debugfs: Implement debugfs_create_str()

Implement debugfs_create_str() to easily display names and such in
debugfs.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20210412102001.415407080@infradead.org

fs/debugfs/file.c

@@ -865,6 +865,97 @@ struct dentry *debugfs_create_bool(const char *name, umode_t mode,
 }
 EXPORT_SYMBOL_GPL(debugfs_create_bool);
 
+ssize_t debugfs_read_file_str(struct file *file, char __user *user_buf,
+			      size_t count, loff_t *ppos)
+{
+	struct dentry *dentry = F_DENTRY(file);
+	char *str, *copy = NULL;
+	int copy_len, len;
+	ssize_t ret;
+
+	ret = debugfs_file_get(dentry);
+	if (unlikely(ret))
+		return ret;
+
+	str = *(char **)file->private_data;
+	len = strlen(str) + 1;
+	copy = kmalloc(len, GFP_KERNEL);
+	if (!copy) {
+		debugfs_file_put(dentry);
+		return -ENOMEM;
+	}
+
+	copy_len = strscpy(copy, str, len);
+	debugfs_file_put(dentry);
+	if (copy_len < 0) {
+		kfree(copy);
+		return copy_len;
+	}
+
+	copy[copy_len] = '\n';
+
+	ret = simple_read_from_buffer(user_buf, count, ppos, copy, copy_len);
+	kfree(copy);
+
+	return ret;
+}
+
+static ssize_t debugfs_write_file_str(struct file *file, const char __user *user_buf,
+				      size_t count, loff_t *ppos)
+{
+	/* This is really only for read-only strings */
+	return -EINVAL;
+}
+
+static const struct file_operations fops_str = {
+	.read =		debugfs_read_file_str,
+	.write =	debugfs_write_file_str,
+	.open =		simple_open,
+	.llseek =	default_llseek,
+};
+
+static const struct file_operations fops_str_ro = {
+	.read =		debugfs_read_file_str,
+	.open =		simple_open,
+	.llseek =	default_llseek,
+};
+
+static const struct file_operations fops_str_wo = {
+	.write =	debugfs_write_file_str,
+	.open =		simple_open,
+	.llseek =	default_llseek,
+};
+
+/**
+ * debugfs_create_str - create a debugfs file that is used to read and write a string value
+ * @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.
+ * @value: a pointer to the variable that the file should read to and write
+ *         from.
+ *
+ * This function creates a file in debugfs with the given name that
+ * contains the value of the variable @value.  If the @mode variable is so
+ * set, it can be read from, and written to.
+ *
+ * This function will return a pointer to a dentry if it succeeds.  This
+ * pointer must be passed to the debugfs_remove() function when the file is
+ * to be removed (no automatic cleanup happens if your module is unloaded,
+ * you are responsible here.)  If an error occurs, ERR_PTR(-ERROR) will be
+ * returned.
+ *
+ * If debugfs is not enabled in the kernel, the value ERR_PTR(-ENODEV) will
+ * be returned.
+ */
+void debugfs_create_str(const char *name, umode_t mode,
+			struct dentry *parent, char **value)
+{
+	debugfs_create_mode_unsafe(name, mode, parent, value, &fops_str,
+				   &fops_str_ro, &fops_str_wo);
+}
+
 static ssize_t read_file_blob(struct file *file, char __user *user_buf,
 			      size_t count, loff_t *ppos)
 {

include/linux/debugfs.h

@@ -128,6 +128,8 @@ void debugfs_create_atomic_t(const char *name, umode_t mode,
 			     struct dentry *parent, atomic_t *value);
 struct dentry *debugfs_create_bool(const char *name, umode_t mode,
 				  struct dentry *parent, bool *value);
+void debugfs_create_str(const char *name, umode_t mode,
+			struct dentry *parent, char **value);
 
 struct dentry *debugfs_create_blob(const char *name, umode_t mode,
 				  struct dentry *parent,
@@ -156,6 +158,9 @@ ssize_t debugfs_read_file_bool(struct file *file, char __user *user_buf,
 ssize_t debugfs_write_file_bool(struct file *file, const char __user *user_buf,
 				size_t count, loff_t *ppos);
 
+ssize_t debugfs_read_file_str(struct file *file, char __user *user_buf,
+			      size_t count, loff_t *ppos);
+
 #else
 
 #include <linux/err.h>
@@ -297,6 +302,11 @@ static inline struct dentry *debugfs_create_bool(const char *name, umode_t mode,
 	return ERR_PTR(-ENODEV);
 }
 
+static inline void debugfs_create_str(const char *name, umode_t mode,
+				      struct dentry *parent,
+				      char **value)
+{ }
+
 static inline struct dentry *debugfs_create_blob(const char *name, umode_t mode,
 				  struct dentry *parent,
 				  struct debugfs_blob_wrapper *blob)
@@ -348,6 +358,13 @@ static inline ssize_t debugfs_write_file_bool(struct file *file,
 	return -ENODEV;
 }
 
+static inline ssize_t debugfs_read_file_str(struct file *file,
+					    char __user *user_buf,
+					    size_t count, loff_t *ppos)
+{
+	return -ENODEV;
+}
+
 #endif
 
 /**