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

       memfd_create - create an anonymous file

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <sys/mman.h>

       int memfd_create(const char *name, unsigned int flags);

       memfd_create()  creates an anonymous file and returns a file descriptor
       that refers to it.  The file behaves like a regular file, and so can be
       modified, truncated, memory-mapped, and so on.  However, unlike a regu-
       lar file, it lives in RAM and has a volatile backing storage.  Once all
       references  to  the  file  are  dropped,  it is automatically released.
       Anonymous memory is used for all backing pages of the file.  Therefore,
       files created by memfd_create() have the same semantics as other anony-
       mous memory allocations such as those allocated using mmap(2) with  the
       MAP_ANONYMOUS flag.

       The initial size of the file is set to 0.  Following the call, the file
       size should be set using ftruncate(2).  (Alternatively, the file may be
       populated by calls to write(2) or similar.)

       The  name  supplied in name is used as a filename and will be displayed
       as the target of the  corresponding  symbolic  link  in  the  directory
       /proc/self/fd/.   The displayed name is always prefixed with memfd: and
       serves only for debugging purposes.  Names do not affect  the  behavior
       of  the  file  descriptor, and as such multiple files can have the same
       name without any side effects.

       The following values may be bitwise ORed in flags to change the  behav-
       ior of memfd_create():

              Set the close-on-exec (FD_CLOEXEC) flag on the new file descrip-
              tor.  See the description of the O_CLOEXEC flag in  open(2)  for
              reasons why this may be useful.

              Allow  sealing  operations  on this file.  See the discussion of
              the F_ADD_SEALS and F_GET_SEALS operations in fcntl(2), and also
              NOTES,  below.  The initial set of seals is empty.  If this flag
              is not set, the initial set of seals will be F_SEAL_SEAL,  mean-
              ing that no other seals can be set on the file.

       MFD_HUGETLB (since Linux 4.14)
              The  anonymous  file will be created in the hugetlbfs filesystem
              using huge pages.  See the Linux kernel source  file  Documenta-
              tion/admin-guide/mm/hugetlbpage.rst  for  more information about
              hugetlbfs.  Specifying both MFD_HUGETLB and MFD_ALLOW_SEALING in
              flags is supported since Linux 4.16.

       MFD_HUGE_2MB, MFD_HUGE_1GB, ...
              Used  in  conjunction  with  MFD_HUGETLB  to  select alternative
              hugetlb page sizes (respectively, 2 MB, 1 GB, ...)   on  systems
              that support multiple hugetlb page sizes.  Definitions for known
              huge page sizes are included in the header file <linux/memfd.h>.

              For details on encoding huge page  sizes  not  included  in  the
              header file, see the discussion of the similarly named constants
              in mmap(2).

       Unused bits in flags must be 0.

       As its return value, memfd_create() returns a new file descriptor  that
       can  be  used to refer to the file.  This file descriptor is opened for
       both reading and writing (O_RDWR) and O_LARGEFILE is set for  the  file

       With  respect  to  fork(2) and execve(2), the usual semantics apply for
       the file descriptor created by memfd_create().  A copy of the file  de-
       scriptor  is  inherited  by the child produced by fork(2) and refers to
       the same file.  The file descriptor is preserved across execve(2),  un-
       less the close-on-exec flag has been set.

       On success, memfd_create() returns a new file descriptor.  On error, -1
       is returned and errno is set to indicate the error.

       EFAULT The address in name points to invalid memory.

       EINVAL flags included unknown bits.

       EINVAL name was too long.  (The limit is 249 bytes, excluding the  ter-
              minating null byte.)

       EINVAL Both MFD_HUGETLB and MFD_ALLOW_SEALING were specified in flags.

       EMFILE The per-process limit on the number of open file descriptors has
              been reached.

       ENFILE The system-wide limit on the total number of open files has been

       ENOMEM There was insufficient memory to create a new anonymous file.

       The memfd_create() system call first appeared in Linux 3.17; glibc sup-
       port was added in version 2.27.

       The memfd_create() system call is Linux-specific.

       The memfd_create() system call provides a simple alternative  to  manu-
       ally  mounting a tmpfs(5) filesystem and creating and opening a file in
       that filesystem.  The primary purpose of memfd_create()  is  to  create
       files and associated file descriptors that are used with the file-seal-
       ing APIs provided by fcntl(2).

