Function create

vmm/src/uffd.rs:96–129

Create a userfaultfd file descriptor and perform the API handshake. Prefers `/dev/userfaultfd` (no capability/sysctl requirements, just file permissions) and falls back to the `userfaultfd(2)` syscall.

(required_features: u64)

/// Prefers `/dev/userfaultfd` (no capability/sysctl requirements, just file
/// permissions) and falls back to the `userfaultfd(2)` syscall.
pub(crate) fn create(required_features: u64) -> Result<OwnedFd, Error> {
    let fd = match try_dev_userfaultfd() {
        Ok(fd) => fd,
        Err(_) => {
            // SAFETY: `userfaultfd` syscall with O_CLOEXEC | O_NONBLOCK flags.
            let raw =
                unsafe { libc::syscall(libc::SYS_userfaultfd, libc::O_CLOEXEC | libc::O_NONBLOCK) };
            if raw < 0 {
                return Err(Error::last_os_error());
            }
            // SAFETY: the syscall returned a valid fd above.
            unsafe { OwnedFd::from_raw_fd(raw as RawFd) }
        }
    };

    let mut api = UffdioApi {
        api: userfaultfd::UFFD_API,
        features: required_features,
        ioctls: 0,
    };
    // SAFETY: `api` is a valid, correctly-sized struct for this ioctl.
    let ret = unsafe {
        libc::ioctl(
            fd.as_raw_fd(),
            userfaultfd::UFFDIO_API as libc::Ioctl,
            &mut api,
        )
    };
    if ret < 0 {
        return Err(Error::last_os_error());
    }

    Ok(fd)
}

/// Register a memory range for missing-page fault handling.
pub(crate) fn register(fd: BorrowedFd<'_>, addr: u64, len: u64) -> Result<u64, Error> {