From: Chuck Lever <chuck.lever@oracle.com>
Date: Tue, 21 Dec 2021 11:52:06 -0500
Subject: [PATCH] NFSD: Fix zero-length NFSv3 WRITEs
Git-commit: 6a2f774424bfdcc2df3e17de0cefe74a4269cad5
Patch-mainline: v5.17-rc1
References: git-fixes

The Linux NFS server currently responds to a zero-length NFSv3 WRITE
request with NFS3ERR_IO. It responds to a zero-length NFSv4 WRITE
with NFS4_OK and count of zero.

RFC 1813 says of the WRITE procedure's @count argument:

count
         The number of bytes of data to be written. If count is
         0, the WRITE will succeed and return a count of 0,
         barring errors due to permissions checking.

RFC 8881 has similar language for NFSv4, though NFSv4 removed the
explicit @count argument because that value is already contained in
the opaque payload array.

The synthetic client pynfs's WRT4 and WRT15 tests do emit zero-
length WRITEs to exercise this spec requirement. Commit fdec6114ee1f
("nfsd4: zero-length WRITE should succeed") addressed the same
problem there with the same fix.

But interestingly the Linux NFS client does not appear to emit zero-
length WRITEs, instead squelching them. I'm not aware of a test that
can generate such WRITEs for NFSv3, so I wrote a naive C program to
generate a zero-length WRITE and test this fix.

Fixes: 8154ef2776aa ("NFSD: Clean up legacy NFS WRITE argument XDR decoders")
Reported-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: stable@vger.kernel.org
Acked-by: NeilBrown <neilb@suse.com>

---
 fs/nfsd/nfs3proc.c |    3 +--
 fs/nfsd/nfsproc.c  |    2 --
 2 files changed, 1 insertion(+), 4 deletions(-)

--- a/fs/nfsd/nfs3proc.c
+++ b/fs/nfsd/nfs3proc.c
@@ -204,8 +204,7 @@ nfsd3_proc_write(struct svc_rqst *rqstp)
 	resp->committed = argp->stable;
 	nvecs = svc_fill_write_vector(rqstp, rqstp->rq_arg.pages,
 				      &argp->first, cnt);
-	if (!nvecs)
-		RETURN_STATUS(nfserr_io);
+
 	nfserr = nfsd_write(rqstp, &resp->fh, argp->offset,
 			    rqstp->rq_vec, nvecs, &cnt,
 			    resp->committed);
--- a/fs/nfsd/nfsproc.c
+++ b/fs/nfsd/nfsproc.c
@@ -220,8 +220,6 @@ nfsd_proc_write(struct svc_rqst *rqstp)
 
 	nvecs = svc_fill_write_vector(rqstp, rqstp->rq_arg.pages,
 				      &argp->first, cnt);
-	if (!nvecs)
-		return nfserr_io;
 	nfserr = nfsd_write(rqstp, fh_copy(&resp->fh, &argp->fh),
 			    argp->offset, rqstp->rq_vec, nvecs,
 			    &cnt, NFS_DATA_SYNC);