       The memfd_create() system call  also  has  uses  without  file  sealing
       (which  is  why  file-sealing  is disabled, unless explicitly requested
       with the MFD_ALLOW_SEALING flag).  In particular, it can be used as  an
       alternative  to creating files in tmp or as an alternative to using the
       open(2) O_TMPFILE in cases where there is no intention to actually link
       the resulting file into the filesystem.

   File sealing
       In  the  absence of file sealing, processes that communicate via shared
       memory must either trust each other, or take measures to deal with  the
       possibility that an untrusted peer may manipulate the shared memory re-
       gion in problematic ways.  For example, an untrusted peer might  modify
       the  contents  of  the  shared memory at any time, or shrink the shared
       memory region.  The former possibility leaves the local process vulner-
       able  to  time-of-check-to-time-of-use race conditions (typically dealt
       with by copying data from the shared memory region before checking  and
       using  it).  The latter possibility leaves the local process vulnerable
       to SIGBUS signals when an attempt is made to access  a  now-nonexistent
       location  in  the shared memory region.  (Dealing with this possibility
       necessitates the use of a handler for the SIGBUS signal.)

       Dealing with untrusted peers imposes extra complexity on code that  em-
       ploys  shared  memory.  Memory sealing enables that extra complexity to
       be eliminated, by allowing a process to operate secure in the knowledge
       that its peer can't modify the shared memory in an undesired fashion.

       An example of the usage of the sealing mechanism is as follows:

       1. The first process creates a tmpfs(5) file using memfd_create().  The
          call yields a file descriptor used in subsequent steps.

       2. The first process sizes the file created in the previous step  using
          ftruncate(2), maps it using mmap(2), and populates the shared memory
          with the desired data.

       3. The first process uses the fcntl(2) F_ADD_SEALS operation  to  place
          one  or more seals on the file, in order to restrict further modifi-
          cations on the file.  (If placing the  seal  F_SEAL_WRITE,  then  it
          will be necessary to first unmap the shared writable mapping created
          in the previous step.  Otherwise, behavior similar  to  F_SEAL_WRITE
          can be achieved by using F_SEAL_FUTURE_WRITE, which will prevent fu-
          ture writes via mmap(2) and write(2) from succeeding  while  keeping
          existing shared writable mappings).

       4. A second process obtains a file descriptor for the tmpfs(5) file and
          maps it.  Among the possible ways in which this could happen are the

          *  The process that called memfd_create() could transfer the result-
             ing file descriptor to the  second  process  via  a  UNIX  domain
             socket  (see  unix(7) and cmsg(3)).  The second process then maps
             the file using mmap(2).

          *  The second process is created via fork(2) and thus  automatically
             inherits  the  file  descriptor  and mapping.  (Note that in this
             case and the next, there is a natural trust relationship  between
             the two processes, since they are running under the same user ID.
             Therefore, file sealing would not normally be necessary.)

          *  The second process  opens  the  file  /proc/<pid>/fd/<fd>,  where
             <pid>  is  the  PID  of  the  first  process (the one that called
             memfd_create()), and <fd> is the number of  the  file  descriptor
             returned by the call to memfd_create() in that process.  The sec-
             ond process then maps the file using mmap(2).

       5. The second process uses the fcntl(2) F_GET_SEALS  operation  to  re-
          trieve  the  bit  mask  of  seals that has been applied to the file.
          This bit mask can be inspected in order to determine what  kinds  of
          restrictions  have  been  placed on file modifications.  If desired,
          the second process can apply further seals to impose additional  re-
          strictions  (so  long  as  the F_SEAL_SEAL seal has not yet been ap-

       Below are shown two  example  programs  that  demonstrate  the  use  of
       memfd_create() and the file sealing API.

       The  first  program,  t_memfd_create.c,  creates  a tmpfs(5) file using
       memfd_create(), sets a size for the file, maps it into memory, and  op-
       tionally  places  some  seals  on  the file.  The program accepts up to
       three command-line arguments, of which the first two are required.  The
       first argument is the name to associate with the file, the second argu-
       ment is the size to be set for the file, and the optional  third  argu-
       ment is a string of characters that specify seals to be set on file.

       The second program, t_get_seals.c, can be used to open an existing file
       that was created via memfd_create() and inspect the set of  seals  that
       have been applied to that file.

