nfsd: opt in to labeled nfs per export

Currently turning on NFSv4.2 results in 4.2 clients suddenly seeing the
individual file labels as they're set on the server. This is not what
they've previously seen, and not appropriate in may cases. (In
particular, if clients have heterogenous security policies then one
client's labels may not even make sense to another.) Labeled NFS should
be opted in only in those cases when the administrator knows it makes
sense.

It's helpful to be able to turn 4.2 on by default, and otherwise the
protocol upgrade seems free of regressions. So, default labeled NFS to
off and provide an export flag to reenable it.

Users wanting labeled NFS support on an export will henceforth need to:

- make sure 4.2 support is enabled on client and server (as
before), and
- upgrade the server nfs-utils to a version supporting the new
"security_label" export flag.
- set that "security_label" flag on the export.

This is commit may be seen as a regression to anyone currently depending
on security labels. We believe those cases are currently rare.

Reported-by: [e-mail]
Signed-off-by: J. Bruce Fields <[e-mail]>

fs/nfsd/export.c

···
 	{ NFSEXP_NOAUTHNLM, {"insecure_locks", ""}},
 	{ NFSEXP_V4ROOT, {"v4root", ""}},
 	{ NFSEXP_PNFS, {"pnfs", ""}},
+	{ NFSEXP_SECURITY_LABEL, {"security_label", ""}},
 	{ 0, {"", ""}}
 };
 

fs/nfsd/nfs4proc.c

···
 		   u32 *bmval, u32 *writable)
 {
 	struct dentry *dentry = cstate->current_fh.fh_dentry;
+	struct svc_export *exp = cstate->current_fh.fh_export;
 
 	if (!nfsd_attrs_supported(cstate->minorversion, bmval))
 		return nfserr_attrnotsupp;
 	if ((bmval[0] & FATTR4_WORD0_ACL) && !IS_POSIXACL(d_inode(dentry)))
+		return nfserr_attrnotsupp;
+	if ((bmval[2] & FATTR4_WORD2_SECURITY_LABEL) &&
+			!(exp->ex_flags & NFSEXP_SECURITY_LABEL))
 		return nfserr_attrnotsupp;
 	if (writable && !bmval_is_subset(bmval, writable))
 		return nfserr_inval;

fs/nfsd/nfs4xdr.c

···
 #ifdef CONFIG_NFSD_V4_SECURITY_LABEL
 	if ((bmval2 & FATTR4_WORD2_SECURITY_LABEL) ||
 	     bmval0 & FATTR4_WORD0_SUPPORTED_ATTRS) {
-		err = security_inode_getsecctx(d_inode(dentry),
+		if (exp->ex_flags & NFSEXP_SECURITY_LABEL)
+			err = security_inode_getsecctx(d_inode(dentry),
 						&context, &contextlen);
+		else
+			err = -EOPNOTSUPP;
 		contextsupport = (err == 0);
 		if (bmval2 & FATTR4_WORD2_SECURITY_LABEL) {
 			if (err == -EOPNOTSUPP)

include/uapi/linux/nfsd/export.h

···
 #define NFSEXP_ASYNC		0x0010
 #define NFSEXP_GATHERED_WRITES	0x0020
 #define NFSEXP_NOREADDIRPLUS    0x0040
-/* 80 100 currently unused */
+#define NFSEXP_SECURITY_LABEL	0x0080
+/* 0x100 currently unused */
 #define NFSEXP_NOHIDE		0x0200
 #define NFSEXP_NOSUBTREECHECK	0x0400
 #define	NFSEXP_NOAUTHNLM	0x0800		/* Don't authenticate NLM requests - just trust */
···
 #define NFSEXP_PNFS		0x20000
 
 /* All flags that we claim to support.  (Note we don't support NOACL.) */
-#define NFSEXP_ALLFLAGS		0x3FE7F
+#define NFSEXP_ALLFLAGS		0x3FEFF
 
 /* The flags that may vary depending on security flavor: */
 #define NFSEXP_SECINFO_FLAGS	(NFSEXP_READONLY | NFSEXP_ROOTSQUASH \