cifsd: add server handler for central processing and tranport layers

This adds server handler for central processing,
transport layers(tcp, rdma, ipc) and a document describing cifsd
architecture.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Hyunchul Lee <hyc.lee@gmail.com>
Acked-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: namjaejeon

Documentation/filesystems/cifs/cifsd.rst

@@ -0,0 +1,136 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=========================
+CIFSD - SMB3 Kernel Server
+=========================
+
+CIFSD is a linux kernel server which implements SMB3 protocol in kernel space
+for sharing files over network.
+
+CIFSD architecture
+==================
+
+The subset of performance related operations belong in kernelspace and
+the other subset which belong to operations which are not really related with
+performance in userspace. So, DCE/RPC management that has historically resulted
+into number of buffer overflow issues and dangerous security bugs and user
+account management are implemented in user space as ksmbd.mountd.
+File operations that are related with performance (open/read/write/close etc.)
+in kernel space (ksmbd). This also allows for easier integration with VFS
+interface for all file operations.
+
+ksmbd (kernel daemon)
+---------------------
+
+When the server daemon is started, It starts up a forker thread
+(ksmbd/interface name) at initialization time and open a dedicated port 445
+for listening to SMB requests. Whenever new clients make request, Forker
+thread will accept the client connection and fork a new thread for dedicated
+communication channel between the client and the server. It allows for parallel
+processing of SMB requests(commands) from clients as well as allowing for new
+clients to make new connections. Each instance is named ksmbd/1~n(port number)
+to indicate connected clients. Depending on the SMB request types, each new
+thread can decide to pass through the commands to the user space (ksmbd.mountd),
+currently DCE/RPC commands are identified to be handled through the user space.
+To further utilize the linux kernel, it has been chosen to process the commands
+as workitems and to be executed in the handlers of the ksmbd-io kworker threads.
+It allows for multiplexing of the handlers as the kernel take care of initiating
+extra worker threads if the load is increased and vice versa, if the load is
+decreased it destroys the extra worker threads. So, after connection is
+established with client. Dedicated ksmbd/1..n(port number) takes complete
+ownership of receiving/parsing of SMB commands. Each received command is worked
+in parallel i.e., There can be multiple clients commands which are worked in
+parallel. After receiving each command a separated kernel workitem is prepared
+for each command which is further queued to be handled by ksmbd-io kworkers.
+So, each SMB workitem is queued to the kworkers. This allows the benefit of load
+sharing to be managed optimally by the default kernel and optimizing client
+performance by handling client commands in parallel.
+
+ksmbd.mountd (user space daemon)
+--------------------------------
+
+ksmbd.mountd is userspace process to, transfer user account and password that
+are registered using ksmbd.adduser(part of utils for user space). Further it
+allows sharing information parameters that parsed from smb.conf to ksmbd in
+kernel. For the execution part it has a daemon which is continuously running
+and connected to the kernel interface using netlink socket, it waits for the
+requests(dcerpc and share/user info). It handles RPC calls (at a minimum few
+dozen) that are most important for file server from NetShareEnum and
+NetServerGetInfo. Complete DCE/RPC response is prepared from the user space
+and passed over to the associated kernel thread for the client.
+
+Key Features
+============
+
+The supported features are:
+ * SMB3 protocols for basic file sharing
+ * Auto negotiation
+ * Compound requests
+ * Oplock/Lease
+ * Large MTU
+ * NTLM/NTLMv2
+ * HMAC-SHA256 Signing
+ * Secure negotiate
+ * Signing Update
+ * Pre-authentication integrity(SMB 3.1.1)
+ * SMB3 encryption(CCM, GCM)
+ * SMB direct(RDMA)
+ * SMB3.1.1 POSIX extension support
+ * ACLs
+ * Kerberos
+
+The features that are planned or not supported:
+ * SMB3 Multi-channel
+ * Durable handle v1,v2
+ * Persistent handles
+ * Directory lease
+ * SMB2 notify
+
+How to run
+==========
+
+1. Download ksmbd-tools and compile them.
+	- https://github.com/cifsd-team/ksmbd-tools
+
+2. Create user/password for SMB share.
+
+	# mkdir /etc/ksmbd/
+	# ksmbd.adduser -a <Enter USERNAME for SMB share access>
+
+3. Create /etc/ksmbd/smb.conf file, add SMB share in smb.conf file
+	- Refer smb.conf.example and Documentation/configuration.txt
+	  in ksmbd-tools
+
+4. Insert ksmbd.ko module
+
+	# insmod ksmbd.ko
+
+5. Start ksmbd user space daemon
+	# ksmbd.mountd
+
+6. Access share from Windows or Linux using CIFS
+
+Shutdown CIFSD
+==============
+
+1. kill user and kernel space daemon
+	# sudo ksmbd.control -s
+
+How to turn debug print on
+==========================
+
+Each layer
+/sys/class/ksmbd-control/debug
+
+1. Enable all component prints
+	# sudo ksmbd.control -d "all"
+
+2. Enable one of components(smb, auth, vfs, oplock, ipc, conn, rdma)
+	# sudo ksmbd.control -d "smb"
+
+3. Show what prints are enable.
+	# cat/sys/class/ksmbd-control/debug
+	  [smb] auth vfs oplock ipc conn [rdma]
+
+4. Disable prints:
+	If you try the selected component once more, It is disabled without brackets.