Ubuntu Manpage: ioctl_userfaultfd - create a file descriptor for handling page faults in user space

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NAME

       ioctl_userfaultfd - create a file descriptor for handling page faults in user space

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <linux/userfaultfd.h>  /* Definition of UFFD* constants */
       #include <sys/ioctl.h>

       int ioctl(int fd, int op, ...);

DESCRIPTION

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, op, argp);

In the above, fd is a file descriptor referring to a userfaultfd object, op is one of the operations
listed below, and argp is a pointer to a data structure that is specific to op.

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.

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 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.

This ioctl(2) operation returns 0 on success. On error, -1 is returned and errno is set to indicate the
error. 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.
Possible errors include:

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.

UFFDIO_REGISTER
(Since Linux 4.3.) Register a memory address range with the userfaultfd object. The pages in the range
must be “compatible”. Please refer to the list of register modes below for the compatible memory
backends for each mode.

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. Since Linux 4.3, only private anonymous ranges are
compatible. Since Linux 4.11, hugetlbfs and shared memory ranges are also compatible.

UFFDIO_REGISTER_MODE_WP
Track page faults on write-protected pages. Since Linux 5.7, only private anonymous ranges are
compatible.

UFFDIO_REGISTER_MODE_MINOR
Track minor page faults. Since Linux 5.13, only hugetlbfs ranges are compatible. Since Linux
5.14, compatibility with shmem ranges was added.

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 can contain the following bits:

1 << _UFFDIO_COPY
The UFFDIO_COPY operation is supported.

1 << _UFFDIO_WAKE
The UFFDIO_WAKE operation is supported.

1 << _UFFDIO_WRITEPROTECT
The UFFDIO_WRITEPROTECT operation is supported.

1 << _UFFDIO_ZEROPAGE
The UFFDIO_ZEROPAGE operation is supported.

1 << _UFFDIO_CONTINUE
The UFFDIO_CONTINUE operation is supported.

1 << _UFFDIO_POISON
The UFFDIO_POISON 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:

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.

UFFDIO_CONTINUE
(Since Linux 5.13.) Resolve a minor page fault by installing page table entries for existing pages in
the page cache.

The argp argument is a pointer to a uffdio_continue structure as shown below:

struct uffdio_continue {
struct uffdio_range range;
/* Range to install PTEs for and continue */
__u64 mode; /* Flags controlling the behavior of continue */
__s64 mapped; /* Number of bytes mapped, or negated error */
};

The following value may be bitwise ORed in mode to change the behavior of the UFFDIO_CONTINUE operation:

UFFDIO_CONTINUE_MODE_DONTWAKE
Do not wake up the thread that waits for page-fault resolution.

The mapped field is used by the kernel to return the number of bytes that were actually mapped, or an
error in the same manner as UFFDIO_COPY. If the value returned in the mapped field doesn't match the
value that was specified in range.len, the operation fails with the error EAGAIN. The mapped field is
output-only; it is not read by the UFFDIO_CONTINUE operation.

This ioctl(2) operation returns 0 on success. In this case, the entire area was mapped. On error, -1 is
returned and errno is set to indicate the error. Possible errors include:

EAGAIN The number of bytes mapped (i.e., the value returned in the mapped field) does not equal the value
that was specified in the range.len field.

EEXIST One or more pages were already mapped in the given range.

EFAULT No existing page could be found in the page cache for the given range.

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.

ENOENT The faulting process has changed its virtual memory layout simultaneously with an outstanding
UFFDIO_CONTINUE operation.

ENOMEM Allocating memory needed to setup the page table mappings failed.

ESRCH The faulting process has exited at the time of a UFFDIO_CONTINUE operation.

UFFDIO_POISON
(Since Linux 6.6.) Mark an address range as "poisoned". Future accesses to these addresses will raise a
SIGBUS signal. Unlike MADV_HWPOISON this works by installing page table entries, rather than "really"
poisoning the underlying physical pages. This means it only affects this particular address space.

The argp argument is a pointer to a uffdio_poison structure as shown below:

struct uffdio_poison {
struct uffdio_range range;
/* Range to install poison PTE markers in */
__u64 mode; /* Flags controlling the behavior of poison */
__s64 updated; /* Number of bytes poisoned, or negated error */
};

The following value may be bitwise ORed in mode to change the behavior of the UFFDIO_POISON operation:

UFFDIO_POISON_MODE_DONTWAKE
Do not wake up the thread that waits for page-fault resolution.

The updated field is used by the kernel to return the number of bytes that were actually poisoned, or an
error in the same manner as UFFDIO_COPY. If the value returned in the updated field doesn't match the
value that was specified in range.len, the operation fails with the error EAGAIN. The updated field is
output-only; it is not read by the UFFDIO_POISON operation.

This ioctl(2) operation returns 0 on success. In this case, the entire area was poisoned. On error, -1
is returned and errno is set to indicate the error. Possible errors include:

EAGAIN The number of bytes mapped (i.e., the value returned in the updated 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.

EEXIST One or more pages were already mapped in the given range.

ENOENT The faulting process has changed its virtual memory layout simultaneously with an outstanding
UFFDIO_POISON operation.

ENOMEM Allocating memory for page table entries failed.

ESRCH The faulting process has exited at the time of a UFFDIO_POISON operation.

RETURN VALUE

       See descriptions of the individual operations, above.

ERRORS

       See descriptions of the individual operations, above.  In addition,  the  following  general  errors  can
       occur for all of the operations described above:

       EFAULT argp does not point to a valid memory address.

       EINVAL (For  all operations except UFFDIO_API.)  The userfaultfd object has not yet been enabled (via the
              UFFDIO_API operation).

STANDARDS

       Linux.

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).

NAME

LIBRARY

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

STANDARDS

BUGS

EXAMPLES

SEE ALSO