UFFDIO_API(2const) UFFDIO_API(2const)
NAME
UFFDIO_API - enable operation of the userfaultfd and perform API hand-
shake
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 avail-
able 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 fea-
tures 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 dur-
ing fork(2) and a UFFD_EVENT_FORK event is delivered to the user-
faultfd 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 mad-
vise(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 vir-
tual memory either explicitly with munmap(2), or implicitly dur-
ing either mmap(2) or mremap(2), the userfaultfd monitor will re-
ceive 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 user-
faultfd ranges on hugetlbfs virtual memory areas
UFFD_FEATURE_MISSING_SHMEM (since Linux 4.11)
If this feature bit is set, the kernel supports registering user-
faultfd ranges on shared memory areas. This includes all kernel
shared memory APIs: System V shared memory, tmpfs(5), shared map-
pings 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 sig-
nal 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 sup-
port is indicated by a separate feature.)
UFFD_FEATURE_MINOR_HUGETLBFS (since Linux 5.13)
If this feature bit is set, the kernel supports registering user-
faultfd 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 user-
faultfd 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 mem-
ory 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 ver-
sion.
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 struc-
ture. The caller should treat its contents as unspecified, and reini-
tialize 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).
SEE ALSO
ioctl(2), ioctl_userfaultfd(2), mmap(2), userfaultfd(2)
linux.git/Documentation/admin-guide/mm/userfaultfd.rst
Linux man-pages 6.9.1 2024-06-17 UFFDIO_API(2const)
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