fs: avoid empty option when generating legacy mount string

As each option string fragment is always prepended with a comma it would
happen that the whole string always starts with a comma. This could be
interpreted by filesystem drivers as an empty option and may produce
errors.

For example the NTFS driver from ntfs.ko behaves like this and fails
when mounted via the new API.

Link: https://github.com/util-linux/util-linux/issues/2298
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Fixes: 3e1aeb00e6d1 ("vfs: Implement a filesystem superblock creation/configuration context")
Cc: stable@vger.kernel.org
Message-Id: <20230607-fs-empty-option-v1-1-20c8dbf4671b@weissschuh.net>
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/fs_context.c b/fs/fs_context.c
index 24ce12f..851214d 100644 (file)
--- a/fs/fs_context.c
+++ b/fs/fs_context.c
@@ -561,7 +561,8 @@ static int legacy_parse_param(struct fs_context *fc, struct fs_parameter *param)
                        return -ENOMEM;
        }
 
-       ctx->legacy_data[size++] = ',';
+       if (size)
+               ctx->legacy_data[size++] = ',';
        len = strlen(param->key);
        memcpy(ctx->legacy_data + size, param->key, len);
        size += len;