Documentation/filesystems/smb/ksmbd.rst

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Documentation/filesystems/smb/ksmbd.rst
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.. SPDX-License-Identifier: GPL-2.0

==========================
KSMBD - SMB3 Kernel Server
==========================

KSMBD is a linux kernel server which implements SMB3 protocol in kernel space
for sharing files over network.

KSMBD 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 a 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 a request, the Forker
thread will accept the client connection and fork a new thread for a 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 takes 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 the connection is
established with the 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 client 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 a userspace process to, transfer the user account and password that
are registered using ksmbd.adduser (part of utils for user space). Further it
allows sharing information parameters that are 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.


KSMBD Feature Status
====================

============================== =================================================
Feature name                   Status
============================== =================================================
Dialects                       Supported. SMB2.1 SMB3.0, SMB3.1.1 dialects
                               (intentionally excludes security vulnerable SMB1

Annotation

Implementation Notes