đŸ’Ÿ Archived View for gmi.noulin.net â€ș man â€ș man2 â€ș ioctl_userfaultfd.2.gmi captured on 2023-12-28 at 16:02:52. Gemini links have been rewritten to link to archived content

View Raw

More Information

âŹ…ïž Previous capture (2022-06-12)

-=-=-=-=-=-=-

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  behav‐
       ior.  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 op‐
       erations 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: Sys‐
              tem 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 user‐
              faultfd.

       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 speci‐
       fied 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 des‐
       tination 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 UFF‐
       DIO_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.   Pos‐
       sible 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 re‐
       turned 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.  Pos‐
       sible 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_ZE‐
       ROPAGE_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 explic‐
       itly 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  UFF‐
              DIO_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  UFF‐
       DIO_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

Linux                                                                          2021-03-22                                                           IOCTL_USERFAULTFD(2)