NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

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

NAME         top

       memfd_create - create an anonymous file

SYNOPSIS         top

       #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         top

       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 regular 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 anonymous 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
       behavior of memfd_create():

       MFD_CLOEXEC
              Set the close-on-exec (FD_CLOEXEC) flag on the new file
              descriptor.  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, meaning 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         top

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

ERRORS         top

       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         top

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

CONFORMING TO         top

       The memfd_create() system call is Linux-specific.

NOTES         top

       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
       manually 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-sealing 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 modify the contents of the shared memory at any time, or shrink
       the shared memory region.  The former possibility leaves the local
       process vulnerable 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
          modifications 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
             resulting 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
             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         top

       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
       argument is the size to be set for the file, and the optional third
       argument 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
       background.  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_cre‐
       ate().  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         top

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

COLOPHON         top

       This page is part of release 4.13 of the Linux man-pages project.  A
       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                            2017-09-15                  MEMFD_CREATE(2)

Pages that refer to this page: fcntl(2)ioctl_userfaultfd(2)mmap(2)shmget(2)syscalls(2)sd_bus_message_append_array(3)sd_notify(3)shm_open(3)systemd.service(5)tmpfs(5)