FAILED: patch "[PATCH] btrfs: reject log replay if there is unsupported RO compat" failed to apply to 4.14-stable tree

The patch below does not apply to the 4.14-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From dc4d31684974d140250f3ee612c3f0cab13b3146 Mon Sep 17 00:00:00 2001
From: Qu Wenruo <[email protected]>
Date: Tue, 7 Jun 2022 19:48:24 +0800
Subject: [PATCH] btrfs: reject log replay if there is unsupported RO compat
flag

[BUG]
If we have a btrfs image with dirty log, along with an unsupported RO compatible flag:

log_root 30474240
...
compat_flags 0x0
compat_ro_flags 0x40000003

			( FREE_SPACE_TREE |
			  FREE_SPACE_TREE_VALID |
			  unknown flag: 0x40000000 )

Then even if we can only mount it RO, we will still cause metadata update for log replay:

BTRFS info (device dm-1): flagging fs with big metadata feature BTRFS info (device dm-1): using free space tree
BTRFS info (device dm-1): has skinny extents
BTRFS info (device dm-1): start tree-log replay

This is definitely against RO compact flag requirement.

[CAUSE]
RO compact flag only forces us to do RO mount, but we will still do log replay for plain RO mount.

Thus this will result us to do log replay and update metadata.

This can be very problematic for new RO compat flag, for example older kernel can not understand v2 cache, and if we allow metadata update on RO mount and invalidate/corrupt v2 cache.

[FIX]
Just reject the mount unless rescue=nologreplay is provided:

BTRFS error (device dm-1): cannot replay dirty log with unsupport optional features (0x40000000), try rescue=nologreplay instead

We don't want to set rescue=nologreply directly, as this would make the end user to read the old data, and cause confusion.

Since the such case is really rare, we're mostly fine to just reject the mount with an error message, which also includes the proper workaround.

index ed1d92b370db..32b88a227734 100644
@@ -3556,6 +3556,20 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 		err = -EINVAL;
 		goto fail_alloc;
 	}
+	/*
+	 * We have unsupported RO compat features, although RO mounted, we
+	 * should not cause any metadata write, including log replay.
+	 * Or we could screw up whatever the new feature requires.
+	 */
+	if (unlikely(features && btrfs_super_log_root(disk_super) &&
+		     !btrfs_test_opt(fs_info, NOLOGREPLAY))) {
+		btrfs_err(fs_info,
+"cannot replay dirty log with unsupported compat_ro features (0x%llx), try rescue=nologreplay",
+			  features);
+		err = -EINVAL;
+		goto fail_alloc;
+	}
+
 
 	if (sectorsize < PAGE_SIZE) {
 		struct btrfs_subpage_info *subpage_info;