memfd_create(2) - Linux manual page (original) (raw)
memfdcreate(2) System Calls Manual memfdcreate(2)
NAME top
memfd_create - create an anonymous fileLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#define _GNU_SOURCE** /* See feature_test_macros(7) */
**#include <sys/mman.h>**
**int memfd_create(const char ***_name_**, unsigned int** _flags_**);**DESCRIPTION top
**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)](../man2/mmap.2.html) 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)](../man2/ftruncate.2.html). (Alternatively, the
file may be populated by calls to [write(2)](../man2/write.2.html) 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)](../man2/open.2.html) 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)](../man2/fcntl.2.html),
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)](../man2/mmap.2.html).
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)](../man2/fork.2.html) and [execve(2)](../man2/execve.2.html), 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)](../man2/fork.2.html) and
refers to the same file. The file descriptor is preserved across
[execve(2)](../man2/execve.2.html), unless the close-on-exec flag has been set.RETURN VALUE top
On success, **memfd_create**() returns a new file descriptor. On
error, -1 is returned and _[errno](../man3/errno.3.html)_ is set to indicate the error.ERRORS top
**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 _sysctlhugetlbshmgroup_ group; see
the description of _/proc/sys/vm/sysctlhugetlbshmgroup_ in
[proc(5)](../man5/proc.5.html).STANDARDS top
Linux.HISTORY top
Linux 3.17, glibc 2.27.NOTES top
The **memfd_create**() system call provides a simple alternative to
manually mounting a [tmpfs(5)](../man5/tmpfs.5.html) 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)](../man2/fcntl.2.html).
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)](../man2/open.2.html) **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)](../man5/tmpfs.5.html) 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)](../man2/ftruncate.2.html), maps it using [mmap(2)](../man2/mmap.2.html), and populates the
shared memory with the desired data.
(3) The first process uses the [fcntl(2)](../man2/fcntl.2.html) **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)](../man2/mmap.2.html) and [write(2)](../man2/write.2.html) from succeeding while keeping
existing shared writable mappings).
(4) A second process obtains a file descriptor for the [tmpfs(5)](../man5/tmpfs.5.html)
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)](../man7/unix.7.html) and [cmsg(3)](../man3/cmsg.3.html)). The second
process then maps the file using [mmap(2)](../man2/mmap.2.html).
• The second process is created via [fork(2)](../man2/fork.2.html) 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)](../man2/mmap.2.html).
(5) The second process uses the [fcntl(2)](../man2/fcntl.2.html) **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 top
Below are shown two example programs that demonstrate the use of
**memfd_create**() and the file sealing API.
The first program, _tmemfdcreate.c_, creates a [tmpfs(5)](../man5/tmpfs.5.html) 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, _tgetseals.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)](../man5/tmpfs.5.html) 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 _tmemfdcreate_ 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 _tgetseals_ 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 SHRINKProgram 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 size;
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];
size = 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, size) == -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 top
[fcntl(2)](../man2/fcntl.2.html), [ftruncate(2)](../man2/ftruncate.2.html), [memfd_secret(2)](../man2/memfd%5Fsecret.2.html), [mmap(2)](../man2/mmap.2.html), [shmget(2)](../man2/shmget.2.html),
[shm_open(3)](../man3/shm%5Fopen.3.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-11-17 memfdcreate(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)