cifs: Add support for root file systems

Introduce a new CONFIG_CIFS_ROOT option to handle root file systems
over a SMB share.

In order to mount the root file system during the init process, make
cifs.ko perform non-blocking socket operations while mounting and
accessing it.

Cc: Steve French <[e-mail]>
Reviewed-by: Aurelien Aptel <[e-mail]>
Signed-off-by: Paulo Alcantara (SUSE) <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

Documentation/filesystems/cifs/cifsroot.txt

···
+Mounting root file system via SMB (cifs.ko)
+===========================================
+
+Written 2019 by Paulo Alcantara <[e-mail]>
+Written 2019 by Aurelien Aptel <[e-mail]>
+
+The CONFIG_CIFS_ROOT option enables experimental root file system
+support over the SMB protocol via cifs.ko.
+
+It introduces a new kernel command-line option called 'cifsroot='
+which will tell the kernel to mount the root file system over the
+network by utilizing SMB or CIFS protocol.
+
+In order to mount, the network stack will also need to be set up by
+using 'ip=' config option. For more details, see
+Documentation/filesystems/nfs/nfsroot.txt.
+
+A CIFS root mount currently requires the use of SMB1+UNIX Extensions
+which is only supported by the Samba server. SMB1 is the older
+deprecated version of the protocol but it has been extended to support
+POSIX features (See [1]). The equivalent extensions for the newer
+recommended version of the protocol (SMB3) have not been fully
+implemented yet which means SMB3 doesn't support some required POSIX
+file system objects (e.g. block devices, pipes, sockets).
+
+As a result, a CIFS root will default to SMB1 for now but the version
+to use can nonetheless be changed via the 'vers=' mount option.  This
+default will change once the SMB3 POSIX extensions are fully
+implemented.
+
+Server configuration
+====================
+
+To enable SMB1+UNIX extensions you will need to set these global
+settings in Samba smb.conf:
+
+    [global]
+    server min protocol = NT1
+    unix extension = yes        # default
+
+Kernel command line
+===================
+
+root=/dev/cifs
+
+This is just a virtual device that basically tells the kernel to mount
+the root file system via SMB protocol.
+
+cifsroot=//<server-ip>/<share>[,options]
+
+Enables the kernel to mount the root file system via SMB that are
+located in the <server-ip> and <share> specified in this option.
+
+The default mount options are set in fs/cifs/cifsroot.c.
+
+server-ip
+	IPv4 address of the server.
+
+share
+	Path to SMB share (rootfs).
+
+options
+	Optional mount options. For more information, see mount.cifs(8).
+
+Examples
+========
+
+Export root file system as a Samba share in smb.conf file.
+
+...
+[linux]
+	path = /path/to/rootfs
+	read only = no
+	guest ok = yes
+	force user = root
+	force group = root
+	browseable = yes
+	writeable = yes
+	admin users = root
+	public = yes
+	create mask = 0777
+	directory mask = 0777
+...
+
+Restart smb service.
+
+# systemctl restart smb
+
+Test it under QEMU on a kernel built with CONFIG_CIFS_ROOT and
+CONFIG_IP_PNP options enabled.
+
+# qemu-system-x86_64 -enable-kvm -cpu host -m 1024 \
+  -kernel /path/to/linux/arch/x86/boot/bzImage -nographic \
+  -append "root=/dev/cifs rw ip=dhcp cifsroot=//10.0.2.2/linux,username=foo,password=bar console=ttyS0 3"
+
+
+1: https://wiki.samba.org/index.php/UNIX_Extensions

fs/cifs/Kconfig

···
 	  Makes CIFS FS-Cache capable. Say Y here if you want your CIFS data
 	  to be cached locally on disk through the general filesystem cache
 	  manager. If unsure, say N.
+
+config CIFS_ROOT
+	bool "SMB root file system (Experimental)"
+	depends on CIFS=y && IP_PNP
+	help
+	  Enables root file system support over SMB protocol.
+
+	  Most people say N here.

fs/cifs/Makefile

···
 cifs-$(CONFIG_CIFS_FSCACHE) += fscache.o cache.o
 
 cifs-$(CONFIG_CIFS_SMB_DIRECT) += smbdirect.o
+
+cifs-$(CONFIG_CIFS_ROOT) += cifsroot.o

fs/cifs/cifsglob.h

···
 	__u32 handle_timeout; /* persistent and durable handle timeout in ms */
 	unsigned int max_credits; /* smb3 max_credits 10 < credits < 60000 */
 	__u16 compression; /* compression algorithm 0xFFFF default 0=disabled */
+	bool rootfs:1; /* if it's a SMB root file system */
 };
 
