nfsd: new interface to advertise export features

Soon we will add the new V4ROOT flag, and allow the INSECURE flag to
vary by pseudoflavor.  It would be useful for nfs-utils (for example,
for improved exportfs error reporting) to be able to know when this
happens.  Use this new interface for that purpose.
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>

fs/nfsd/nfsctl.c

@@ -31,6 +31,7 @@ enum {
 	NFSD_Getfd,
 	NFSD_Getfs,
 	NFSD_List,
+	NFSD_Export_features,
 	NFSD_Fh,
 	NFSD_FO_UnlockIP,
 	NFSD_FO_UnlockFS,
@@ -149,6 +150,24 @@ static const struct file_operations exports_operations = {
 	.owner		= THIS_MODULE,
 };
 
+static int export_features_show(struct seq_file *m, void *v)
+{
+	seq_printf(m, "0x%x 0x%x\n", NFSEXP_ALLFLAGS, NFSEXP_SECINFO_FLAGS);
+	return 0;
+}
+
+static int export_features_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, export_features_show, NULL);
+}
+
+static struct file_operations export_features_operations = {
+	.open		= export_features_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= single_release,
+};
+
 extern int nfsd_pool_stats_open(struct inode *inode, struct file *file);
 extern int nfsd_pool_stats_release(struct inode *inode, struct file *file);
 
@@ -1306,6 +1325,8 @@ static int nfsd_fill_super(struct super_block * sb, void * data, int silent)
 		[NFSD_Getfd] = {".getfd", &transaction_ops, S_IWUSR|S_IRUSR},
 		[NFSD_Getfs] = {".getfs", &transaction_ops, S_IWUSR|S_IRUSR},
 		[NFSD_List] = {"exports", &exports_operations, S_IRUGO},
+		[NFSD_Export_features] = {"export_features",
+					&export_features_operations, S_IRUGO},
 		[NFSD_FO_UnlockIP] = {"unlock_ip",
 					&transaction_ops, S_IWUSR|S_IRUSR},
 		[NFSD_FO_UnlockFS] = {"unlock_filesystem",

include/linux/nfsd/export.h

@@ -39,7 +39,8 @@
 #define NFSEXP_FSID		0x2000
 #define	NFSEXP_CROSSMOUNT	0x4000
 #define	NFSEXP_NOACL		0x8000	/* reserved for possible ACL related use */
-#define NFSEXP_ALLFLAGS		0xFE3F
+/* All flags that we claim to support.  (Note we don't support NOACL.) */
+#define NFSEXP_ALLFLAGS		0x7E3F
 
 /* The flags that may vary depending on security flavor: */
 #define NFSEXP_SECINFO_FLAGS	(NFSEXP_READONLY | NFSEXP_ROOTSQUASH \