noble (2) ioctl_userfaultfd.2.gz

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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).

SEE ALSO

       ioctl(2), mmap(2), userfaultfd(2)

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