[PATCH v2] nfsd: use short read rather than i_size to set eof

[Benjamin Coddington <bcodding@redhat.com>]

On Mon, 21 Mar 2016, J. Bruce Fields wrote:
> On Mon, Mar 21, 2016 at 10:42:25AM -0400, Benjamin Coddington wrote:
> > Use the result of a local read to determine when to set the eof flag.
> > This
> > allows us to return the location of the end of the file atomically at
> > the
> > time of the read.
>
> My only question is whether we should instead do something like:
>
>   eof = (res > cnt) || (offset + cnt >= i_size)

Yes, let's do that.

> That'd fix the reported bug without changing existing behavior
> otherwise.
>
> But maybe it's unlikely any client depends on existing behavior.
>
> The only test failure I'm seeing is on pynfs WRT13, which literally just
> checks that a 6-byte read of a 6-byte file returns with eof set.  The
> test is correct (the spec does clearly require eof to be set in that
> case), but maybe it's not important.

The above change will fix this up and be correct in the absence of races
which saves the client from having to perform another full RPC just to
retrieve eof.  This passes WRT13:

8<------------------------------------------------------------------------

Use the result of a local read to determine when to set the eof flag.  This
allows us to return the location of the end of the file atomically at the
time of the read.

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
---
 fs/nfsd/nfs3proc.c |    7 ++++---
 fs/nfsd/nfs4xdr.c  |   11 +++++++----
 2 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/fs/nfsd/nfs3proc.c b/fs/nfsd/nfs3proc.c
index 7b755b7..83c9abb 100644
--- a/fs/nfsd/nfs3proc.c
+++ b/fs/nfsd/nfs3proc.c
@@ -147,6 +147,7 @@ nfsd3_proc_read(struct svc_rqst *rqstp, struct nfsd3_readargs *argp,
 {
 	__be32	nfserr;
 	u32	max_blocksize = svc_max_payload(rqstp);
+	unsigned long cnt = min(argp->count, max_blocksize);
 
 	dprintk("nfsd: READ(3) %s %lu bytes at %Lu\n",
 				SVCFH_fmt(&argp->fh),
@@ -157,7 +158,7 @@ nfsd3_proc_read(struct svc_rqst *rqstp, struct nfsd3_readargs *argp,
 	 * 1 (status) + 22 (post_op_attr) + 1 (count) + 1 (eof)
 	 * + 1 (xdr opaque byte count) = 26
 	 */
-	resp->count = min(argp->count, max_blocksize);
+	resp->count = cnt;
 	svc_reserve_auth(rqstp, ((1 + NFS3_POST_OP_ATTR_WORDS + 3)<<2) + resp->count +4);
 
 	fh_copy(&resp->fh, &argp->fh);
@@ -167,8 +168,8 @@ nfsd3_proc_read(struct svc_rqst *rqstp, struct nfsd3_readargs *argp,
 				  &resp->count);
 	if (nfserr == 0) {
 		struct inode	*inode = d_inode(resp->fh.fh_dentry);
-
-		resp->eof = (argp->offset + resp->count) >= inode->i_size;
+		resp->eof = (cnt > resp->count) ||
+			((argp->offset + resp->count) >= inode->i_size);
 	}
 
 	RETURN_STATUS(nfserr);
diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
index d6ef095..1d26b2b 100644
--- a/fs/nfsd/nfs4xdr.c
+++ b/fs/nfsd/nfs4xdr.c
@@ -3362,6 +3362,7 @@ static __be32 nfsd4_encode_splice_read(
 	struct xdr_stream *xdr = &resp->xdr;
 	struct xdr_buf *buf = xdr->buf;
 	u32 eof;
+	long len;
 	int space_left;
 	__be32 nfserr;
 	__be32 *p = xdr->p - 2;
@@ -3370,6 +3371,7 @@ static __be32 nfsd4_encode_splice_read(
 	if (xdr->end - xdr->p < 1)
 		return nfserr_resource;
 
+	len = maxcount;
 	nfserr = nfsd_splice_read(read->rd_rqstp, file,
 				  read->rd_offset, &maxcount);
 	if (nfserr) {
@@ -3382,8 +3384,8 @@ static __be32 nfsd4_encode_splice_read(
 		return nfserr;
 	}
 
-	eof = (read->rd_offset + maxcount >=
-	       d_inode(read->rd_fhp->fh_dentry)->i_size);
+	eof = (len > maxcount) ||
+		((read->rd_offset + maxcount >= d_inode(read->rd_fhp->fh_dentry)->i_size));
 
 	*(p++) = htonl(eof);
 	*(p++) = htonl(maxcount);
@@ -3453,14 +3455,15 @@ static __be32 nfsd4_encode_readv(struct nfsd4_compoundres *resp,
 	}
 	read->rd_vlen = v;
 
+	len = maxcount;
 	nfserr = nfsd_readv(file, read->rd_offset, resp->rqstp->rq_vec,
 			read->rd_vlen, &maxcount);
 	if (nfserr)
 		return nfserr;
 	xdr_truncate_encode(xdr, starting_len + 8 + ((maxcount+3)&~3));
 
-	eof = (read->rd_offset + maxcount >=
-	       d_inode(read->rd_fhp->fh_dentry)->i_size);
+	eof = (len > maxcount) ||
+		((read->rd_offset + maxcount >= d_inode(read->rd_fhp->fh_dentry)->i_size));
 
 	tmp = htonl(eof);
 	write_bytes_to_xdr_buf(xdr->buf, starting_len    , &tmp, 4);
-- 
1.7.1