Prefer /dev/userfaultfd (Linux 6.1+) over the userfaultfd(2) syscall
for obtaining userfaultfd file descriptors. The device path bypasses
the capable(CAP_SYS_PTRACE) and vm.unprivileged_userfaultfd sysctl
checks that block the syscall in user-namespaced containers, using
file permissions instead.
Falls back to the syscall on older kernels or when the device node
does not exist.
Signed-off-by: Roberto Campesato <render@metalabs.org>
Assisted-by: Claude:claude-opus-4-6
Cross-check each UFFDIO_* constant against the Linux _IOC(dir, type,
nr, size) encoding formula at compile time so that transposed direction
bits or struct sizes are caught immediately rather than silently
producing wrong ioctl numbers at runtime.
Signed-off-by: Shayon Mukherjee <shayonj@gmail.com>
UFFDIO_WAKE was 0x4010_aa02 (_IOW) but should be 0x8010_aa02,
causing every wake call to silently fail with -EINVAL.
UFFD_FEATURE_MISSING_HUGETLBFS was (1<<6) but should be (1<<4),
colliding with UFFD_FEATURE_EVENT_UNMAP.
Signed-off-by: Zhiheng Tao <junchuan.tzh@antgroup.com>
Add a small constants module with the ioctl numbers and protocol
constants needed for userfaultfd-based demand-paged snapshot restore.
These are derived from the kernel's include/uapi/linux/userfaultfd.h.
Signed-off-by: Shayon Mukherjee <shayonj@gmail.com>
