openat2(2) System Calls Manual openat2(2)
NAME
openat2 - open and possibly create a file (extended)
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <fcntl.h> /* Definition of O_* and S_* constants */
#include <linux/openat2.h> /* Definition of RESOLVE_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
long syscall(SYS_openat2, int dirfd, const char *pathname,
struct open_how *how, size_t size);
Note: glibc provides no wrapper for openat2(), necessitating the use of
syscall(2).
DESCRIPTION
The openat2() system call is an extension of openat(2) and provides a
superset of its functionality.
The openat2() system call opens the file specified by pathname. If the
specified file does not exist, it may optionally (if O_CREAT is speci-
fied in how.flags) be created.
As with openat(2), if pathname is a relative pathname, then it is in-
terpreted relative to the directory referred to by the file descriptor
dirfd (or the current working directory of the calling process, if
dirfd is the special value AT_FDCWD). If pathname is an absolute path-
name, then dirfd is ignored (unless how.resolve contains RE-
SOLVE_IN_ROOT, in which case pathname is resolved relative to dirfd).
Rather than taking a single flags argument, an extensible structure
(how) is passed to allow for future extensions. The size argument must
be specified as sizeof(struct open_how).
The open_how structure
The how argument specifies how pathname should be opened, and acts as a
superset of the flags and mode arguments to openat(2). This argument
is a pointer to an open_how structure, described in open_how(2type).
Any future extensions to openat2() will be implemented as new fields
appended to the open_how structure, with a zero value in a new field
resulting in the kernel behaving as though that extension field was not
present. Therefore, the caller must zero-fill this structure on ini-
tialization. (See the "Extensibility" section of the NOTES for more
detail on why this is necessary.)
The fields of the open_how structure are as follows:
flags This field specifies the file creation and file status flags to
use when opening the file. All of the O_* flags defined for
openat(2) are valid openat2() flag values.
Whereas openat(2) ignores unknown bits in its flags argument,
openat2() returns an error if unknown or conflicting flags are
specified in how.flags.
mode This field specifies the mode for the new file, with identical
semantics to the mode argument of openat(2).
Whereas openat(2) ignores bits other than those in the range
07777 in its mode argument, openat2() returns an error if
how.mode contains bits other than 07777. Similarly, an error is
returned if openat2() is called with a nonzero how.mode and
how.flags does not contain O_CREAT or O_TMPFILE.
resolve
This is a bit-mask of flags that modify the way in which all
components of pathname will be resolved. (See path_resolu-
tion(7) for background information.)
The primary use case for these flags is to allow trusted pro-
grams to restrict how untrusted paths (or paths inside untrusted
directories) are resolved. The full list of resolve flags is as
follows:
RESOLVE_BENEATH
Do not permit the path resolution to succeed if any com-
ponent of the resolution is not a descendant of the di-
rectory indicated by dirfd. This causes absolute sym-
bolic links (and absolute values of pathname) to be re-
jected.
Currently, this flag also disables magic-link resolution
(see below). However, this may change in the future.
Therefore, to ensure that magic links are not resolved,
the caller should explicitly specify RESOLVE_NO_MAGI-
CLINKS.
RESOLVE_IN_ROOT
Treat the directory referred to by dirfd as the root di-
rectory while resolving pathname. Absolute symbolic
links are interpreted relative to dirfd. If a prefix
component of pathname equates to dirfd, then an immedi-
ately following .. component likewise equates to dirfd
(just as /.. is traditionally equivalent to /). If path-
name is an absolute path, it is also interpreted relative
to dirfd.
The effect of this flag is as though the calling process
had used chroot(2) to (temporarily) modify its root di-
rectory (to the directory referred to by dirfd). How-
ever, unlike chroot(2) (which changes the filesystem root
permanently for a process), RESOLVE_IN_ROOT allows a pro-
gram to efficiently restrict path resolution on a per-
open basis.
Currently, this flag also disables magic-link resolution.
However, this may change in the future. Therefore, to
ensure that magic links are not resolved, the caller
should explicitly specify RESOLVE_NO_MAGICLINKS.
RESOLVE_NO_MAGICLINKS
Disallow all magic-link resolution during path resolu-
tion.
Magic links are symbolic link-like objects that are most
notably found in proc(5); examples include /proc/pid/exe
and /proc/pid/fd/*. (See symlink(7) for more details.)
Unknowingly opening magic links can be risky for some ap-
plications. Examples of such risks include the follow-
ing:
• If the process opening a pathname is a controlling
process that currently has no controlling terminal
(see credentials(7)), then opening a magic link inside
/proc/pid/fd that happens to refer to a terminal would
cause the process to acquire a controlling terminal.
• In a containerized environment, a magic link inside
/proc may refer to an object outside the container,
and thus may provide a means to escape from the con-
tainer.
Because of such risks, an application may prefer to dis-
able magic link resolution using the RESOLVE_NO_MAGI-
CLINKS flag.
If the trailing component (i.e., basename) of pathname is
a magic link, how.resolve contains RESOLVE_NO_MAGICLINKS,
and how.flags contains both O_PATH and O_NOFOLLOW, then
an O_PATH file descriptor referencing the magic link will
be returned.
RESOLVE_NO_SYMLINKS
Disallow resolution of symbolic links during path resolu-
tion. This option implies RESOLVE_NO_MAGICLINKS.
If the trailing component (i.e., basename) of pathname is
a symbolic link, how.resolve contains RESOLVE_NO_SYM-
LINKS, and how.flags contains both O_PATH and O_NOFOLLOW,
then an O_PATH file descriptor referencing the symbolic
link will be returned.
