RPC: add wrapper for svc_reserve to account for checksum

When the kernel calls svc_reserve to downsize the expected size of an RPC
reply, it fails to account for the possibility of a checksum at the end of
the packet.  If a client mounts a NFSv2/3 with sec=krb5i/p, and does I/O
then you'll generally see messages similar to this in the server's ring
buffer:

RPC request reserved 164 but used 208

While I was never able to verify it, I suspect that this problem is also
the root cause of some oopses I've seen under these conditions:

https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=227726

This is probably also a problem for other sec= types and for NFSv4.  The
large reserved size for NFSv4 compound packets seems to generally paper
over the problem, however.

This patch adds a wrapper for svc_reserve that accounts for the possibility
of a checksum.  It also fixes up the appropriate callers of svc_reserve to
call the wrapper.  For now, it just uses a hardcoded value that I
determined via testing.  That value may need to be revised upward as things
change, or we may want to eventually add a new auth_op that attempts to
calculate this somehow.

Unfortunately, there doesn't seem to be a good way to reliably determine
the expected checksum length prior to actually calculating it, particularly
with schemes like spkm3.
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Neil Brown <neilb@suse.de>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Acked-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/nfsd/nfs3proc.c

@@ -177,7 +177,7 @@ nfsd3_proc_read(struct svc_rqst *rqstp, struct nfsd3_readargs *argp,
 	if (max_blocksize < resp->count)
 		resp->count = max_blocksize;
 
-	svc_reserve(rqstp, ((1 + NFS3_POST_OP_ATTR_WORDS + 3)<<2) + resp->count +4);
+	svc_reserve_auth(rqstp, ((1 + NFS3_POST_OP_ATTR_WORDS + 3)<<2) + resp->count +4);
 
 	fh_copy(&resp->fh, &argp->fh);
 	nfserr = nfsd_read(rqstp, &resp->fh, NULL,

fs/nfsd/nfsproc.c

@@ -155,7 +155,7 @@ nfsd_proc_read(struct svc_rqst *rqstp, struct nfsd_readargs *argp,
 				argp->count);
 		argp->count = NFSSVC_MAXBLKSIZE_V2;
 	}
-	svc_reserve(rqstp, (19<<2) + argp->count + 4);
+	svc_reserve_auth(rqstp, (19<<2) + argp->count + 4);
 
 	resp->count = argp->count;
 	nfserr = nfsd_read(rqstp, fh_copy(&resp->fh, &argp->fh), NULL,

include/linux/sunrpc/svc.h

@@ -396,4 +396,23 @@ char *		   svc_print_addr(struct svc_rqst *, char *, size_t);
 
 #define	RPC_MAX_ADDRBUFLEN	(63U)
 
+/*
+ * When we want to reduce the size of the reserved space in the response
+ * buffer, we need to take into account the size of any checksum data that
+ * may be at the end of the packet. This is difficult to determine exactly
+ * for all cases without actually generating the checksum, so we just use a
+ * static value.
+ */
+static inline void
+svc_reserve_auth(struct svc_rqst *rqstp, int space)
+{
+	int			added_space = 0;
+
+	switch(rqstp->rq_authop->flavour) {
+		case RPC_AUTH_GSS:
+			added_space = RPC_MAX_AUTH_SIZE;
+	}
+	return svc_reserve(rqstp, space + added_space);
+}
+
 #endif /* SUNRPC_SVC_H */

net/sunrpc/svc.c

@@ -907,7 +907,7 @@ svc_process(struct svc_rqst *rqstp)
 	 * better idea of reply size
 	 */
 	if (procp->pc_xdrressize)
-		svc_reserve(rqstp, procp->pc_xdrressize<<2);
+		svc_reserve_auth(rqstp, procp->pc_xdrressize<<2);
 
 	/* Call the function that processes the request. */
 	if (!versp->vs_dispatch) {