UFFDIO_API(2const) — Linux manual page

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | CAVEATS | BUGS | EXAMPLES | SEE ALSO | COLOPHON

UFFDIO_API(2const)                                    UFFDIO_API(2const)

NAME         top

       UFFDIO_API - enable operation of the userfaultfd and perform API
       handshake

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

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

       int ioctl(int fd, UFFDIO_API, struct uffdio_api *argp);

       #include <linux/userfaultfd.h>

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

DESCRIPTION         top

       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.

       Since Linux 4.11, applications should use the features field to
       perform a two-step handshake.  First, UFFDIO_API is called with
       the features field set to zero.  The kernel responds by setting
       all supported feature bits.

       Applications which do not require any specific features can begin
       using the userfaultfd immediately.  Applications which do need
       specific features should call UFFDIO_API again with a subset of
       the reported feature bits set to enable those features.

       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).

       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.

       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.

RETURN VALUE         top

       On success, 0 is returned.

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

ERRORS         top

       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.

STANDARDS         top

       Linux.

HISTORY         top

       Linux 4.3.

CAVEATS         top

       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.

BUGS         top

       In order to detect available userfault features and enable some
       subset of those features the userfaultfd file descriptor must be
       closed after the first UFFDIO_API operation that queries features
       availability and reopened before the second UFFDIO_API operation
       that actually enables the desired features.

EXAMPLES         top

       See userfaultfd(2).

SEE ALSO         top

       ioctl(2), ioctl_userfaultfd(2), mmap(2), userfaultfd(2)

       linux.git/Documentation/admin-guide/mm/userfaultfd.rst

COLOPHON         top

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Linux man-pages 6.9.1          2024-06-17             UFFDIO_API(2const)

Pages that refer to this page: ioctl_userfaultfd(2)