Various ioctl(2) operations can be performed on a userfaultfd
object (created by a call to userfaultfd(2)) using calls of the
form:
ioctl(fd, cmd, argp);
In the above, fd is a file descriptor referring to a userfaultfd
object, cmd is one of the commands listed below, and argp is a
pointer to a data structure that is specific to cmd.
The various ioctl(2) operations are described below. The
UFFDIO_API, UFFDIO_REGISTER, and UFFDIO_UNREGISTER operations are
used to configure userfaultfd behavior. These operations allow
the caller to choose what features will be enabled and what kinds
of events will be delivered to the application. The remaining
operations are range operations. These operations enable the
calling application to resolve page-fault events.
UFFDIO_API
(Since Linux 4.3.) Enable operation of the userfaultfd and
perform API handshake.
The argp argument is a pointer to a uffdio_api structure, defined
as:
struct uffdio_api {
__u64 api; /* Requested API version (input) */
__u64 features; /* Requested features (input/output) */
__u64 ioctls; /* Available ioctl() operations (output) */
};
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.
For Linux kernel versions before 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).
Starting from Linux 4.11, the features field can be used to ask
whether particular features are supported and explicitly enable
userfaultfd features that are disabled by default. The kernel
always reports all the available features in the features field.
To enable userfaultfd features the application should set a bit
corresponding to each feature it wants to enable in the features
field. If the kernel supports all the requested features it will
enable them. Otherwise it 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.
The returned 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.
1 << _UFFDIO_WRITEPROTECT
The UFFDIO_WRITEPROTECT operation is supported.
This ioctl(2) operation returns 0 on success. On error, -1 is
returned and errno is set to indicate the error. Possible errors
include:
EFAULT argp refers to an address that is outside the calling
process's accessible address space.
EINVAL The userfaultfd has already been enabled by a previous
UFFDIO_API operation.
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.
UFFDIO_REGISTER
(Since Linux 4.3.) Register a memory address range with the
userfaultfd object. The pages in the range must be "compatible".
Up to Linux kernel 4.11, only private anonymous ranges are
compatible for registering with UFFDIO_REGISTER.
Since Linux 4.11, hugetlbfs and shared memory ranges are also
compatible with UFFDIO_REGISTER.
The argp argument is a pointer to a uffdio_register structure,
defined as:
struct uffdio_range {
__u64 start; /* Start of range */
__u64 len; /* Length of range (bytes) */
};
struct uffdio_register {
struct uffdio_range range;
__u64 mode; /* Desired mode of operation (input) */
__u64 ioctls; /* Available ioctl() operations (output) */
};
The range field defines a memory range starting at start and
continuing for len bytes that should be handled by the
userfaultfd.
The mode field defines the mode of operation desired for this
memory region. The following values may be bitwise ORed to set
the userfaultfd mode for the specified range:
UFFDIO_REGISTER_MODE_MISSING
Track page faults on missing pages.
UFFDIO_REGISTER_MODE_WP
Track page faults on write-protected pages.
If the operation is successful, the kernel modifies the ioctls
bit-mask field to indicate which ioctl(2) operations are
available for the specified range. This returned bit mask is as
for UFFDIO_API.
This ioctl(2) operation returns 0 on success. On error, -1 is
returned and errno is set to indicate the error. Possible errors
include:
EBUSY A mapping in the specified range is registered with
another userfaultfd object.
EFAULT argp refers to an address that is outside the calling
process's accessible address space.
EINVAL An invalid or unsupported bit was specified in the mode
field; or the mode field was zero.
EINVAL There is no mapping in the specified address range.
EINVAL range.start or range.len is not a multiple of the system
page size; or, range.len is zero; or these fields are
otherwise invalid.
EINVAL There as an incompatible mapping in the specified address
range.
UFFDIO_UNREGISTER
(Since Linux 4.3.) Unregister a memory address range from
userfaultfd. The pages in the range must be "compatible" (see
the description of UFFDIO_REGISTER.)
The address range to unregister is specified in the uffdio_range
structure pointed to by argp.
This ioctl(2) operation returns 0 on success. On error, -1 is
returned and errno is set to indicate the error. Possible errors
include:
EINVAL Either the start or the len field of the ufdio_range
structure was not a multiple of the system page size; or
the len field was zero; or these fields were otherwise
invalid.
EINVAL There as an incompatible mapping in the specified address
range.
EINVAL There was no mapping in the specified address range.
UFFDIO_COPY
(Since Linux 4.3.) Atomically copy a continuous memory chunk
into the userfault registered range and optionally wake up the
blocked thread. The source and destination addresses and the
number of bytes to copy are specified by the src, dst, and len
fields of the uffdio_copy structure pointed to by argp:
struct uffdio_copy {
__u64 dst; /* Destination of copy */
__u64 src; /* Source of copy */
__u64 len; /* Number of bytes to copy */
__u64 mode; /* Flags controlling behavior of copy */
__s64 copy; /* Number of bytes copied, or negated error */
};
The following value may be bitwise ORed in mode to change the
behavior of the UFFDIO_COPY operation:
UFFDIO_COPY_MODE_DONTWAKE
Do not wake up the thread that waits for page-fault
resolution
UFFDIO_COPY_MODE_WP
Copy the page with read-only permission. This allows the
user to trap the next write to the page, which will block
and generate another write-protect userfault message.
