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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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memfd_create(2) System Calls Manual memfd_create(2)
memfd_create - create an anonymous file
Standard C library (libc, -lc)
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sys/mman.h>
int memfd_create(const char *name, unsigned int flags);
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.
On success, memfd_create() returns a new file descriptor. On
error, -1 is returned and errno is set to indicate the error.
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.
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).
Linux.
Linux 3.17, glibc 2.27.
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).
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
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 <err.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <unistd.h>
int
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");
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)
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. */
pause();
exit(EXIT_SUCCESS);
}
Program source: t_get_seals.c
#define _GNU_SOURCE
#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
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)
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");
printf("\n");
/* Code to map the file and access the contents of the
resulting mapping omitted. */
exit(EXIT_SUCCESS);
}
fcntl(2), ftruncate(2), memfd_secret(2), mmap(2), shmget(2),
shm_open(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.9.1 2024-06-15 memfd_create(2)
Pages that refer to this page: fcntl(2), memfd_secret(2), mmap(2), shmget(2), syscalls(2), UFFDIO_API(2const), sd_bus_message_append_array(3), sd_notify(3), shm_open(3), proc_sys_vm(5), systemd.service(5), tmpfs(5), capabilities(7), shm_overview(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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