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

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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).