1 .. SPDX-License-Identifier: GPL-2.0 2 3 =========================================== 4 Mounting root file system via SMB (cifs.ko) 5 =========================================== 6 7 Written 2019 by Paulo Alcantara <palcantara@suse.de> 8 9 Written 2019 by Aurelien Aptel <aaptel@suse.com> 10 11 The CONFIG_CIFS_ROOT option enables experimental root file system 12 support over the SMB protocol via cifs.ko. 13 14 It introduces a new kernel command-line option called 'cifsroot=' 15 which will tell the kernel to mount the root file system over the 16 network by utilizing SMB or CIFS protocol. 17 18 In order to mount, the network stack will also need to be set up by 19 using 'ip=' config option. For more details, see 20 Documentation/admin-guide/nfs/nfsroot.rst. 21 22 A CIFS root mount currently requires the use of SMB1+UNIX Extensions 23 which is only supported by the Samba server. SMB1 is the older 24 deprecated version of the protocol but it has been extended to support 25 POSIX features (See [1]). The equivalent extensions for the newer 26 recommended version of the protocol (SMB3) have not been fully 27 implemented yet which means SMB3 doesn't support some required POSIX 28 file system objects (e.g. block devices, pipes, sockets). 29 30 As a result, a CIFS root will default to SMB1 for now but the version 31 to use can nonetheless be changed via the 'vers=' mount option. This 32 default will change once the SMB3 POSIX extensions are fully 33 implemented. 34 35 Server configuration 36 ==================== 37 38 To enable SMB1+UNIX extensions you will need to set these global 39 settings in Samba smb.conf:: 40 41 [global] 42 server min protocol = NT1 43 unix extension = yes # default 44 45 Kernel command line 46 =================== 47 48 :: 49 50 root=/dev/cifs 51 52 This is just a virtual device that basically tells the kernel to mount 53 the root file system via SMB protocol. 54 55 :: 56 57 cifsroot=//<server-ip>/<share>[,options] 58 59 Enables the kernel to mount the root file system via SMB that are 60 located in the <server-ip> and <share> specified in this option. 61 62 The default mount options are set in fs/smb/client/cifsroot.c. 63 64 server-ip 65 IPv4 address of the server. 66 67 share 68 Path to SMB share (rootfs). 69 70 options 71 Optional mount options. For more information, see mount.cifs(8). 72 73 Examples 74 ======== 75 76 Export root file system as a Samba share in smb.conf file:: 77 78 ... 79 [linux] 80 path = /path/to/rootfs 81 read only = no 82 guest ok = yes 83 force user = root 84 force group = root 85 browseable = yes 86 writeable = yes 87 admin users = root 88 public = yes 89 create mask = 0777 90 directory mask = 0777 91 ... 92 93 Restart smb service:: 94 95 # systemctl restart smb 96 97 Test it under QEMU on a kernel built with CONFIG_CIFS_ROOT and 98 CONFIG_IP_PNP options enabled:: 99 100 # qemu-system-x86_64 -enable-kvm -cpu host -m 1024 \ 101 -kernel /path/to/linux/arch/x86/boot/bzImage -nographic \ 102 -append "root=/dev/cifs rw ip=dhcp cifsroot=//10.0.2.2/linux,username=foo,password=bar console=ttyS0 3" 103 104 105 1: https://wiki.samba.org/index.php/UNIX_Extensions
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