NFSD: Replace /proc/fs/nfsd/supported_krb5_enctypes with a symlink

Now that I've added a file under /proc/net/rpc that is managed by
the SunRPC's Kerberos mechanism, replace NFSD's
supported_krb5_enctypes file with a symlink to the new SunRPC proc
file, which contains exactly the same content.

Remarkably, commit b0b0c0a2 ("nfsd: add proc file listing
kernel's gss_krb5 enctypes") added the nfsd_supported_krb5_enctypes
file in 2011, but this file has never been documented in nfsd(7).
Tested-by: Scott Mayhew <smayhew@redhat.com>
Reviewed-by: Simo Sorce <simo@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfsctl.c

@@ -14,7 +14,6 @@
 #include <linux/lockd/lockd.h>
 #include <linux/sunrpc/addr.h>
 #include <linux/sunrpc/gss_api.h>
-#include <linux/sunrpc/gss_krb5_enctypes.h>
 #include <linux/sunrpc/rpc_pipe_fs.h>
 #include <linux/module.h>
 #include <linux/fsnotify.h>
@@ -47,7 +46,6 @@ enum {
 	NFSD_MaxBlkSize,
 	NFSD_MaxConnections,
 	NFSD_Filecache,
-	NFSD_SupportedEnctypes,
 	/*
 	 * The below MUST come last.  Otherwise we leave a hole in nfsd_files[]
 	 * with !CONFIG_NFSD_V4 and simple_fill_super() goes oops
@@ -187,16 +185,6 @@ static int export_features_show(struct seq_file *m, void *v)
 
 DEFINE_SHOW_ATTRIBUTE(export_features);
 
-#if defined(CONFIG_SUNRPC_GSS) || defined(CONFIG_SUNRPC_GSS_MODULE)
-static int supported_enctypes_show(struct seq_file *m, void *v)
-{
-	seq_printf(m, KRB5_SUPPORTED_ENCTYPES);
-	return 0;
-}
-
-DEFINE_SHOW_ATTRIBUTE(supported_enctypes);
-#endif /* CONFIG_SUNRPC_GSS or CONFIG_SUNRPC_GSS_MODULE */
-
 static const struct file_operations pool_stats_operations = {
 	.open		= nfsd_pool_stats_open,
 	.read		= seq_read,
@@ -1150,6 +1138,9 @@ static struct inode *nfsd_get_inode(struct super_block *sb, umode_t mode)
 		inode->i_op = &simple_dir_inode_operations;
 		inc_nlink(inode);
 		break;
+	case S_IFLNK:
+		inode->i_op = &simple_symlink_inode_operations;
+		break;
 	default:
 		break;
 	}
@@ -1195,6 +1186,59 @@ static struct dentry *nfsd_mkdir(struct dentry *parent, struct nfsdfs_client *nc
 	goto out;
 }
 
+#if IS_ENABLED(CONFIG_SUNRPC_GSS)
+static int __nfsd_symlink(struct inode *dir, struct dentry *dentry,
+			  umode_t mode, const char *content)
+{
+	struct inode *inode;
+
+	inode = nfsd_get_inode(dir->i_sb, mode);
+	if (!inode)
+		return -ENOMEM;
+
+	inode->i_link = (char *)content;
+	inode->i_size = strlen(content);
+
+	d_add(dentry, inode);
+	inc_nlink(dir);
+	fsnotify_create(dir, dentry);
+	return 0;
+}
+
+/*
+ * @content is assumed to be a NUL-terminated string that lives
+ * longer than the symlink itself.
+ */
+static void nfsd_symlink(struct dentry *parent, const char *name,
+			 const char *content)
+{
+	struct inode *dir = parent->d_inode;
+	struct dentry *dentry;
+	int ret = -ENOMEM;
+
+	inode_lock(dir);
+	dentry = d_alloc_name(parent, name);
+	if (!dentry)
+		goto out_err;
+	ret = __nfsd_symlink(d_inode(parent), dentry, S_IFLNK | 0777, content);
+	if (ret)
+		goto out_err;
+out:
+	inode_unlock(dir);
+	return;
+out_err:
+	dput(dentry);
+	dentry = ERR_PTR(ret);
+	goto out;
+}
+#else
+static inline void nfsd_symlink(struct dentry *parent, const char *name,
+				const char *content)
+{
+}
+
+#endif
+
 static void clear_ncl(struct inode *inode)
 {
 	struct nfsdfs_client *ncl = inode->i_private;
@@ -1355,10 +1399,6 @@ static int nfsd_fill_super(struct super_block *sb, struct fs_context *fc)
 		[NFSD_MaxBlkSize] = {"max_block_size", &transaction_ops, S_IWUSR|S_IRUGO},
 		[NFSD_MaxConnections] = {"max_connections", &transaction_ops, S_IWUSR|S_IRUGO},
 		[NFSD_Filecache] = {"filecache", &nfsd_file_cache_stats_fops, S_IRUGO},
-#if defined(CONFIG_SUNRPC_GSS) || defined(CONFIG_SUNRPC_GSS_MODULE)
-		[NFSD_SupportedEnctypes] = {"supported_krb5_enctypes",
-					&supported_enctypes_fops, S_IRUGO},
-#endif /* CONFIG_SUNRPC_GSS or CONFIG_SUNRPC_GSS_MODULE */
 #ifdef CONFIG_NFSD_V4
 		[NFSD_Leasetime] = {"nfsv4leasetime", &transaction_ops, S_IWUSR|S_IRUSR},
 		[NFSD_Gracetime] = {"nfsv4gracetime", &transaction_ops, S_IWUSR|S_IRUSR},
@@ -1371,6 +1411,8 @@ static int nfsd_fill_super(struct super_block *sb, struct fs_context *fc)
 	ret = simple_fill_super(sb, 0x6e667364, nfsd_files);
 	if (ret)
 		return ret;
+	nfsd_symlink(sb->s_root, "supported_krb5_enctypes",
+		     "/proc/net/rpc/gss_krb5_enctypes");
 	dentry = nfsd_mkdir(sb->s_root, NULL, "clients");
 	if (IS_ERR(dentry))
 		return PTR_ERR(dentry);