       The  following  shell  session  demonstrates the use of these programs.
       First we create a tmpfs(5) file and set some seals on it:

           $ ./t_memfd_create my_memfd_file 4096 sw &
           [1] 11775
           PID: 11775; fd: 3; /proc/11775/fd/3

       At this point, the t_memfd_create program continues to run in the back-
       ground.   From another program, we can obtain a file descriptor for the
       file created by memfd_create() by opening the /proc/[pid]/fd file  that
       corresponds  to  the  file  descriptor opened by memfd_create().  Using
       that pathname, we inspect the content of  the  /proc/[pid]/fd  symbolic
       link,  and use our t_get_seals program to view the seals that have been
       placed on the file:

           $ readlink /proc/11775/fd/3
           /memfd:my_memfd_file (deleted)
           $ ./t_get_seals /proc/11775/fd/3
           Existing seals: WRITE SHRINK

   Program source: t_memfd_create.c

       #define _GNU_SOURCE
       #include <sys/mman.h>
       #include <fcntl.h>
       #include <stdlib.h>
       #include <unistd.h>
       #include <string.h>
       #include <stdio.h>

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       main(int argc, char *argv[])
           int fd;
           unsigned int seals;
           char *addr;
           char *name, *seals_arg;
           ssize_t len;

           if (argc < 3) {
               fprintf(stderr, "%s name size [seals]\n", argv[0]);
               fprintf(stderr, "\t'seals' can contain any of the "
                       "following characters:\n");
               fprintf(stderr, "\t\tg - F_SEAL_GROW\n");
               fprintf(stderr, "\t\ts - F_SEAL_SHRINK\n");
               fprintf(stderr, "\t\tw - F_SEAL_WRITE\n");
               fprintf(stderr, "\t\tW - F_SEAL_FUTURE_WRITE\n");
               fprintf(stderr, "\t\tS - F_SEAL_SEAL\n");

           name = argv[1];
           len = atoi(argv[2]);
           seals_arg = argv[3];

           /* Create an anonymous file in tmpfs; allow seals to be
              placed on the file */

           fd = memfd_create(name, MFD_ALLOW_SEALING);
           if (fd == -1)

           /* Size the file as specified on the command line */

           if (ftruncate(fd, len) == -1)

           printf("PID: %ld; fd: %d; /proc/%ld/fd/%d\n",
                   (long) getpid(), fd, (long) getpid(), fd);

           /* Code to map the file and populate the mapping with data
              omitted */

           /* If a 'seals' command-line argument was supplied, set some
              seals on the file */

           if (seals_arg != NULL) {
               seals = 0;

               if (strchr(seals_arg, 'g') != NULL)
                   seals |= F_SEAL_GROW;
               if (strchr(seals_arg, 's') != NULL)
                   seals |= F_SEAL_SHRINK;
               if (strchr(seals_arg, 'w') != NULL)
                   seals |= F_SEAL_WRITE;
               if (strchr(seals_arg, 'W') != NULL)
                   seals |= F_SEAL_FUTURE_WRITE;
               if (strchr(seals_arg, 'S') != NULL)
                   seals |= F_SEAL_SEAL;

               if (fcntl(fd, F_ADD_SEALS, seals) == -1)

           /* Keep running, so that the file created by memfd_create()
              continues to exist */



   Program source: t_get_seals.c

       #define _GNU_SOURCE
       #include <sys/mman.h>
       #include <fcntl.h>
       #include <unistd.h>
       #include <stdlib.h>
       #include <string.h>
       #include <stdio.h>

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       main(int argc, char *argv[])
           int fd;
           unsigned int seals;

           if (argc != 2) {
               fprintf(stderr, "%s /proc/PID/fd/FD\n", argv[0]);


           fd = open(argv[1], O_RDWR);
           if (fd == -1)

           seals = fcntl(fd, F_GET_SEALS);
           if (seals == -1)

           printf("Existing seals:");
           if (seals & F_SEAL_SEAL)
               printf(" SEAL");
           if (seals & F_SEAL_GROW)
               printf(" GROW");
           if (seals & F_SEAL_WRITE)
               printf(" WRITE");
           if (seals & F_SEAL_FUTURE_WRITE)
               printf(" FUTURE_WRITE");
           if (seals & F_SEAL_SHRINK)
               printf(" SHRINK");

           /* Code to map the file and access the contents of the
              resulting mapping omitted */


       fcntl(2), ftruncate(2), mmap(2), shmget(2), shm_open(3)

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Linux                             2020-02-09                   MEMFD_CREATE(2)
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