[PATCH] fs: avoid empty option when generating legacy mount string

[Thomas Weißschuh]

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
Fixes: 3e1aeb00e6d1 ("vfs: Implement a filesystem superblock creation/configuration context")
Signed-off-by: Thomas Weißschuh <linux@xxxxxxxxxxxxxx>
---
 fs/fs_context.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/fs_context.c b/fs/fs_context.c
index 24ce12f0db32..851214d1d013 100644
--- 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;

---
base-commit: 9561de3a55bed6bdd44a12820ba81ec416e705a7
change-id: 20230607-fs-empty-option-265622371023

Best regards,
-- 
Thomas Weißschuh <linux@xxxxxxxxxxxxxx>