization of kobjects

-Code which creates a kobject must, of course, initialize that object. Some
-of the internal fields are setup with a (mandatory) call to kobject_init():
+Code which creates a kobject must, of course, initialize that object.
+
+First, ensure the memory for the kobject is initialized to zero; this is
+typically accomplished by obtaining the memory for the kobject (and the
+structure it is embedded in) from a call to kzalloc().
+
+Then, some of the internal fields must be setup with a (mandatory) call
+to kobject_init():

     void kobject_init(struct kobject *kobj, struct kobj_type *ktype);

diff --git a/lib/kobject.c b/lib/kobject.c
index 58751bb..46fc865 100644
--- a/lib/kobject.c
+++ b/lib/kobject.c
@@ -307,6 +307,8 @@ EXPORT_SYMBOL(kobject_set_name);
  * This function will properly initialize a kobject such that it can then
  * be passed to the kobject_add() call.
  *
+ * The kobject must be initialized to zero prior to calling this function.
+ *
  * After this function is called, the kobject MUST be cleaned up by a call
  * to kobject_put(), not by a call to kfree directly to ensure that all of
  * the memory is cleaned up properly.
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