Date:	Wed, 7 Dec 2011 02:01:07 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	akpm@...ux-foundation.org
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH] Add refcount type and refcount misuse debugging

There is quite a lot of idiomatic code which does

	if (atomic_dec_and_test(&obj->refcnt))
		[destroy obj]

Bugs like double-frees in this case are dereferred and it may not be
immediately obvious that double-free has happened.

The answer is to wrap reference count debugging to every such operation.

Enter _refcnt_t (non-atomic version), refcnt_t (atomic version) datatypes
and CONFIG_DEBUG_REFCNT config option.

The latter directly checks for
a) GET on dead object
b) PUT on dead object (aka double PUT)
(and indirectly for memory corruptions turning positive integers into negative)

All of this has basic idea coming from grsecurity/PaX's CONFIG_PAX_REFCOUNT code.
The main difference is that developer has to opt in into new code.

Convert struct proc_dir_entry for a start.

Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
---

 Very ligthly tested.

 fs/proc/generic.c       |    4 +-
 fs/proc/internal.h      |    2 -
 fs/proc/root.c          |    3 +
 include/linux/proc_fs.h |    4 +-
 include/linux/refcnt.h  |   89 ++++++++++++++++++++++++++++++++++++++++++++++++
 lib/Kconfig.debug       |    3 +
 6 files changed, 99 insertions(+), 6 deletions(-)

--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -624,7 +624,7 @@ static struct proc_dir_entry *__proc_create(struct proc_dir_entry **parent,
 	ent->namelen = len;
 	ent->mode = mode;
 	ent->nlink = nlink;
-	atomic_set(&ent->count, 1);
+	refcnt_init(&ent->refcnt);
 	ent->pde_users = 0;
 	spin_lock_init(&ent->pde_unload_lock);
 	ent->pde_unload_completion = NULL;
@@ -774,7 +774,7 @@ static void free_proc_entry(struct proc_dir_entry *de)
 
 void pde_put(struct proc_dir_entry *pde)
 {
-	if (atomic_dec_and_test(&pde->count))
+	if (refcnt_put(&pde->refcnt))
 		free_proc_entry(pde);
 }
 
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -110,7 +110,7 @@ void task_mem(struct seq_file *, struct mm_struct *);
 
 static inline struct proc_dir_entry *pde_get(struct proc_dir_entry *pde)
 {
-	atomic_inc(&pde->count);
+	refcnt_get(&pde->refcnt);
 	return pde;
 }
 void pde_put(struct proc_dir_entry *pde);
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -9,6 +9,7 @@
 #include <asm/uaccess.h>
 
 #include <linux/errno.h>
+#include <linux/refcnt.h>
 #include <linux/time.h>
 #include <linux/proc_fs.h>
 #include <linux/stat.h>
@@ -188,7 +189,7 @@ struct proc_dir_entry proc_root = {
 	.namelen	= 5, 
 	.mode		= S_IFDIR | S_IRUGO | S_IXUGO, 
 	.nlink		= 2, 
-	.count		= ATOMIC_INIT(1),
+	.refcnt		= REFCNT_INIT,
 	.proc_iops	= &proc_root_inode_operations, 
 	.proc_fops	= &proc_root_operations,
 	.parent		= &proc_root,
--- a/include/linux/proc_fs.h
+++ b/include/linux/proc_fs.h
@@ -1,11 +1,11 @@
 #ifndef _LINUX_PROC_FS_H
 #define _LINUX_PROC_FS_H
 
+#include <linux/refcnt.h>
 #include <linux/slab.h>
 #include <linux/fs.h>
 #include <linux/spinlock.h>
 #include <linux/magic.h>
-#include <linux/atomic.h>
 
 struct net;
 struct completion;
@@ -69,7 +69,7 @@ struct proc_dir_entry {
 	void *data;
 	read_proc_t *read_proc;
 	write_proc_t *write_proc;
-	atomic_t count;		/* use count */
+	refcnt_t refcnt;
 	int pde_users;	/* number of callers into module in progress */
 	struct completion *pde_unload_completion;
 	struct list_head pde_openers;	/* who did ->open, but not ->release */
--- /dev/null
+++ b/include/linux/refcnt.h
@@ -0,0 +1,89 @@
+/*
+ * Use these types iff
+ * a) object is created with refcount 1, and
+ * b) every GET does +1, and
+ * c) every PUT does -1, and
+ * d) once refcount reaches 0, object is destroyed.
+ *
+ * Do not use otherwise.
+ *
+ * Use underscored version if refcount manipulations are under
+ * some sort of locking already making additional atomicity unnecessary.
+ */
+#ifndef _LINUX_REFCNT_H
+#define _LINUX_REFCNT_H
+#include <linux/types.h>
+#include <asm/atomic.h>
+#ifdef CONFIG_DEBUG_REFCNT
+#include <asm/bug.h>
+#endif
+
+typedef struct {
+	int _n;
+} _refcnt_t;
+#define _REFCNT_INIT	((_refcnt_t){ ._n = 1 })
+
+static inline void _refcnt_init(_refcnt_t *refcnt)
+{
+	refcnt->_n = 1;
+}
+
+static inline void _refcnt_get(_refcnt_t *refcnt)
+{
+#ifdef CONFIG_DEBUG_REFCNT
+	BUG_ON(refcnt->_n < 1);
+#endif
+	refcnt->_n++;
+}
+
+static inline int _refcnt_put(_refcnt_t *refcnt)
+{
+#ifdef CONFIG_DEBUG_REFCNT
+	BUG_ON(refcnt->_n < 1);
+#endif
+	refcnt->_n--;
+	return refcnt->_n == 0;
+}
+
+typedef struct {
+	atomic_t _n;
+} refcnt_t;
+#define REFCNT_INIT	((refcnt_t){ ._n = ATOMIC_INIT(1) })
+
+static inline void refcnt_init(refcnt_t *refcnt)
+{
+	atomic_set(&refcnt->_n, 1);
+}
+
+static inline void refcnt_get(refcnt_t *refcnt)
+{
+#ifdef CONFIG_DEBUG_REFCNT
+	int rv;
+
+	rv = atomic_inc_return(&refcnt->_n);
+	BUG_ON(rv < 2);
+#else
+	atomic_inc(&refcnt->_n);
+#endif
+}
+
+/*
+ * Return 1 if last PUT was done, return 0 otherwise.
+ *
+ *	if (refcnt_put(&obj->refcnt)) {
+ *		[destroy object]
+ *	}
+ */
+static inline int refcnt_put(refcnt_t *refcnt)
+{
+#ifdef CONFIG_DEBUG_REFCNT
+	int rv;
+
+	rv = atomic_dec_return(&refcnt->_n);
+	BUG_ON(rv < 0);
+	return rv == 0;
+#else
+	return atomic_dec_and_test(&refcnt->_n);
+#endif
+}
+#endif
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1276,3 +1276,6 @@ source "lib/Kconfig.kmemcheck"
 
 config TEST_KSTRTOX
 	tristate "Test kstrto*() family of functions at runtime"
+
+config DEBUG_REFCNT
+	bool "Debug reference count objects"
--
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