[PATCH] fs, nilfs: convert nilfs_root.count from atomic_t to refcount_t

[Elena Reshetova]

atomic_t variables are currently used to implement reference
counters with the following properties:
 - counter is initialized to 1 using atomic_set()
 - a resource is freed upon counter reaching zero
 - once counter reaches zero, its further
   increments aren't allowed
 - counter schema uses basic atomic operations
   (set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.

The variable nilfs_root.count is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

Suggested-by: Kees Cook <keesc...@chromium.org>
Reviewed-by: David Windsor <dwind...@gmail.com>
Reviewed-by: Hans Liljestrand <ishkam...@gmail.com>
Signed-off-by: Elena Reshetova <elena.reshet...@intel.com>
---
 fs/nilfs2/the_nilfs.c | 8 ++++----
 fs/nilfs2/the_nilfs.h | 5 +++--
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/fs/nilfs2/the_nilfs.c b/fs/nilfs2/the_nilfs.c
index 2dd75bf..afebb50 100644
--- a/fs/nilfs2/the_nilfs.c
+++ b/fs/nilfs2/the_nilfs.c
@@ -737,7 +737,7 @@ struct nilfs_root *nilfs_lookup_root(struct the_nilfs 
*nilfs, __u64 cno)
                } else if (cno > root->cno) {
                        n = n->rb_right;
                } else {
-                       atomic_inc(&root->count);
+                       refcount_inc(&root->count);
                        spin_unlock(&nilfs->ns_cptree_lock);
                        return root;
                }
@@ -776,7 +776,7 @@ nilfs_find_or_create_root(struct the_nilfs *nilfs, __u64 
cno)
                } else if (cno > root->cno) {
                        p = &(*p)->rb_right;
                } else {
-                       atomic_inc(&root->count);
+                       refcount_inc(&root->count);
                        spin_unlock(&nilfs->ns_cptree_lock);
                        kfree(new);
                        return root;
@@ -786,7 +786,7 @@ nilfs_find_or_create_root(struct the_nilfs *nilfs, __u64 
cno)
        new->cno = cno;
        new->ifile = NULL;
        new->nilfs = nilfs;
-       atomic_set(&new->count, 1);
+       refcount_set(&new->count, 1);
        atomic64_set(&new->inodes_count, 0);
        atomic64_set(&new->blocks_count, 0);
 
@@ -806,7 +806,7 @@ nilfs_find_or_create_root(struct the_nilfs *nilfs, __u64 
cno)
 
 void nilfs_put_root(struct nilfs_root *root)
 {
-       if (atomic_dec_and_test(&root->count)) {
+       if (refcount_dec_and_test(&root->count)) {
                struct the_nilfs *nilfs = root->nilfs;
 
                nilfs_sysfs_delete_snapshot_group(root);
diff --git a/fs/nilfs2/the_nilfs.h b/fs/nilfs2/the_nilfs.h
index b305c6f..883d732 100644
--- a/fs/nilfs2/the_nilfs.h
+++ b/fs/nilfs2/the_nilfs.h
@@ -27,6 +27,7 @@
 #include <linux/blkdev.h>
 #include <linux/backing-dev.h>
 #include <linux/slab.h>
+#include <linux/refcount.h>
 
 struct nilfs_sc_info;
 struct nilfs_sysfs_dev_subgroups;
@@ -246,7 +247,7 @@ struct nilfs_root {
        __u64 cno;
        struct rb_node rb_node;
 
-       atomic_t count;
+       refcount_t count;
        struct the_nilfs *nilfs;
        struct inode *ifile;
 
@@ -299,7 +300,7 @@ void nilfs_swap_super_block(struct the_nilfs *);
 
 static inline void nilfs_get_root(struct nilfs_root *root)
 {
-       atomic_inc(&root->count);
+       refcount_inc(&root->count);
 }
 
 static inline int nilfs_valid_fs(struct the_nilfs *nilfs)
-- 
2.7.4