Ubuntu Manpage: UFFDIO_API - enable operation of the userfaultfd and perform API handshake

NAME

       UFFDIO_API - enable operation of the userfaultfd and perform API handshake

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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

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

       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.

STANDARDS

       Linux.

HISTORY

       Linux 4.3.

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.

BUGS

       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

       See userfaultfd(2).