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



NAME
       memfd_create - create an anonymous file

SYNOPSIS
       #include <sys/memfd.h>

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

       Note: There is no glibc wrapper for this system call; see NOTES.

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 file
       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.

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

ERRORS
       EFAULT The address in name points to invalid memory.

       EINVAL An  unsupported  value  was  specified  in one of the arguments:
	      flags included unknown bits, or name was too long.

       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
	      reached.

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

VERSIONS
       The memfd_create() system call first appeared in Linux 3.17.

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

NOTES
       Glibc does not provide a wrapper for this system call;  call  it	 using
       syscall(2).

       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
       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(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.)

       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
	  following:

	  *  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
	  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(5) 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	 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

       #include <sys/memfd.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)

       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;

	       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, 'S') != NULL)
		   seals |= F_SEAL_SEAL;

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

	   /* 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 */

	   exit(EXIT_SUCCESS);
       }

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

COLOPHON
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       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				  2016-10-08		       MEMFD_CREATE(2)