UFFDIO_API2const

Usage

#include <linux/userfaultfd.h>  /* Definition of UFFD* constants */
#include <sys/ioctl.h>

struct uffdio_api {
    __u64 api;       /* Requested API version (input) */
    __u64 features;  /* Requested features (input/output) */
    __u64 ioctls;    /* Available ioctl() operations (output) */
};

Library

Standard C library (libc, -lc)

Overview

Enable operation of the userfaultfd and perform API handshake.

The api field denotes the API version requested by the application. The kernel verifies that it can support the requested API version, and sets the features and ioctls fields to bit masks representing all the available features and the generic ioctl(2) operations available.

Before Linux 4.11, the features field must be initialized to zero before the call to UFFDIO_API, and zero (i.e., no feature bits) is placed in the features field by the kernel upon return from ioctl(2).

Since Linux 4.11, userfaultfd supports features that need to be enabled explicitly. To enable any of the features, one needs to set the corresponding feature bits in features when issuing the UFFDIO_API ioctl.

For historical reasons, a temporary userfaultfd is needed to probe what userfaultfd features the kernel supports. The application needs to create a temporary userfaultfd, issue an UFFDIO_API ioctl with features set to zero. After the UFFDIO_API ioctl returns successfully, features should contain all the userfaultfd features that the kernel supports. The temporary userfaultfd can be safely closed after the probe.

If the application sets unsupported feature bits, the kernel will zero out the returned uffdio_api structure and return EINVAL.

The following feature bits may be set:

UFFD_FEATURE_EVENT_FORK (since Linux 4.11)

When this feature is enabled, the userfaultfd objects associated with a parent process are duplicated into the child process during fork(2) and a UFFD_EVENT_FORK event is delivered to the userfaultfd monitor

UFFD_FEATURE_EVENT_REMAP (since Linux 4.11)

If this feature is enabled, when the faulting process invokes mremap(2), the userfaultfd monitor will receive an event of type UFFD_EVENT_REMAP.

UFFD_FEATURE_EVENT_REMOVE (since Linux 4.11)

If this feature is enabled, when the faulting process calls madvise(2) with the MADV_DONTNEED or MADV_REMOVE advice value to free a virtual memory area the userfaultfd monitor will receive an event of type UFFD_EVENT_REMOVE.

UFFD_FEATURE_EVENT_UNMAP (since Linux 4.11)

If this feature is enabled, when the faulting process unmaps virtual memory either explicitly with munmap(2), or implicitly during either mmap(2) or mremap(2), the userfaultfd monitor will receive an event of type UFFD_EVENT_UNMAP.

UFFD_FEATURE_MISSING_HUGETLBFS (since Linux 4.11)

If this feature bit is set, the kernel supports registering userfaultfd ranges on hugetlbfs virtual memory areas

UFFD_FEATURE_MISSING_SHMEM (since Linux 4.11)

If this feature bit is set, the kernel supports registering userfaultfd ranges on shared memory areas. This includes all kernel shared memory APIs: System V shared memory, tmpfs(5), shared mappings of /dev/zero, mmap(2) with the MAP_SHARED flag set, memfd_create(2), and so on.

UFFD_FEATURE_SIGBUS (since Linux 4.14)

If this feature bit is set, no page-fault events (UFFD_EVENT_PAGEFAULT) will be delivered. Instead, a SIGBUS signal will be sent to the faulting process. Applications using this feature will not require the use of a userfaultfd monitor for processing memory accesses to the regions registered with userfaultfd.

UFFD_FEATURE_THREAD_ID (since Linux 4.14)

If this feature bit is set, uffd_msg.pagefault.feat.ptid will be set to the faulted thread ID for each page-fault message.

UFFD_FEATURE_PAGEFAULT_FLAG_WP (since Linux 5.10)

If this feature bit is set, userfaultfd supports write-protect faults for anonymous memory. (Note that shmem / hugetlbfs support is indicated by a separate feature.)

UFFD_FEATURE_MINOR_HUGETLBFS (since Linux 5.13)

If this feature bit is set, the kernel supports registering userfaultfd ranges in minor mode on hugetlbfs-backed memory areas.

UFFD_FEATURE_MINOR_SHMEM (since Linux 5.14)

If this feature bit is set, the kernel supports registering userfaultfd ranges in minor mode on shmem-backed memory areas.

UFFD_FEATURE_EXACT_ADDRESS (since Linux 5.18)

If this feature bit is set, uffd_msg.pagefault.address will be set to the exact page-fault address that was reported by the hardware, and will not mask the offset within the page. Note that old Linux versions might indicate the exact address as well, even though the feature bit is not set.

UFFD_FEATURE_WP_HUGETLBFS_SHMEM (since Linux 5.19)

If this feature bit is set, userfaultfd supports write-protect faults for hugetlbfs and shmem / tmpfs memory.

UFFD_FEATURE_WP_UNPOPULATED (since Linux 6.4)

If this feature bit is set, the kernel will handle anonymous memory the same way as file memory, by allowing the user to write-protect unpopulated page table entries.

UFFD_FEATURE_POISON (since Linux 6.6)

If this feature bit is set, the kernel supports resolving faults with the UFFDIO_POISON ioctl.

UFFD_FEATURE_WP_ASYNC (since Linux 6.7)

If this feature bit is set, the write protection faults would be asynchronously resolved by the kernel.

UFFD_FEATURE_MOVE (since Linux 6.8)

If this feature bit is set, the kernel supports resolving faults with the UFFDIO_MOVE ioctl.

UFFD_FEATURE_RWP (since Linux 7.2)

If this feature bit is set, the kernel supports read-write-protection tracking, and the UFFDIO_REGISTER_MODE_RWP registration mode and the UFFDIO_RWPROTECT ioctl become available.

UFFD_FEATURE_RWP_ASYNC (since Linux 7.2)

If this feature bit is set, the kernel will resolve read-write-protect faults in place without delivering a notification, automatically restoring page permissions and letting the faulted thread continue. This bit requires UFFD_FEATURE_RWP to be set in the same UFFDIO_API call. The async mode can also be toggled at runtime using the UFFDIO_SET_MODE(2const) ioctl.

The returned argp->ioctls field can contain the following bits:

1 << _UFFDIO_API

The UFFDIO_API operation is supported.

1 << _UFFDIO_REGISTER

The UFFDIO_REGISTER operation is supported.

1 << _UFFDIO_UNREGISTER

The UFFDIO_UNREGISTER operation is supported.

1 << _UFFDIO_SET_MODE (since Linux 7.2)

The UFFDIO_SET_MODE operation is supported. This is a file-descriptor-level ioctl and is reported once per userfaultfd, independent of any registered range.

Return value

On success, 0 is returned.

On error, -1 is returned and errno is set to indicate the error.

Errors

EFAULT

argp refers to an address that is outside the calling process's accessible address space.

EINVAL

The API version requested in the api field is not supported by this kernel, or the features field passed to the kernel includes feature bits that are not supported by the current kernel version.

EINVAL

A previous UFFDIO_API call already enabled one or more features for this userfaultfd. Calling UFFDIO_API twice, the first time with no features set, is explicitly allowed as per the two-step feature detection handshake.

EPERM

The UFFD_FEATURE_EVENT_FORK feature was enabled, but the calling process doesn't have the CAP_SYS_PTRACE capability.

Caveats

If an error occurs, the kernel may zero the provided uffdio_api structure. The caller should treat its contents as unspecified, and reinitialize it before re-attempting another UFFDIO_API call.

Examples

See userfaultfd(2).