debugfs: Add write support to debugfs_create_str()

Currently, debugfs_create_str() only supports reading strings from
debugfs. Add support for writing them as well.

Based on original implementation by Peter Zijlstra [0]. Write support
was present in the initial patch version, but dropped in v2 due to lack
of users. We have a user now, so reintroduce it.

[0] https://lore.kernel.org/all/YF3Hv5zXb%2F6lauzs@hirez.programming.kicks-ass.net/Signed-off-by: Mike Tipton <quic_mdtipton@quicinc.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20230807142914.12480-2-quic_mdtipton@quicinc.comSigned-off-by: Georgi Djakov <djakov@kernel.org>

fs/debugfs/file.c

@@ -904,8 +904,52 @@ EXPORT_SYMBOL_GPL(debugfs_create_str);
 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;
+	struct dentry *dentry = F_DENTRY(file);
+	char *old, *new = NULL;
+	int pos = *ppos;
+	int r;
+
+	r = debugfs_file_get(dentry);
+	if (unlikely(r))
+		return r;
+
+	old = *(char **)file->private_data;
+
+	/* only allow strict concatenation */
+	r = -EINVAL;
+	if (pos && pos != strlen(old))
+		goto error;
+
+	r = -E2BIG;
+	if (pos + count + 1 > PAGE_SIZE)
+		goto error;
+
+	r = -ENOMEM;
+	new = kmalloc(pos + count + 1, GFP_KERNEL);
+	if (!new)
+		goto error;
+
+	if (pos)
+		memcpy(new, old, pos);
+
+	r = -EFAULT;
+	if (copy_from_user(new + pos, user_buf, count))
+		goto error;
+
+	new[pos + count] = '\0';
+	strim(new);
+
+	rcu_assign_pointer(*(char **)file->private_data, new);
+	synchronize_rcu();
+	kfree(old);
+
+	debugfs_file_put(dentry);
+	return count;
+
+error:
+	kfree(new);
+	debugfs_file_put(dentry);
+	return r;
 }
 
 static const struct file_operations fops_str = {