fhandle: hoist copy_from_user() above get_path_from_fd()

In follow-up patches we need access to @file_handle->handle_type
before we start caring about get_path_from_fd().

Link: https://lore.kernel.org/20250624-work-pidfs-fhandle-v2-2-d02a04858fe3@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

fs/fhandle.c

@@ -323,13 +323,24 @@ static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
 {
 	int retval = 0;
 	struct file_handle f_handle;
-	struct file_handle *handle = NULL;
+	struct file_handle *handle __free(kfree) = NULL;
 	struct handle_to_path_ctx ctx = {};
 	const struct export_operations *eops;
 
+	if (copy_from_user(&f_handle, ufh, sizeof(struct file_handle)))
+		return -EFAULT;
+
+	if ((f_handle.handle_bytes > MAX_HANDLE_SZ) ||
+	    (f_handle.handle_bytes == 0))
+		return -EINVAL;
+
+	if (f_handle.handle_type < 0 ||
+	    FILEID_USER_FLAGS(f_handle.handle_type) & ~FILEID_VALID_USER_FLAGS)
+		return -EINVAL;
+
 	retval = get_path_from_fd(mountdirfd, &ctx.root);
 	if (retval)
-		goto out_err;
+		return retval;
 
 	eops = ctx.root.mnt->mnt_sb->s_export_op;
 	if (eops && eops->permission)
@@ -339,21 +350,6 @@ static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
 	if (retval)
 		goto out_path;
 
-	if (copy_from_user(&f_handle, ufh, sizeof(struct file_handle))) {
-		retval = -EFAULT;
-		goto out_path;
-	}
-	if ((f_handle.handle_bytes > MAX_HANDLE_SZ) ||
-	    (f_handle.handle_bytes == 0)) {
-		retval = -EINVAL;
-		goto out_path;
-	}
-	if (f_handle.handle_type < 0 ||
-	    FILEID_USER_FLAGS(f_handle.handle_type) & ~FILEID_VALID_USER_FLAGS) {
-		retval = -EINVAL;
-		goto out_path;
-	}
-
 	handle = kmalloc(struct_size(handle, f_handle, f_handle.handle_bytes),
 			 GFP_KERNEL);
 	if (!handle) {
@@ -366,7 +362,7 @@ static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
 			   &ufh->f_handle,
 			   f_handle.handle_bytes)) {
 		retval = -EFAULT;
-		goto out_handle;
+		goto out_path;
 	}
 
 	/*
@@ -384,11 +380,8 @@ static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
 	handle->handle_type &= ~FILEID_USER_FLAGS_MASK;
 	retval = do_handle_to_path(handle, path, &ctx);
 
-out_handle:
-	kfree(handle);
 out_path:
 	path_put(&ctx.root);
-out_err:
 	return retval;
 }