Even if it was tested extensively, why would I want to run any server in the kernel?
this post was submitted on 16 Sep 2023
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I guess this is to get some kind of parity with nfsd
Yeah, another thing where I never understood why it was in the kernel.
Yeah, not sure about that either. Unless you have some kind of use case where you need every bit of performance out of this, this stuff belongs in user space.
And if performance was an issue, just throw more/better hardware at it.
The above text says that the aim is to do RDMA, to let the NIC access memory directly, but I'd think that existing Linux zero-copy interfaces would be sufficient for that.
https://en.wikipedia.org/wiki/Zero-copy
The Linux kernel supports zero-copy through various system calls, such as:
- sendfile, sendfile64;[9]
- splice;[10]
- tee;[11]
- vmsplice;[12]
- process_vm_readv;[13]
- process_vm_writev;[14]
- copy_file_range;[15]
- raw sockets with packet mmap[16] or AF_XDP.
So I'd think that the target workload has to be one where you can't just fetch a big chunk of pre-existing data, where you have to interject server-generated data in response to small requests, and even the overhead of switching to userspace to generate some kind of server-generated response is too high.
Which seems like a heck of a niche case.
But it obviously got approval from the kernel team.
googles
https://www.kernel.org/doc/html/next/filesystems/smb/ksmbd.html
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.
I guess you could accelerate open and close too.
In all seriousness, I feel like if you're in such a niche situation that you can't afford the overhead of going to userspace for that, (a) there's likely room to optimize your application to request different things and (b) CIFS might not be the best option to be sharing data over the network either.
The above text says that the aim is to do RDMA, to let the NIC access memory directly
Oh, so the attack surface is much bigger than I realized. The NIC is probably the last thing I'd want writing directly to memory and bypassing the kernel.
I guess none of this will be enabled in desktop distros or even the majority of server distros...right?
I was under the impression this is already the norm for network equipment because the vast amount of data is no longer processable by the kernel. In fairness though that equipment most likely doesn't really consume the data but rather just forwards.
Shouldn't io_uring solve the issues with speed between usermode and kernelmode?
KSMBD is also important in that placing such core server functionality right inside the kernel represents a significant potential attack surface for crackers. As one comment on Hacker News said "Unless this is formally proven or rewritten in a safer language, you'll have to pay me in solid gold to use such a CVE factory waiting to happen."
Words to live by.
There have been vulnerabilities on the TCP/IP stack on a number of platforms (maybe all?), and that's a rather smaller attack surface.
EDIT: It also looks like ksmbd has already built itself a bit of a security history:
https://access.redhat.com/solutions/6991749
EDIT2: A bad security history:
https://lwn.net/Articles/871866/
The commit history for ksmbd shows a steady stream of fixes, as expected. Worryingly, though, many of the problems being fixed are clearly security issues — not a good thing in a network filesystem implementation. Examples include:
- The code to change ownership and permissions did not check existing file permissions first.
- Failure to validate data lengths could lead to access to invalid data.
- The server would blindly follow symbolic links during pathname lookup.
- Numerous failures to validate buffer lengths, such as this one or this one.
All of those fixes were applied after ksmbd landed in the mainline; there are others that came before. Currently, twelve fixes to ksmbd credit Coverity scans in their changelogs.
Those would worry me if they showed up in a production userspace network filesystem.
I guess if you're just running it in a VM with a passthrough storage device with nothing else running in the VM that could be okay.
Inb4 systemd-smbd
KSMBD is an opensource In-kernel CIFS/SMB3 server created by Namjae Jeon for Linux Kernel. It's an implementation of SMB/CIFS protocol in kernel space for sharing files and IPC services over network. Initially the target is to provide improved file I/O performances, but the bigger goal is to have some new features which are much easier to develop and maintain inside the kernel and expose the layers fully. Directions can be attributed to sections where SAMBA is moving to few modules inside the kernel to have features like RDMA(Remote direct memory access) to work with actual performance gain.
I'm moving to BSD!! Seriously though the Linux kernel shouldn't just continually get more and more bloated... Keep this shit in user space!!