CIFSD In-Kernel SMB3 File-Sharing Server Lands In Linux-Next

Written by Michael Larabel in Linux Networking on 18 March 2021 at 08:45 AM EDT. 23 Comments
Samsung for some time now has been working on an in-kernel SMB3 protocol implementation for file sharing across the network with "CIFSD" and it's now been queued into Linux-Next meaning it will likely go for mainline in a coming cycle.

There's long been CIFSD on GitHub for the in-kernel CIFS/SMB3 server with it being designed to run within the kernel for greater I/O performance and better handling some features such as RDMA integration.

Queued up this week in linux-next is that CIFSD kernel code. The SMB3 kernel server is more than 31k lines of new code thanks to Samsung. .
Key Features

The supported features are:
* SMB3 protocols for basic file sharing
* Auto negotiation
* Compound requests
* Oplock/Lease
* Large MTU
* HMAC-SHA256 Signing
* Secure negotiate
* Signing Update
* Pre-authentication integrity(SMB 3.1.1)
* SMB3 encryption(CCM, GCM)
* SMB direct(RDMA)
* The part of POSIX extension support (posix context, modebit)

With the patch, the CIFSD code is considered experimental and hidden behind the SMB_SERVER Kconfig switch.

The SMB_SERVER_SMBDIRECT mode allows for transferring SMB packets over RDMA. In the name of security is also a Kconfig option that would prevent unprivileged processes from being able to start the CIFSD server.

It will be interesting to see how the performance and adoption of CIFSD is moving forward compared to Samba in user-space. For now this CIFSD code is baking in linux-next while we'll see for Linux 5.13 or a near-term kernel cycle if it's ultimately submitted as a pull to Linus during the merge window.
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