[PATCH RFC] NFSD: Fix zero-length NFSv3 WRITEs

[Chuck Lever <chuck.lever@xxxxxxxxxx>]

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, 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: 14168d678a0f ("NFSD: Remove the RETURN_STATUS() macro")
Reported-by: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
---

Here's an alternate approach to addressing the zero-length NFSv3
WRITE failures.


 fs/nfsd/nfs3proc.c |    6 +-----
 fs/nfsd/nfsproc.c  |    5 -----
 2 files changed, 1 insertion(+), 10 deletions(-)

diff --git a/fs/nfsd/nfs3proc.c b/fs/nfsd/nfs3proc.c
index 4418517f6f12..2c681785186f 100644
--- a/fs/nfsd/nfs3proc.c
+++ b/fs/nfsd/nfs3proc.c
@@ -202,15 +202,11 @@ nfsd3_proc_write(struct svc_rqst *rqstp)
 	fh_copy(&resp->fh, &argp->fh);
 	resp->committed = argp->stable;
 	nvecs = svc_fill_write_vector(rqstp, &argp->payload);
-	if (!nvecs) {
-		resp->status = nfserr_io;
-		goto out;
-	}
+
 	resp->status = nfsd_write(rqstp, &resp->fh, argp->offset,
 				  rqstp->rq_vec, nvecs, &cnt,
 				  resp->committed, resp->verf);
 	resp->count = cnt;
-out:
 	return rpc_success;
 }
 
diff --git a/fs/nfsd/nfsproc.c b/fs/nfsd/nfsproc.c
index eea5b59b6a6c..1743ed04197e 100644
--- a/fs/nfsd/nfsproc.c
+++ b/fs/nfsd/nfsproc.c
@@ -235,10 +235,6 @@ nfsd_proc_write(struct svc_rqst *rqstp)
 		argp->len, argp->offset);
 
 	nvecs = svc_fill_write_vector(rqstp, &argp->payload);
-	if (!nvecs) {
-		resp->status = nfserr_io;
-		goto out;
-	}
 
 	resp->status = nfsd_write(rqstp, fh_copy(&resp->fh, &argp->fh),
 				  argp->offset, rqstp->rq_vec, nvecs,
@@ -247,7 +243,6 @@ nfsd_proc_write(struct svc_rqst *rqstp)
 		resp->status = fh_getattr(&resp->fh, &resp->stat);
 	else if (resp->status == nfserr_jukebox)
 		return rpc_drop_reply;
-out:
 	return rpc_success;
 }