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memfd_create(2) System Calls Manual memfd_create(2)
NAME
memfd_create - create an anonymous file
LIBRARY
Standard C library (libc, -lc)
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 regu-
lar 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 anony-
mous 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 behav-
ior of memfd_create():
MFD_CLOEXEC
Set the close-on-exec (FD_CLOEXEC) flag on the new file descrip-
tor. 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, mean-
ing 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 Documenta-
tion/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 de-
scriptor is inherited by the child produced by fork(2) and refers to
the same file. The file descriptor is preserved across execve(2), un-
less 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 ter-
minating 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 priv-
ileged (did not have the CAP_IPC_LOCK capability) and is not a
member of the sysctl_hugetlb_shm_group group; see the descrip-
tion of /proc/sys/vm/sysctl_hugetlb_shm_group in proc(5).
STANDARDS
Linux.
HISTORY
Linux 3.17, glibc 2.27.
NOTES
The memfd_create() system call provides a simple alternative to manu-
ally 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-seal-
ing 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 re-
gion 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 vulner-
able 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 em-
ploys 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 us-
ing 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 modi-
fications on the file. (If placing the seal F_SEAL_WRITE, then it
will be necessary to first unmap the shared writable mapping cre-
ated 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 succeed-
ing 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:
o The process that called memfd_create() could transfer the re-
sulting 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).
o The second process is created via fork(2) and thus automati-
cally 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.)
o The second process opens the file /proc/pid/fd/fd, where <pid>
is the PID of the first process (the one that called memfd_cre-
ate()), 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 re-
trieve 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 ap-
plied).
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 op-
tionally 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 argu-
ment is the size to be set for the file, and the optional third argu-
ment 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 back-
ground. 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>
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);
}
SEE ALSO
fcntl(2), ftruncate(2), memfd_secret(2), mmap(2), shmget(2),
shm_open(3)
Linux man-pages 6.04 2023-04-03 memfd_create(2)
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