We can mount OpenBSD's just fine but for partitions created in NetBSD 10.0+ we get: $ sudo mount -r /dev/ada0p5 /mnt mount: /dev/ada0p5: No such file or directory And this line on dmesg: [969] UFS2 superblock failed: fs->fs_metaspace (131072) > fs->fs_fpg / 2 (94980) The check was added in this commit: d22531d572829 I also tried this command on NetBSD to convert the fs to support extattrs and ACLs: # fsck_ffs -c ea / https://man.netbsd.org/fsck_ffs.8
I ran this command on NetBSD: # dumpfs -s / file system: /dev/rdk6 format FFSv2 endian little-endian location 65536 (-b 128) magic 19540119 time Fri Jun 12 16:24:17 2026 superblock location 65536 id [ 68d06135 787b968e ] nbfree 9260990 ndir 51861 nifree 18957676 nffree 16418 ncg 414 size 78643200 blocks 77424358 bsize 32768 shift 15 mask 0xffff8000 fsize 4096 shift 12 mask 0xfffff000 frag 8 shift 3 fsbtodb 3 bpg 23745 fpg 189960 ipg 46848 minfree 5% optim time maxcontig 2 maxbpg 4096 symlinklen 120 contigsumsize 2 maxfilesize 0x000800800805ffff nindir 4096 inopb 128 avgfilesize 16384 avgfpdir 64 sblkno 24 cblkno 32 iblkno 40 dblkno 2968 sbsize 4096 cgsize 32768 csaddr 2968 cssize 8192 cgrotor 0 fmod 0 ronly 0 clean 0x02 wapbl version 0x1 location 2 flags 0x0 wapbl loc0 314597568 loc1 131072 loc2 512 loc3 3 snapshot inodes 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 usrquota 0 grpquota 0 flags wapbl fsmnt / volname swuid 0 It seems FreeBSD is reading NetBSD WAPBL fields as UFS2 fields due to struct layout divergence: wapbl loc1 131072 FreeBSD reports fs_metaspace = 131072. That's wapbl loc1. FreeBSD's fs_metaspace field offset in struct fs corresponds to a NetBSD WAPBL journal location field.
Fix: https://github.com/freebsd/freebsd-src/pull/2279
AFAIK NetBSD did a noisy mod to FFS code to allow it to support endianess different from the host. So we already do not support the NetBSD FFS mounts in some cases. Might be we should just claim that NetBSD FFS is not FFS.
(In reply to Konstantin Belousov from comment #3) We already fail on endian mismatch: https://github.com/freebsd/freebsd-src/blob/main/sys/ufs/ffs/ffs_subr.c#L544
Apart from a minor style nit, I think the suggested patch in the pull request is good.
A commit in branch main references this bug: URL: https://cgit.FreeBSD.org/src/commit/?id=3eb80d8d7daf4f14c22dd462d8c4e5b6fc818bd4 commit 3eb80d8d7daf4f14c22dd462d8c4e5b6fc818bd4 Author: Ricardo Branco <rbranco@suse.de> AuthorDate: 2026-06-13 08:26:58 +0000 Commit: Warner Losh <imp@FreeBSD.org> CommitDate: 2026-07-08 05:16:01 +0000 ufs: Allow read-only mounting of NetBSD FFSv2 WAPBL filesystems Skip UFS2 fs_metaspace upper-bound validation that rejects NetBSD FFSv2 WAPBL filesystems due to differing superblock layouts. Detect the condition during mount instead and permit read-only mounts while rejecting read-write mounts with EROFS. This follows NetBSD's recommendation for systems without WAPBL support and avoids modifying unsupported journal metadata. PR: 296022 Signed-off-by: Ricardo Branco <rbranco@suse.de> Reviewed by: imp, kirk Pull Request: https://github.com/freebsd/freebsd-src/pull/2279 sys/ufs/ffs/ffs_subr.c | 14 +++++++++++++- sys/ufs/ffs/ffs_vfsops.c | 27 +++++++++++++++++++++++++++ 2 files changed, 40 insertions(+), 1 deletion(-)