Io_uring

io_uring

io_uring

Linux kernel interface for storage devices

io_uring (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.[1][2]: 2 

Development is ongoing, worked on primarily by Jens Axboe at Meta.[1]

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).[3] 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.[1][3][4] 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.[1]: 3 

eBPF can be combined with io_uring.[5]

History

The Linux kernel has supported asynchronous I/O since version 2.5, but it was seen as difficult to use and inefficient.[6] This older API only supported certain niche use cases,[7] 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.[8]

The io_uring kernel interface was adopted in Linux kernel version 5.1 to resolve the deficiencies of Linux AIO.[1][4][9] The liburing library provides an API to interact with the kernel interface easily from userspace.[1][1]: 12 

Security

io_uring has been noted for exposing a significant attack surface and structural difficulties integrating it with the Linux security subsystem.[10]

In June 2023, Google's security team reported that 60% of Linux kernel exploits submitted to their bug bounty program in 2022 were exploits of io_uring vulnerabilities. As a result, io_uring was disabled for apps in Android, and disabled entirely in ChromeOS as well as Google servers.[11] Docker also consequently disabled io_uring from their default seccomp profile.[12]

References

  1. [1]
    "Linux Kernel Getting io_uring To Deliver Fast & Efficient I/O - Phoronix". Phoronix. Retrieved 2021-03-14.
  2. [2]
    Axboe, Jens (October 15, 2019). "Efficient IO with io_uring" (PDF).
  3. [3]
    "Getting Hands-on with io_uring using Go". developers.mattermost.com. Retrieved 2021-11-20.
  4. [4]
    "The rapid growth of io_uring [LWN.net]". lwn.net. Retrieved 2021-11-20.
  5. [5]
    "BPF meets io_uring [LWN.net]". LWN.net. Retrieved 2023-04-17.
  6. [6]
    Corbet, Jonathan. "Ringing in a new asynchronous I/O API". LWN.net. Retrieved 2021-03-14.
  7. [7]
    "What's new with io_uring" (PDF). Retrieved 2022-06-01.
  8. [8]
    "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.
  9. [9]
    "Faster IO through io_uring | Kernel Recipes 2019". Retrieved 2021-03-14.
  10. [10]
    Corbet, Jonathan (2022-07-28). "Security requirements for new kernel features". LWN.net. Retrieved 2023-06-16.
  11. [11]
    Koczka, Tamás. "Learnings from kCTF VRP's 42 Linux kernel exploits submissions". Google Online Security Blog. Google. Retrieved 14 June 2023.
  12. [12]
    "seccomp: block io_uring_* syscalls in default profile by akerouanton · Pull Request #46762 · moby/moby". GitHub. Retrieved 2023-11-02.
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