debugfs: add support for self-protecting attribute file fops

In order to protect them against file removal issues, debugfs_create_file()
creates a lifetime managing proxy around each struct file_operations
handed in.

In cases where this struct file_operations is able to manage file lifetime
by itself already, the proxy created by debugfs is a waste of resources.

The most common class of struct file_operations given to debugfs are those
defined by means of the DEFINE_SIMPLE_ATTRIBUTE() macro.

Introduce a DEFINE_DEBUGFS_ATTRIBUTE() macro to allow any
struct file_operations of this class to be easily made file lifetime aware
and thus, to be operated unproxied.

Specifically, introduce debugfs_attr_read() and debugfs_attr_write()
which wrap simple_attr_read() and simple_attr_write() under the protection
of a debugfs_use_file_start()/debugfs_use_file_finish() pair.

Make DEFINE_DEBUGFS_ATTRIBUTE() set the defined struct file_operations'
->read() and ->write() members to these wrappers.

Export debugfs_create_file_unsafe() in order to allow debugfs users to
create their files in non-proxying operation mode.

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

2 files changed, 56 insertions, 0 deletions

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index 6eb58a8ed03c..8ef56d9499a4 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -284,6 +284,34 @@ const struct file_operations debugfs_full_proxy_file_operations = {
         .open = full_proxy_open,
 };
 
+ssize_t debugfs_attr_read(struct file *file, char __user *buf,
+                        size_t len, loff_t *ppos)
+{
+        ssize_t ret;
+        int srcu_idx;
+
+        ret = debugfs_use_file_start(F_DENTRY(file), &srcu_idx);
+        if (likely(!ret))
+                ret = simple_attr_read(file, buf, len, ppos);
+        debugfs_use_file_finish(srcu_idx);
+        return ret;
+}
+EXPORT_SYMBOL_GPL(debugfs_attr_read);
+
+ssize_t debugfs_attr_write(struct file *file, const char __user *buf,
+                         size_t len, loff_t *ppos)
+{
+        ssize_t ret;
+        int srcu_idx;
+
+        ret = debugfs_use_file_start(F_DENTRY(file), &srcu_idx);
+        if (likely(!ret))
+                ret = simple_attr_write(file, buf, len, ppos);
+        debugfs_use_file_finish(srcu_idx);
+        return ret;
+}
+EXPORT_SYMBOL_GPL(debugfs_attr_write);
+
 static struct dentry *debugfs_create_mode(const char *name, umode_t mode,
                                           struct dentry *parent, void *value,
                                           const struct file_operations *fops,
diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index 136f269f01de..41e079a8da26 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -369,6 +369,33 @@ struct dentry *debugfs_create_file(const char *name, umode_t mode,
 }
 EXPORT_SYMBOL_GPL(debugfs_create_file);
 
+/**
+ * debugfs_create_file_unsafe - create a file in the debugfs filesystem
+ * @name: a pointer to a string containing the name of the file to create.
+ * @mode: the permission that the file should have.
+ * @parent: a pointer to the parent dentry for this file.  This should be a
+ *          directory dentry if set.  If this parameter is NULL, then the
+ *          file will be created in the root of the debugfs filesystem.
+ * @data: a pointer to something that the caller will want to get to later
+ *        on.  The inode.i_private pointer will point to this value on
+ *        the open() call.
+ * @fops: a pointer to a struct file_operations that should be used for
+ *        this file.
+ *
+ * debugfs_create_file_unsafe() is completely analogous to
+ * debugfs_create_file(), the only difference being that the fops
+ * handed it will not get protected against file removals by the
+ * debugfs core.
+ *
+ * It is your responsibility to protect your struct file_operation
+ * methods against file removals by means of debugfs_use_file_start()
+ * and debugfs_use_file_finish(). ->open() is still protected by
+ * debugfs though.
+ *
+ * Any struct file_operations defined by means of
+ * DEFINE_DEBUGFS_ATTRIBUTE() is protected against file removals and
+ * thus, may be used here.
+ */
 struct dentry *debugfs_create_file_unsafe(const char *name, umode_t mode,
                                    struct dentry *parent, void *data,
                                    const struct file_operations *fops)
@@ -379,6 +406,7 @@ struct dentry *debugfs_create_file_unsafe(const char *name, umode_t mode,
                                         &debugfs_noop_file_operations,
                                 fops);
 }
+EXPORT_SYMBOL_GPL(debugfs_create_file_unsafe);
 
 /**
  * debugfs_create_file_size - create a file in the debugfs filesystem