The patch below does not apply to the 5.15-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 stable@vger.kernel.org.

Possible dependencies:

2ba48b20049b ("btrfs: fix compat_ro checks against remount") d7f67ac9a928 ("btrfs: relax block-group-tree feature dependency checks") a05d3c915314 ("btrfs: check superblock to ensure the fs was not modified at thaw time") 49f468c93807 ("btrfs: don't print 'has skinny extents' anymore on mount") 6b769dac219d ("btrfs: don't print 'flagging with big metadata' anymore on mount") e26b04c4c919 ("btrfs: properly flag filesystem with BTRFS_FEATURE_INCOMPAT_BIG_METADATA") dc4d31684974 ("btrfs: reject log replay if there is unsupported RO compat flag") 0591f0403621 ("btrfs: prevent remounting to v1 space cache for subpage mount") d5321a0fa8bc ("btrfs: add "0x" prefix for unsupported optional features") 8481dd80ab1e ("btrfs: subpage: introduce btrfs_subpage_bitmap_info")

thanks,

greg k-h

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

From 2ba48b20049b5a76f34a85f853c9496d1b10533a Mon Sep 17 00:00:00 2001 From: Qu Wenruo wqu@suse.com Date: Thu, 22 Dec 2022 07:59:17 +0800 Subject: [PATCH] btrfs: fix compat_ro checks against remount

[BUG] Even with commit 81d5d61454c3 ("btrfs: enhance unsupported compat RO flags handling"), btrfs can still mount a fs with unsupported compat_ro flags read-only, then remount it RW:

# btrfs ins dump-super /dev/loop0 | grep compat_ro_flags -A 3 compat_ro_flags 0x403 ( FREE_SPACE_TREE | FREE_SPACE_TREE_VALID | unknown flag: 0x400 )

# mount /dev/loop0 /mnt/btrfs mount: /mnt/btrfs: wrong fs type, bad option, bad superblock on /dev/loop0, missing codepage or helper program, or other error. dmesg(1) may have more information after failed mount system call. ^^^ RW mount failed as expected ^^^

# dmesg -t | tail -n5 loop0: detected capacity change from 0 to 1048576 BTRFS: device fsid cb5b82f5-0fdd-4d81-9b4b-78533c324afa devid 1 transid 7 /dev/loop0 scanned by mount (1146) BTRFS info (device loop0): using crc32c (crc32c-intel) checksum algorithm BTRFS info (device loop0): using free space tree BTRFS error (device loop0): cannot mount read-write because of unknown compat_ro features (0x403) BTRFS error (device loop0): open_ctree failed

# mount /dev/loop0 -o ro /mnt/btrfs # mount -o remount,rw /mnt/btrfs ^^^ RW remount succeeded unexpectedly ^^^

[CAUSE] Currently we use btrfs_check_features() to check compat_ro flags against our current mount flags.

That function get reused between open_ctree() and btrfs_remount().

But for btrfs_remount(), the super block we passed in still has the old mount flags, thus btrfs_check_features() still believes we're mounting read-only.

[FIX] Replace the existing @sb argument with @is_rw_mount.

As originally we only use @sb to determine if the mount is RW.

Now it's callers' responsibility to determine if the mount is RW, and since there are only two callers, the check is pretty simple:

- caller in open_ctree() Just pass !sb_rdonly().

- caller in btrfs_remount() Pass !(*flags & SB_RDONLY), as our check should be against the new flags.

Now we can correctly reject the RW remount:

# mount /dev/loop0 -o ro /mnt/btrfs # mount -o remount,rw /mnt/btrfs mount: /mnt/btrfs: mount point not mounted or bad option. dmesg(1) may have more information after failed mount system call. # dmesg -t | tail -n 1 BTRFS error (device loop0: state M): cannot mount read-write because of unknown compat_ro features (0x403)

Reported-by: Chung-Chiang Cheng shepjeng@gmail.com Fixes: 81d5d61454c3 ("btrfs: enhance unsupported compat RO flags handling") CC: stable@vger.kernel.org # 5.15+ Reviewed-by: Anand Jain anand.jain@oracle.com Signed-off-by: Qu Wenruo wqu@suse.com Reviewed-by: David Sterba dsterba@suse.com Signed-off-by: David Sterba dsterba@suse.com

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c index 9940cc39dbc9..8aeaada1fcae 100644 --- a/fs/btrfs/disk-io.c +++ b/fs/btrfs/disk-io.c @@ -3384,6 +3384,8 @@ int btrfs_start_pre_rw_mount(struct btrfs_fs_info *fs_info) /* * Do various sanity and dependency checks of different features. * + * @is_rw_mount: If the mount is read-write. + * * This is the place for less strict checks (like for subpage or artificial * feature dependencies). * @@ -3394,7 +3396,7 @@ int btrfs_start_pre_rw_mount(struct btrfs_fs_info *fs_info) * (space cache related) can modify on-disk format like free space tree and * screw up certain feature dependencies. */ -int btrfs_check_features(struct btrfs_fs_info *fs_info, struct super_block *sb) +int btrfs_check_features(struct btrfs_fs_info *fs_info, bool is_rw_mount) { struct btrfs_super_block *disk_super = fs_info->super_copy; u64 incompat = btrfs_super_incompat_flags(disk_super); @@ -3433,7 +3435,7 @@ int btrfs_check_features(struct btrfs_fs_info *fs_info, struct super_block *sb) if (btrfs_super_nodesize(disk_super) > PAGE_SIZE) incompat |= BTRFS_FEATURE_INCOMPAT_BIG_METADATA;

- if (compat_ro_unsupp && !sb_rdonly(sb)) { + if (compat_ro_unsupp && is_rw_mount) { btrfs_err(fs_info, "cannot mount read-write because of unknown compat_ro features (0x%llx)", compat_ro); @@ -3636,7 +3638,7 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device goto fail_alloc; }

- ret = btrfs_check_features(fs_info, sb); + ret = btrfs_check_features(fs_info, !sb_rdonly(sb)); if (ret < 0) { err = ret; goto fail_alloc; diff --git a/fs/btrfs/disk-io.h b/fs/btrfs/disk-io.h index 363935cfc084..f2f295eb6103 100644 --- a/fs/btrfs/disk-io.h +++ b/fs/btrfs/disk-io.h @@ -50,7 +50,7 @@ int __cold open_ctree(struct super_block *sb, void __cold close_ctree(struct btrfs_fs_info *fs_info); int btrfs_validate_super(struct btrfs_fs_info *fs_info, struct btrfs_super_block *sb, int mirror_num); -int btrfs_check_features(struct btrfs_fs_info *fs_info, struct super_block *sb); +int btrfs_check_features(struct btrfs_fs_info *fs_info, bool is_rw_mount); int write_all_supers(struct btrfs_fs_info *fs_info, int max_mirrors); struct btrfs_super_block *btrfs_read_dev_super(struct block_device *bdev); struct btrfs_super_block *btrfs_read_dev_one_super(struct block_device *bdev, diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c index d5de18d6517e..433ce221dc5c 100644 --- a/fs/btrfs/super.c +++ b/fs/btrfs/super.c @@ -1705,7 +1705,7 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data) if (ret) goto restore;

- ret = btrfs_check_features(fs_info, sb); + ret = btrfs_check_features(fs_info, !(*flags & SB_RDONLY)); if (ret < 0) goto restore;