Re: crossmnt exports partially broken in 7.2?

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2026-09-07 at 14:33 +0300, Mantas Mikulėnas wrote:
> Hello,
> 
> After upgrading an NFS server to kernel 7.2.2 (from 7.1.11), it is no longer
> possible to mount filesystems that should be handled by a "crossmnt" export.
> 
> For example, with /etc/exports being:
> 
>          /       *(rw,crossmnt,sec=krb5:krb5i:krb5p)
> 
> and all clients using NFSv4.2, it used to be possible to either mount 
> foo:/hdd2
> directly, or to descend into it through a "foo:/" mount.
> 
> With kernel 7.2 this no longer works -- after mounting foo:/hdd2 all 
> attempts
> to access it result in -ESTALE and the client's kernel complains about 
> "fileid
> changed":
> 
>          # mount horizon:/hdd1 /mnt
> 
>          # ls -l /mnt
>          ls: cannot open directory '/mnt': Stale file handle
> 
>          # dmesg | tail -2
>          [374226.359731] NFS: server horizon error: fileid changed
>                          fsid 0:150: expected fileid 0x2, got 0x100
> 
> Mounting "foo:/" on /mnt and trying to stat /mnt/hdd1 likewise returns 
> -ESTALE.
> (Or hangs for a while if it's a autofs mount on the server side, but I 
> switched
> from systemd.automount to regular non-autofs mounts for now.)
> 
> In this case the server's root filesystem is Btrfs, additional disks are 
> Ext4.
> Another system with the same problem has Btrfs for both the rootfs and 
> for the
> secondary disks.
> 
> Bisect points to commit 70b7e3526c53d9dd7caccdbeff5b0485640d8cf1 which 
> adds a
> netlink interface for nfs-utils' rpc.mountd to use. Indeed if I disable 
> netlink
> support there (adding "[mountd] no-netlink=1" to the server's /etc/nfs.conf)
> then crossmnt exports again start working as they should, even with 
> kernel 7.2.
> I've also tried kernel 7.3-rc1 with the same results.
> 
> Does this look like a kernel bug, or a nfs-utils (rpc.mountd) bug, or both?
> The server has nfs-utils 2.9.2.
> 

It's an nfs-utils bug (I think), though I'm still confirming that. I
had Claude draft some patches, but they're still quite raw. If you're
interested in testing them, they're in my nl-crossmnt branch on github:

    https://github.com/jtlayton/nfs-utils/tree/nl-crossmnt

Thanks for the bug report!
-- 
Jeff Layton <jlayton@kernel.org>