debugfs: add support for more elaborate ->d_fsdata

Currently, the user provided fops, "real_fops", are stored directly into
->d_fsdata.

In order to be able to store more per-file state and thus prepare for more
granular file removal protection, wrap the real_fops into a dynamically
allocated container struct, debugfs_fsdata.

A struct debugfs_fsdata gets allocated at file creation and freed from the
newly intoduced ->d_release().

Finally, move the implementation of debugfs_real_fops() out of the public
debugfs header such that struct debugfs_fsdata's declaration can be kept
private.

Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

3 files changed, 35 insertions, 3 deletions

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index 6dabc4a10396..b6f5ddab66bf 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -97,6 +97,18 @@ EXPORT_SYMBOL_GPL(debugfs_use_file_finish);
 
 #define F_DENTRY(filp) ((filp)->f_path.dentry)
 
+const struct file_operations *debugfs_real_fops(const struct file *filp)
+        __must_hold(&debugfs_srcu)
+{
+        struct debugfs_fsdata *fsd = F_DENTRY(filp)->d_fsdata;
+        /*
+         * Neither the pointer to the struct file_operations, nor its
+         * contents ever change -- srcu_dereference() is not needed here.
+         */
+        return fsd->real_fops;
+}
+EXPORT_SYMBOL_GPL(debugfs_real_fops);
+
 static int open_proxy_open(struct inode *inode, struct file *filp)
 {
         const struct dentry *dentry = F_DENTRY(filp);
diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index c59f015f386e..a9c3d3e9af39 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -185,6 +185,11 @@ static const struct super_operations debugfs_super_operations = {
         .evict_inode    = debugfs_evict_inode,
 };
 
+static void debugfs_release_dentry(struct dentry *dentry)
+{
+        kfree(dentry->d_fsdata);
+}
+
 static struct vfsmount *debugfs_automount(struct path *path)
 {
         debugfs_automount_t f;
@@ -194,6 +199,7 @@ static struct vfsmount *debugfs_automount(struct path *path)
 
 static const struct dentry_operations debugfs_dops = {
         .d_delete = always_delete_dentry,
+        .d_release = debugfs_release_dentry,
         .d_automount = debugfs_automount,
 };
 
@@ -341,24 +347,34 @@ static struct dentry *__debugfs_create_file(const char *name, umode_t mode,
 {
         struct dentry *dentry;
         struct inode *inode;
+        struct debugfs_fsdata *fsd;
+
+        fsd = kmalloc(sizeof(*fsd), GFP_KERNEL);
+        if (!fsd)
+                return NULL;
 
         if (!(mode & S_IFMT))
                 mode |= S_IFREG;
         BUG_ON(!S_ISREG(mode));
         dentry = start_creating(name, parent);
 
-        if (IS_ERR(dentry))
+        if (IS_ERR(dentry)) {
+                kfree(fsd);
                 return NULL;
+        }
 
         inode = debugfs_get_inode(dentry->d_sb);
-        if (unlikely(!inode))
+        if (unlikely(!inode)) {
+                kfree(fsd);
                 return failed_creating(dentry);
+        }
 
         inode->i_mode = mode;
         inode->i_private = data;
 
         inode->i_fop = proxy_fops;
-        dentry->d_fsdata = (void *)real_fops;
+        fsd->real_fops = real_fops;
+        dentry->d_fsdata = fsd;
 
         d_instantiate(dentry, inode);
         fsnotify_create(d_inode(dentry->d_parent), dentry);
diff --git a/fs/debugfs/internal.h b/fs/debugfs/internal.h
index b3e8443a1f47..512601eed3ce 100644
--- a/fs/debugfs/internal.h
+++ b/fs/debugfs/internal.h
@@ -19,4 +19,8 @@ extern const struct file_operations debugfs_noop_file_operations;
 extern const struct file_operations debugfs_open_proxy_file_operations;
 extern const struct file_operations debugfs_full_proxy_file_operations;
 
+struct debugfs_fsdata {
+        const struct file_operations *real_fops;
+};
+
 #endif /* _DEBUGFS_INTERNAL_H_ */