Note that the effect of the RESOLVE_NO_SYMLINKS flag,
which affects the treatment of symbolic links in all of
the components of pathname, differs from the effect of
the O_NOFOLLOW file creation flag (in how.flags), which
affects the handling of symbolic links only in the final
component of pathname.
Applications that employ the RESOLVE_NO_SYMLINKS flag are
encouraged to make its use configurable (unless it is
used for a specific security purpose), as symbolic links
are very widely used by end-users. Setting this flag in-
discriminately—i.e., for purposes not specifically re-
lated to security—for all uses of openat2() may result in
spurious errors on previously functional systems. This
may occur if, for example, a system pathname that is used
by an application is modified (e.g., in a new distribu-
tion release) so that a pathname component (now) contains
a symbolic link.
RESOLVE_NO_XDEV
Disallow traversal of mount points during path resolution
(including all bind mounts). Consequently, pathname must
either be on the same mount as the directory referred to
by dirfd, or on the same mount as the current working di-
rectory if dirfd is specified as AT_FDCWD.
Applications that employ the RESOLVE_NO_XDEV flag are en-
couraged to make its use configurable (unless it is used
for a specific security purpose), as bind mounts are
widely used by end-users. Setting this flag indiscrimi-
nately—i.e., for purposes not specifically related to se-
curity—for all uses of openat2() may result in spurious
errors on previously functional systems. This may occur
if, for example, a system pathname that is used by an ap-
plication is modified (e.g., in a new distribution re-
lease) so that a pathname component (now) contains a bind
mount.
RESOLVE_CACHED
Make the open operation fail unless all path components
are already present in the kernel's lookup cache. If any
kind of revalidation or I/O is needed to satisfy the
lookup, openat2() fails with the error EAGAIN. This is
useful in providing a fast-path open that can be per-
formed without resorting to thread offload, or other
mechanisms that an application might use to offload
slower operations.
If any bits other than those listed above are set in how.re-
solve, an error is returned.
RETURN VALUE
On success, a new file descriptor is returned. On error, -1 is re-
turned, and errno is set to indicate the error.
ERRORS
The set of errors returned by openat2() includes all of the errors re-
turned by openat(2), as well as the following additional errors:
E2BIG An extension that this kernel does not support was specified in
how. (See the "Extensibility" section of NOTES for more detail
on how extensions are handled.)
EAGAIN how.resolve contains either RESOLVE_IN_ROOT or RESOLVE_BENEATH,
and the kernel could not ensure that a ".." component didn't es-
cape (due to a race condition or potential attack). The caller
may choose to retry the openat2() call.
EAGAIN RESOLVE_CACHED was set, and the open operation cannot be per-
formed using only cached information. The caller should retry
without RESOLVE_CACHED set in how.resolve.
EINVAL An unknown flag or invalid value was specified in how.
EINVAL mode is nonzero, but how.flags does not contain O_CREAT or
O_TMPFILE.
EINVAL size was smaller than any known version of struct open_how.
ELOOP how.resolve contains RESOLVE_NO_SYMLINKS, and one of the path
components was a symbolic link (or magic link).
ELOOP how.resolve contains RESOLVE_NO_MAGICLINKS, and one of the path
components was a magic link.
EXDEV how.resolve contains either RESOLVE_IN_ROOT or RESOLVE_BENEATH,
and an escape from the root during path resolution was detected.
EXDEV how.resolve contains RESOLVE_NO_XDEV, and a path component
crosses a mount point.
STANDARDS
Linux.
HISTORY
Linux 5.6.
The semantics of RESOLVE_BENEATH were modeled after FreeBSD's O_BE-
NEATH.
NOTES
Extensibility
In order to allow for future extensibility, openat2() requires the
user-space application to specify the size of the open_how structure
that it is passing. By providing this information, it is possible for
openat2() to provide both forwards- and backwards-compatibility, with
size acting as an implicit version number. (Because new extension
fields will always be appended, the structure size will always in-
crease.) This extensibility design is very similar to other system
calls such as sched_setattr(2), perf_event_open(2), and clone3(2).
If we let usize be the size of the structure as specified by the user-
space application, and ksize be the size of the structure which the
kernel supports, then there are three cases to consider:
• If ksize equals usize, then there is no version mismatch and how can
be used verbatim.
• If ksize is larger than usize, then there are some extension fields
that the kernel supports which the user-space application is unaware
of. Because a zero value in any added extension field signifies a
no-op, the kernel treats all of the extension fields not provided by
the user-space application as having zero values. This provides
backwards-compatibility.
• If ksize is smaller than usize, then there are some extension fields
which the user-space application is aware of but which the kernel
does not support. Because any extension field must have its zero
values signify a no-op, the kernel can safely ignore the unsupported
extension fields if they are all-zero. If any unsupported extension
fields are nonzero, then -1 is returned and errno is set to E2BIG.
This provides forwards-compatibility.
Because the definition of struct open_how may change in the future
(with new fields being added when system headers are updated), user-
space applications should zero-fill struct open_how to ensure that re-
compiling the program with new headers will not result in spurious er-
rors at run time. The simplest way is to use a designated initializer:
struct open_how how = { .flags = O_RDWR,
.resolve = RESOLVE_IN_ROOT };
or explicitly using memset(3) or similar:
struct open_how how;
memset(&how, 0, sizeof(how));
how.flags = O_RDWR;
how.resolve = RESOLVE_IN_ROOT;
A user-space application that wishes to determine which extensions the
running kernel supports can do so by conducting a binary search on size
with a structure which has every byte nonzero (to find the largest
value which doesn't produce an error of E2BIG).
SEE ALSO
openat(2), open_how(2type), path_resolution(7), symlink(7)
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