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authorKees Cook <kees+treewide@kernel.org>2026-09-02 15:31:14 -0700
committerKees Cook <kees@kernel.org>2026-09-04 21:37:00 -0700
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treec65086f9bdcd48c6360fb7cb4598bca084da1f32 /Documentation/filesystems/smb/cifsroot.rst
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treewide: refresh kmalloc_obj() conversionsgrafted
This is another run of the Coccinelle script for converting kmalloc() family of allocations to kmalloc_obj() via the existing rules in scripts/coccinelle/api/kmalloc_objs.cocci This catches both the set of kmalloc() uses added since the first kmalloc_obj() conversions in v7.0 and adds a large group missed in the first pass due to Coccinelle not interacting well with the cleanup.h scoped_...() family of macros[1]. I worked around this with spatch's "--macro-file" argument to a file with all the scoped_...() macros mapped to Coccinelle's YACFE_ITERATOR[2] as that was the closest viable control flow indicator I could find. Build tested allmodconfig on x86, arm64, arm, loongarch, mips, powerpc, riscv, and s390 with no new warnings. Link: https://lore.kernel.org/lkml/202609021314.8A9C0B8@keescook/ [1] Link: https://github.com/coccinelle/coccinelle/blob/master/standard.h [2] Signed-off-by: Kees Cook <kees+treewide@kernel.org>
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+.. SPDX-License-Identifier: GPL-2.0
+
+===========================================
+Mounting root file system via SMB (cifs.ko)
+===========================================
+
+Written 2019 by Paulo Alcantara <palcantara@suse.de>
+
+Written 2019 by Aurelien Aptel <aaptel@suse.com>
+
+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/admin-guide/nfs/nfsroot.rst.
+
+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/smb/client/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