NFSD: Fix verifier returned in stable WRITEs

RFC 8881 explains the purpose of the write verifier this way:

> The final portion of the result is the field writeverf. This field
> is the write verifier and is a cookie that the client can use to
> determine whether a server has changed instance state (e.g., server
> restart) between a call to WRITE and a subsequent call to either
> WRITE or COMMIT.

But then it says:

> This cookie MUST be unchanged during a single instance of the
> NFSv4.1 server and MUST be unique between instances of the NFSv4.1
> server. If the cookie changes, then the client MUST assume that
> any data written with an UNSTABLE4 value for committed and an old
> writeverf in the reply has been lost and will need to be
> recovered.

RFC 1813 has similar language for NFSv3. NFSv2 does not have a write
verifier since it doesn't implement the COMMIT procedure.

Since commit 19e0663ff9bc ("nfsd: Ensure sampling of the write
verifier is atomic with the write"), the Linux NFS server has
returned a boot-time-based verifier for UNSTABLE WRITEs, but a zero
verifier for FILE_SYNC and DATA_SYNC WRITEs. FILE_SYNC and DATA_SYNC
WRITEs are not followed up with a COMMIT, so there's no need for
clients to compare verifiers for stable writes.

However, by returning a different verifier for stable and unstable
writes, the above commit puts the Linux NFS server a step farther
out of compliance with the first MUST above. At least one NFS client
(FreeBSD) noticed the difference, making this a potential
regression.

Reported-by: Rick Macklem <rmacklem@uoguelph.ca>
Link: https://lore.kernel.org/linux-nfs/YQXPR0101MB096857EEACF04A6DF1FC6D9BDD749@YQXPR0101MB0968.CANPRD01.PROD.OUTLOOK.COM/T/
Fixes: 19e0663ff9bc ("nfsd: Ensure sampling of the write verifier is atomic with the write")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 0faa383..74c3451 100644 (file)
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -995,6 +995,10 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
        iov_iter_kvec(&iter, WRITE, vec, vlen, *cnt);
        if (flags & RWF_SYNC) {
                down_write(&nf->nf_rwsem);
+               if (verf)
+                       nfsd_copy_boot_verifier(verf,
+                                       net_generic(SVC_NET(rqstp),
+                                       nfsd_net_id));
                host_err = vfs_iter_write(file, &iter, &pos, flags);
                if (host_err < 0)
                        nfsd_reset_boot_verifier(net_generic(SVC_NET(rqstp),