memfd_create

       #include <sys/memfd.h>

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

DESCRIPTION
       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():

       MFD_CLOEXEC
              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.

       MFD_ALLOW_SEALING
              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.

       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
       descriptor.

       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
       descriptor is inherited by the child produced by fork(2) and refers  to
       the  same  file.   The  file  descriptor is preserved across execve(2),
       unless the close-on-exec flag has been set.


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

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

VERSIONS
       The memfd_create() system call first appeared in Linux  3.17.   Support
       in the GNU C library is pending.

CONFORMING TO
       The memfd_create() system call is Linux-specific.

NOTES
       The  memfd_create()  system call provides a simple alternative to manu-
       ally mounting a tmpfs 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
       region  in problematic ways.  For example, an untrusted peer might mod-
       ify 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
       employs 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 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.
             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/<pd>/fd/<fd>, where <pid>
             is  the  PID of the first process (the one that called memfd_cre-
             ate()), and <fd> is the number of the file descriptor returned by
             the  call  to memfd_create() in that process.  The second process
             then maps the file using mmap(2).

       5. The second  process  uses  the  fcntl(2)  F_GET_SEALS  operation  to
          retrieve  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
          restrictions  (so  long  as  the  F_SEAL_SEAL  seal has not yet been
          applied).

EXAMPLE
       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  file  using
       memfd_create(), sets a size for the file,  maps  it  into  memory,  and
       optionally  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  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 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  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:
       #include <string.h>
       #include <stdio.h>

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

       int
       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\tS - F_SEAL_SEAL\n");
               exit(EXIT_FAILURE);
           }

           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)
               errExit("memfd_create");

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

           if (ftruncate(fd, len) == -1)
               errExit("truncate");

           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;


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

           pause();

           exit(EXIT_SUCCESS);
       }

   Program source: t_get_seals.c

       #include <sys/memfd.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)

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

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

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

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

           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_SHRINK)
               printf(" SHRINK");
           printf("\n");

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

Linux                             2015-12-28                   MEMFD_CREATE(2)
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