linux-kernel - on static kobjects and double frees...

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Date:	Tue, 10 Jun 2008 08:58:50 -0700
From:	Arthur Jones <ajones@...erbed.com>
To:	Greg KH <greg@...ah.com>
CC:	linux-kernel@...r.kernel.org, Doug Thompson <norsk5@...oo.com>,
	bluesmoke-devel@...ts.sourceforge.net
Subject: on static kobjects and double frees...

Hi Greg,  The edac pci sysfs generic layer uses a static
kobject as a placeholder parent where edac pci drivers
are inserted.  An atomic count is used to know when
to kobject_init_add_add or kobject_put the static kobject.
The issue with this is that the name gets double freed
on the second module load as edac does not clear it, and
kobject_cleanup does not clear it.

The quick fix was to clear the static kobj name before
calling kobject_init, but that seems a bit fragile as it
involves knowing the internals of kobject_put.  Perhaps
the name should be cleared before calling the kobject
release method?  Something like this (not even compile
tested):

diff --git a/lib/kobject.c b/lib/kobject.c
index 718e510..7dfe906 100644
--- a/lib/kobject.c
+++ b/lib/kobject.c
@@ -552,6 +552,9 @@ static void kobject_cleanup(struct kobject *kobj)
 	if (t && t->release) {
 		pr_debug("kobject: '%s' (%p): calling ktype release\n",
 			 kobject_name(kobj), kobj);
+
+		/* avoid double free with static kobjects... */
+		kobj->name = NULL;
 		t->release(kobj);
 	}

What do you think?  I'm happy to implement and test whatever
you think is best...

The edac code in question is drivers/edac/edac_pci_sysfs.c,
the static kobject is called edac_pci_top_main_kobj...

Arthur
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