ioctl_userfaultfd

IOCTL_USERFAULTFD(2)        Linux Programmer's Manual       IOCTL_USERFAULTFD(2)



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

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

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

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, 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.

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).

CONFORMING TO
       These ioctl(2) operations are Linux-specific.

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), mmap(2), userfaultfd(2)

       Documentation/admin-guide/mm/userfaultfd.rst in the Linux kernel source
       tree

COLOPHON
       This page is part of release 5.13 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                              2021-03-22               IOCTL_USERFAULTFD(2)