vfs: protect i_nlink

Prevent direct modification of i_nlink by making it const and adding a
non-const __i_nlink alias.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Tested-by: Toshiyuki Okajima <toshi.okajima@jp.fujitsu.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

diff --git a/fs/inode.c b/fs/inode.c
index ecbb68d..ee4e66b 100644 (file)
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -142,7 +142,7 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
        atomic_set(&inode->i_count, 1);
        inode->i_op = &empty_iops;
        inode->i_fop = &empty_fops;
-       inode->i_nlink = 1;
+       inode->__i_nlink = 1;
        inode->i_opflags = 0;
        inode->i_uid = 0;
        inode->i_gid = 0;

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 48c1f5f..23467d7 100644 (file)
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -768,7 +768,17 @@ struct inode {
 
        /* Stat data, not accessed from path walking */
        unsigned long           i_ino;
-       unsigned int            i_nlink;
+       /*
+        * Filesystems may only read i_nlink directly.  They shall use the
+        * following functions for modification:
+        *
+        *    (set|clear|inc|drop)_nlink
+        *    inode_(inc|dec)_link_count
+        */
+       union {
+               const unsigned int i_nlink;
+               unsigned int __i_nlink;
+       };
        dev_t                   i_rdev;
        loff_t                  i_size;
        struct timespec         i_atime;
@@ -1764,7 +1774,7 @@ static inline void mark_inode_dirty_sync(struct inode *inode)
  */
 static inline void set_nlink(struct inode *inode, unsigned int nlink)
 {
-       inode->i_nlink = nlink;
+       inode->__i_nlink = nlink;
 }
 
 /**
@@ -1777,7 +1787,7 @@ static inline void set_nlink(struct inode *inode, unsigned int nlink)
  */
 static inline void inc_nlink(struct inode *inode)
 {
-       inode->i_nlink++;
+       inode->__i_nlink++;
 }
 
 static inline void inode_inc_link_count(struct inode *inode)
@@ -1799,7 +1809,7 @@ static inline void inode_inc_link_count(struct inode *inode)
  */
 static inline void drop_nlink(struct inode *inode)
 {
-       inode->i_nlink--;
+       inode->__i_nlink--;
 }
 
 /**
@@ -1812,7 +1822,7 @@ static inline void drop_nlink(struct inode *inode)
  */
 static inline void clear_nlink(struct inode *inode)
 {
-       inode->i_nlink = 0;
+       inode->__i_nlink = 0;
 }
 
 static inline void inode_dec_link_count(struct inode *inode)