Io uring

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io_uring [a] (previously known as aioring) is a Linux kernel system call interface for storage device asynchronous I/O operations addressing performance issues with similar interfaces provided by functions like read()/write() or aio_read()/aio_write() etc. for operations on data accessed by file descriptors. [2] [3] : 2 

Contents

Development is ongoing, worked on primarily by Jens Axboe at Meta. [2]

Interface

It works by creating two circular buffers, called "queue rings", for storage of submission and completion of I/O requests, respectively. For storage devices, these are called the submission queue (SQ) and completion queue (CQ). [4] Keeping these buffers shared between the kernel and application helps to boost the I/O performance by eliminating the need to issue extra and expensive system calls to copy these buffers between the two. [2] [4] [5] According to the io_uring design paper, the SQ buffer is writable only by consumer applications, and the CQ buffer is writable only by the kernel. [2] : 3 

eBPF can be combined with io_uring. [6]

History

The Linux kernel has supported asynchronous I/O since version 2.5, but it was seen as difficult to use and inefficient. [7] This older API only supported certain niche use cases, [8] notably it only enables asynchronous operation when using the O_DIRECT flag and while accessing already allocated files. This prevents utilizing the page cache, while also exposing the application to complex O_DIRECT semantics. Linux AIO also does not support sockets, so it cannot be used to multiplex network and disk I/O. [9]

The io_uring kernel interface was adopted in Linux kernel version 5.1 to resolve the deficiencies of Linux AIO. [2] [5] [10] The liburing library provides an API to interact with the kernel interface easily from userspace. [2] : 12 

Security

io_uring has been noted for exposing a significant attack surface and structural difficulties integrating it with the Linux security subsystem. [11]

In June 2023, Google's security team reported that 60% of the exploits submitted to their bug bounty program in 2022 were exploits of the Linux kernel's io_uring vulnerabilities. As a result, io_uring was disabled for apps in Android, and disabled entirely in ChromeOS as well as Google servers. [12] Docker also consequently disabled io_uring from their default seccomp profile. [13]

Notes

  1. ↑ Input/output user ring [1]

Terminology

CQ
Completion Queue
SQ
Submission Queue

References

  1. ↑ Axboe, Jens. "@axboe@fosstodon.org".
  2. 1 2 3 4 5 6 "Linux Kernel Getting io_uring To Deliver Fast & Efficient I/O". Phoronix . 2019-02-14. Retrieved 2021-03-14.
  3. ↑ Axboe, Jens (October 15, 2019). "Efficient IO with io_uring" (PDF).
  4. 1 2 "Getting Hands-on with io_uring using Go". developers.mattermost.com. Retrieved 2021-11-20.
  5. 1 2 "The rapid growth of io_uring [LWN.net]". lwn.net. Retrieved 2021-11-20.
  6. ↑ "BPF meets io_uring [LWN.net]". LWN.net . Retrieved 2023-04-17.
  7. ↑ Corbet, Jonathan. "Ringing in a new asynchronous I/O API". LWN.net . Retrieved 2021-03-14.
  8. ↑ "What's new with io_uring" (PDF). Retrieved 2022-06-01.
  9. ↑ "Linux Asynchronous I/O". 2014-04-21. Archived from the original on 2015-04-06. Retrieved 2023-06-16. Blocking during io_submit on ext4, on buffered operations, network access, pipes, etc. Some operations are not well-represented by the AIO interface. With completely unsupported operations like buffered reads, operations on a socket or pipes, the entire operation will be performed during the io_submit syscall, with the completion available immediately for access with io_getevents. AIO access to a file on a filesystem like ext4 is partially supported: if a metadata read is required to look up the data block (ie if the metadata is not already in memory), then the io_submit call will block on the metadata read. Certain types of file-enlarging writes are completely unsupported and block for the entire duration of the operation.
  10. ↑ "Faster IO through io_uring". Kernel Recipes 2019. Retrieved 2021-03-14.
  11. ↑ Corbet, Jonathan (2022-07-28). "Security requirements for new kernel features". LWN.net . Retrieved 2023-06-16.
  12. ↑ Koczka, Tamás. "Learnings from kCTF VRP's 42 Linux kernel exploits submissions". Google Online Security Blog. Google. Archived from the original on 2024-09-22. Retrieved 14 June 2023. 60% of the submissions exploited the io_uring component of the Linux kernel
  13. ↑ "Update RuntimeDefault seccomp profile to disallow io_uring related syscalls by vinayakankugoyal · Pull Request #9320 · containerd/containerd". GitHub. 2023-11-02. Archived from the original on 2024-01-06. Retrieved 2024-10-20.
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