This is used only when both UFFDIO_REGISTER_MODE_MISSING
and UFFDIO_REGISTER_MODE_WP modes are enabled for the
registered range.
The copy field is used by the kernel to return the number of
bytes that was actually copied, or an error (a negated errno-
style value). If the value returned in copy doesn't match the
value that was specified in len, the operation fails with the
error EAGAIN. The copy field is output-only; it is not read by
the UFFDIO_COPY operation.
This ioctl(2) operation returns 0 on success. In this case, the
entire area was copied. On error, -1 is returned and errno is
set to indicate the error. Possible errors include:
EAGAIN The number of bytes copied (i.e., the value returned in
the copy field) does not equal the value that was
specified in the len field.
EINVAL Either dst or len was not a multiple of the system page
size, or the range specified by src and len or dst and len
was invalid.
EINVAL An invalid bit was specified in the mode field.
ENOENT (since Linux 4.11)
The faulting process has changed its virtual memory layout
simultaneously with an outstanding UFFDIO_COPY operation.
ENOSPC (from Linux 4.11 until Linux 4.13)
The faulting process has exited at the time of a
UFFDIO_COPY operation.
ESRCH (since Linux 4.13)
The faulting process has exited at the time of a
UFFDIO_COPY operation.
UFFDIO_ZEROPAGE
(Since Linux 4.3.) Zero out a memory range registered with
userfaultfd.
The requested range is specified by the range field of the
uffdio_zeropage structure pointed to by argp:
struct uffdio_zeropage {
struct uffdio_range range;
__u64 mode; /* Flags controlling behavior of copy */
__s64 zeropage; /* Number of bytes zeroed, or negated error */
};
The following value may be bitwise ORed in mode to change the
behavior of the UFFDIO_ZEROPAGE operation:
UFFDIO_ZEROPAGE_MODE_DONTWAKE
Do not wake up the thread that waits for page-fault
resolution.
The zeropage field is used by the kernel to return the number of
bytes that was actually zeroed, or an error in the same manner as
UFFDIO_COPY. If the value returned in the zeropage field doesn't
match the value that was specified in range.len, the operation
fails with the error EAGAIN. The zeropage field is output-only;
it is not read by the UFFDIO_ZEROPAGE operation.
This ioctl(2) operation returns 0 on success. In this case, the
entire area was zeroed. On error, -1 is returned and errno is
set to indicate the error. Possible errors include:
EAGAIN The number of bytes zeroed (i.e., the value returned in
the zeropage field) does not equal the value that was
specified in the range.len field.
EINVAL Either range.start or range.len was not a multiple of the
system page size; or range.len was zero; or the range
specified was invalid.
EINVAL An invalid bit was specified in the mode field.
ESRCH (since Linux 4.13)
The faulting process has exited at the time of a
UFFDIO_ZEROPAGE operation.
UFFDIO_WAKE
(Since Linux 4.3.) Wake up the thread waiting for page-fault
resolution on a specified memory address range.
The UFFDIO_WAKE operation is used in conjunction with UFFDIO_COPY
and UFFDIO_ZEROPAGE operations that have the
UFFDIO_COPY_MODE_DONTWAKE or UFFDIO_ZEROPAGE_MODE_DONTWAKE bit
set in the mode field. The userfault monitor can perform several
UFFDIO_COPY and UFFDIO_ZEROPAGE operations in a batch and then
explicitly wake up the faulting thread using UFFDIO_WAKE.
The argp argument is a pointer to a uffdio_range structure (shown
above) that specifies the address range.
This ioctl(2) operation returns 0 on success. On error, -1 is
returned and errno is set to indicate the error. Possible errors
include:
EINVAL The start or the len field of the ufdio_range structure
was not a multiple of the system page size; or len was
zero; or the specified range was otherwise invalid.
UFFDIO_WRITEPROTECT (Since Linux 5.7)
Write-protect or write-unprotect a userfaultfd-registered memory
range registered with mode UFFDIO_REGISTER_MODE_WP.
The argp argument is a pointer to a uffdio_range structure as
shown below:
struct uffdio_writeprotect {
struct uffdio_range range; /* Range to change write permission*/
__u64 mode; /* Mode to change write permission */
};
There are two mode bits that are supported in this structure:
UFFDIO_WRITEPROTECT_MODE_WP
When this mode bit is set, the ioctl will be a write-
protect operation upon the memory range specified by
range. Otherwise it will be a write-unprotect operation
upon the specified range, which can be used to resolve a
userfaultfd write-protect page fault.
UFFDIO_WRITEPROTECT_MODE_DONTWAKE
When this mode bit is set, do not wake up any thread that
waits for page-fault resolution after the operation. This
can be specified only if UFFDIO_WRITEPROTECT_MODE_WP is
not specified.
This ioctl(2) operation returns 0 on success. On error, -1 is
returned and errno is set to indicate the error. Possible errors
include:
EINVAL The start or the len field of the ufdio_range structure
was not a multiple of the system page size; or len was
zero; or the specified range was otherwise invalid.
EAGAIN The process was interrupted; retry this call.
ENOENT The range specified in range is not valid. For example,
the virtual address does not exist, or not registered with
userfaultfd write-protect mode.
EFAULT Encountered a generic fault during processing.
