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.

       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).

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.  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
       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 <stdint.h>
       #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\tW - F_SEAL_FUTURE_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: %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)
                   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_FUTURE_WRITE)
               printf(" FUTURE_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 5.13 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                              2021-03-22                    MEMFD_CREATE(2)