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NAME
memfd_create - create an anonymous file
SYNOPSIS
#include <sys/memfd.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/vm/hugetlbpage.txt for more
information about hugetlbfs. The hugetlbfs filesystem does not support file-
sealing operations. Therefore, specifying both MFD_HUGETLB and MFD_ALLOW_SEALING
in flags is disallowed.
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
<sys/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
#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);
}
SEE ALSO
fcntl(2), ftruncate(2), mmap(2), shmget(2), shm_open(3)
COLOPHON
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