NFSD: Mark exports of NFS as unsupported

It is possible to re-export NFS filesystems, and there's some interest in
doing so within the upstream NFS community.  However there are a number of
tricky problems to addresss before Red Hat is willing to support this
configuration, and these issues are not likely to be immediately apparent
to a customer that might attempt to build a solution on re-export.

Normally we would mark this sort of configuration as TECH PREVIEW, however
doing so implies that Red Hat has committed to fully supporting the feature
in an upcoming release.  Instead of making this promise with
mark_tech_preview(), we will emit a kernel warning to increase customer
awareness of the potential issues with this configuration.

Upstream-status: RHEL-only
Tested: manually
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2150887
Signed-off-by: Benjamin Coddington <bcodding@redhat.com>

Author: bcodding-rh

fs/nfsd/export.c

@@ -438,6 +438,12 @@ static int check_export(struct path *path, int *flags, unsigned char *uuid)
 			__func__, inode->i_sb->s_type->name);
 		return -EINVAL;
 	}
+
+	if (!strcmp(inode->i_sb->s_type->name, "nfs") ||
+		!strcmp(inode->i_sb->s_type->name, "nfs4"))
+		pr_warn("NFSD WARNING: Re-exporting NFS is not a supported "
+				"configuration in RHEL.  Unexpected client behavior may result.\n");
+
 	return 0;
 
 }