memfd_create

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



NAME
       memfd_create - create an anonymous file

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

       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 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
       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 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 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
              reached.

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

VERSIONS
       The memfd_create() system call first appeared in Linux 3.17; glibc
       support was added in version 2.27.

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

NOTES
       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
       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_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)

       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

       #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)

       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
       This page is part of release 5.03 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                             2019-03-06                   MEMFD_CREATE(2)