[PATCH v2 7/8] smb: client: block cache=ro and cache=singleclient on remount

[rajasimandalos@gmail.com]

From: Rajasi Mandal <rajasimandal@microsoft.com>

cache=ro and cache=singleclient are mount-time environment declarations
where the admin promises that the share is read-only or exclusively
accessed.  The client bypasses server-based coherency (oplocks/leases)
and caches aggressively based on this promise.

These modes were intentionally excluded from smb3_update_mnt_flags()
when it was introduced in commit 2d39f50c2b15 ("cifs: move update of
flags into a separate function") — only cache=strict, cache=none and
cache=loose were made reconfigurable.  However, remount currently
silently accepts cache=ro and cache=singleclient without actually
applying them, which is confusing.

Add explicit checks in smb3_verify_reconfigure_ctx() to reject
attempts to change these options during remount with a clear error
message.

Signed-off-by: Rajasi Mandal <rajasimandal@microsoft.com>
---
 fs/smb/client/fs_context.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/smb/client/fs_context.c b/fs/smb/client/fs_context.c
index 01c57adb0ef1..009b9480b5cc 100644
--- a/fs/smb/client/fs_context.c
+++ b/fs/smb/client/fs_context.c
@@ -1207,6 +1207,15 @@ static int smb3_verify_reconfigure_ctx(struct fs_context *fc,
 		cifs_errorf(fc, "can not change rdma during remount\n");
 		return -EINVAL;
 	}
+	/* init default: cache_ro = false, cache_rw = false (i.e. cache=strict) */
+	if (new_ctx->cache_ro != old_ctx->cache_ro) {
+		cifs_errorf(fc, "can not change cache=ro during remount\n");
+		return -EINVAL;
+	}
+	if (new_ctx->cache_rw != old_ctx->cache_rw) {
+		cifs_errorf(fc, "can not change cache=singleclient during remount\n");
+		return -EINVAL;
+	}
 
 	return 0;
 }
-- 
2.43.0