memfd_create(2) — Linux manual page


memfd_create(2)            System Calls Manual           memfd_create(2)

NAME         top

       memfd_create - create an anonymous file

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

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

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

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():

              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.

              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

       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 Documentation/admin-guide/mm/hugetlbpage.rst for more
              information about hugetlbfs.  Specifying both MFD_HUGETLB
              and MFD_ALLOW_SEALING in flags is supported since Linux

       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 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 flags included unknown bits.

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

       EINVAL Both MFD_HUGETLB and MFD_ALLOW_SEALING were specified in

       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

       EPERM  The MFD_HUGETLB flag was specified, but the caller was not
              privileged (did not have the CAP_IPC_LOCK capability) and
              is not a member of the sysctl_hugetlb_shm_group group; see
              the description of /proc/sys/vm/sysctl_hugetlb_shm_group
              in proc(5).

STANDARDS         top


HISTORY         top

       Linux 3.17, glibc 2.27.

NOTES         top

       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

   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

       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.
            Otherwise, behavior similar to F_SEAL_WRITE can be achieved
            by using F_SEAL_FUTURE_WRITE, which will prevent future
            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 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).

EXAMPLES         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 the 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

           $ ./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_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 <err.h>
       #include <fcntl.h>
       #include <stdint.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>
       #include <sys/mman.h>
       #include <unistd.h>

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

           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)
               err(EXIT_FAILURE, "memfd_create");

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

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

           printf("PID: %jd; fd: %d; /proc/%jd/fd/%d\n",
                  (intmax_t) getpid(), fd, (intmax_t) 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)
                   err(EXIT_FAILURE, "fcntl");

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



   Program source: t_get_seals.c

       #define _GNU_SOURCE
       #include <err.h>
       #include <fcntl.h>
       #include <stdio.h>
       #include <stdlib.h>

       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)
               err(EXIT_FAILURE, "open");

           seals = fcntl(fd, F_GET_SEALS);
           if (seals == -1)
               err(EXIT_FAILURE, "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_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. */


SEE ALSO         top

       fcntl(2), ftruncate(2), memfd_secret(2), mmap(2), shmget(2),

Linux man-pages (unreleased)     (date)                  memfd_create(2)

Pages that refer to this page: fcntl(2)ioctl_userfaultfd(2)memfd_secret(2)mmap(2)shmget(2)syscalls(2)sd_bus_message_append_array(3)sd_notify(3)shm_open(3)proc(5)systemd.service(5)tmpfs(5)capabilities(7)shm_overview(7)