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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON |
MEMFD_CREATE(2) Linux Programmer's Manual MEMFD_CREATE(2)
memfd_create - create an anonymous file
#include <sys/memfd.h>
int memfd_create(const char *name, unsigned int flags);
Note: There is no glibc wrapper for this system call; see NOTES.
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.
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 An unsupported value was specified in one of the arguments:
flags included unknown bits, or name was too long.
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.
The memfd_create() system call first appeared in Linux 3.17.
The memfd_create() system call is Linux-specific.
Glibc does not provide a wrapper for this system call; call it using
syscall(2).
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).
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
#include <sys/memfd.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
#include <sys/memfd.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);
}
fcntl(2), ftruncate(2), mmap(2), shmget(2), shm_open(3)
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latest version of this page, can be found at
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Linux 2016-10-08 MEMFD_CREATE(2)
Pages that refer to this page: fcntl(2), ioctl_userfaultfd(2), mmap(2), shmget(2), syscalls(2), sd_bus_message_append_array(3), sd_notify(3), shm_open(3), systemd.service(5), tmpfs(5)