 /**
···
 	 * reconnect.
 	 */
 	int nr_targets;
+	bool noblockcnt; /* use non-blocking connect() */
 };
 
 struct cifs_credits {

fs/cifs/cifsroot.c

···
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * SMB root file system support
+ *
+ * Copyright (c) 2019 Paulo Alcantara <[e-mail]>
+ */
+#include <linux/init.h>
+#include <linux/fs.h>
+#include <linux/types.h>
+#include <linux/ctype.h>
+#include <linux/string.h>
+#include <linux/root_dev.h>
+#include <linux/kernel.h>
+#include <linux/in.h>
+#include <linux/inet.h>
+#include <net/ipconfig.h>
+
+#define DEFAULT_MNT_OPTS \
+	"vers=1.0,cifsacl,mfsymlinks,rsize=1048576,wsize=65536,uid=0,gid=0," \
+	"hard,rootfs"
+
+static char root_dev[2048] __initdata = "";
+static char root_opts[1024] __initdata = DEFAULT_MNT_OPTS;
+
+static __be32 __init parse_srvaddr(char *start, char *end)
+{
+	char addr[sizeof("aaa.bbb.ccc.ddd")];
+	int i = 0;
+
+	while (start < end && i < sizeof(addr) - 1) {
+		if (isdigit(*start) || *start == '.')
+			addr[i++] = *start;
+		start++;
+	}
+	addr[i] = '\0';
+	return in_aton(addr);
+}
+
+/* cifsroot=//<server-ip>/<share>[,options] */
+static int __init cifs_root_setup(char *line)
+{
+	char *s;
+	int len;
+	__be32 srvaddr = htonl(INADDR_NONE);
+
+	ROOT_DEV = Root_CIFS;
+
+	if (strlen(line) > 3 && line[0] == '/' && line[1] == '/') {
+		s = strchr(&line[2], '/');
+		if (!s || s[1] == '\0')
+			return 1;
+
+		s = strchrnul(s, ',');
+		len = s - line + 1;
+		if (len <= sizeof(root_dev)) {
+			strlcpy(root_dev, line, len);
+			srvaddr = parse_srvaddr(&line[2], s);
+			if (*s) {
+				snprintf(root_opts, sizeof(root_opts), "%s,%s",
+					 DEFAULT_MNT_OPTS, s + 1);
+			}
+		}
+	}
+
+	root_server_addr = srvaddr;
+
+	return 1;
+}
+
+__setup("cifsroot=", cifs_root_setup);
+
+int __init cifs_root_data(char **dev, char **opts)
+{
+	if (!root_dev[0] || root_server_addr == htonl(INADDR_NONE)) {
+		printk(KERN_ERR "Root-CIFS: no SMB server address\n");
+		return -1;
+	}
+
+	*dev = root_dev;
+	*opts = root_opts;
+
+	return 0;
+}

fs/cifs/connect.c

···
 	Opt_multiuser, Opt_sloppy, Opt_nosharesock,
 	Opt_persistent, Opt_nopersistent,
 	Opt_resilient, Opt_noresilient,
-	Opt_domainauto, Opt_rdma, Opt_modesid,
+	Opt_domainauto, Opt_rdma, Opt_modesid, Opt_rootfs,
 	Opt_compress,
 
 	/* Mount options which take numeric value */
···
 	{ Opt_ignore, "nomand" },
 	{ Opt_ignore, "relatime" },
 	{ Opt_ignore, "_netdev" },
+	{ Opt_rootfs, "rootfs" },
 
 	{ Opt_err, NULL }
 };
···
 		case Opt_nodfs:
 			vol->nodfs = 1;
 			break;
+		case Opt_rootfs:
+#ifdef CONFIG_CIFS_ROOT
+			vol->rootfs = true;
+#endif
+			break;
 		case Opt_posixpaths:
 			vol->posix_paths = 1;
 			break;
···
 		goto out_err_crypto_release;
 	}
 
-	tcp_ses->noblocksnd = volume_info->noblocksnd;
+	tcp_ses->noblockcnt = volume_info->rootfs;
+	tcp_ses->noblocksnd = volume_info->noblocksnd || volume_info->rootfs;
 	tcp_ses->noautotune = volume_info->noautotune;
 	tcp_ses->tcp_nodelay = volume_info->sockopt_tcp_nodelay;
 	tcp_ses->rdma = volume_info->rdma;
···
 		 socket->sk->sk_sndbuf,
 		 socket->sk->sk_rcvbuf, socket->sk->sk_rcvtimeo);
 
-	rc = socket->ops->connect(socket, saddr, slen, 0);
+	rc = socket->ops->connect(socket, saddr, slen,
+				  server->noblockcnt ? O_NONBLOCK : 0);
+
+	if (rc == -EINPROGRESS)
+		rc = 0;
 	if (rc < 0) {
 		cifs_dbg(FYI, "Error %d connecting to server\n", rc);
 		sock_release(socket);

include/linux/root_dev.h

···
 
 enum {
 	Root_NFS = MKDEV(UNNAMED_MAJOR, 255),
+	Root_CIFS = MKDEV(UNNAMED_MAJOR, 254),
 	Root_RAM0 = MKDEV(RAMDISK_MAJOR, 0),
 	Root_RAM1 = MKDEV(RAMDISK_MAJOR, 1),
 	Root_FD0 = MKDEV(FLOPPY_MAJOR, 0),