UFFDIO_API(2const) - Linux manual page (original) (raw)
UFFDIOAPI(2const) UFFDIOAPI(2const)
NAME top
UFFDIO_API - enable operation of the userfaultfd and perform API
handshakeLIBRARY 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)](../man2/ioctl.2.html) 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)](../man2/ioctl.2.html).
If the application sets unsupported feature bits, the kernel will
zero out the returned _uffdioapi_ 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)](../man2/fork.2.html) 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)](../man2/mremap.2.html), 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)](../man2/madvise.2.html) 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)](../man2/munmap.2.html), or
implicitly during either [mmap(2)](../man2/mmap.2.html) or [mremap(2)](../man2/mremap.2.html), 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)](../man5/tmpfs.5.html), shared mappings of _/dev/zero_, [mmap(2)](../man2/mmap.2.html) with the
**MAP_SHARED** flag set, [memfd_create(2)](../man2/memfd%5Fcreate.2.html), 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, _uffdmsg.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, _uffdmsg.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](../man3/errno.3.html)_ 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 _uffdioapi_
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)](../man2/userfaultfd.2.html).SEE ALSO top
[ioctl(2)](../man2/ioctl.2.html), [ioctl_userfaultfd(2)](../man2/ioctl%5Fuserfaultfd.2.html), [mmap(2)](../man2/mmap.2.html), [userfaultfd(2)](../man2/userfaultfd.2.html)
_linux.git/Documentation/admin-guide/mm/userfaultfd.rst_COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 UFFDIOAPI(2const)
Pages that refer to this page:ioctl_userfaultfd(2)