openat2(2) - Linux manual page (original) (raw)
openat2(2) System Calls Manual openat2(2)
NAME top
openat2 - open and possibly create a file (extended)LIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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)](../man2/syscall.2.html).DESCRIPTION top
The **openat2**() system call is an extension of [openat(2)](../man2/openat.2.html) 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 specified in _how.flags_) be created.
As with [openat(2)](../man2/openat.2.html), if _pathname_ is a relative pathname, then it is
interpreted 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 pathname, then _dirfd_ is ignored (unless _how.resolve_
contains **RESOLVE_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 openhow)_.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 openhow structure, described in open_how(2type).
Any future extensions to **openat2**() will be implemented as new
fields appended to the _openhow_ 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 initialization. (See the "Extensibility"
section of the **NOTES** for more detail on why this is necessary.)
The fields of the _openhow_ 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)](../man2/openat.2.html) are valid **openat2**() flag values.
Whereas [openat(2)](../man2/openat.2.html) 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)](../man2/openat.2.html).
Whereas [openat(2)](../man2/openat.2.html) 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_resolution(7)](../man7/path%5Fresolution.7.html) for background information.)
The primary use case for these flags is to allow trusted
programs 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
component of the resolution is not a descendant of
the directory indicated by _dirfd_. This causes
absolute symbolic links (and absolute values of
_pathname_) to be rejected.
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_MAGICLINKS**.
**RESOLVE_IN_ROOT**
Treat the directory referred to by _dirfd_ as the root
directory while resolving _pathname_. Absolute
symbolic links are interpreted relative to _dirfd_.
If a prefix component of _pathname_ equates to _dirfd_,
then an immediately following _.._ component likewise
equates to _dirfd_ (just as _/.._ is traditionally
equivalent to _/_). If _pathname_ 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)](../man2/chroot.2.html) to (temporarily) modify
its root directory (to the directory referred to by
_dirfd_). However, unlike [chroot(2)](../man2/chroot.2.html) (which changes
the filesystem root permanently for a process),
**RESOLVE_IN_ROOT** allows a program 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
resolution.
Magic links are symbolic link-like objects that are
most notably found in [proc(5)](../man5/proc.5.html); examples include
_/proc/_pid_/exe_ and _/proc/_pid_/fd/*_. (See [symlink(7)](../man7/symlink.7.html)
for more details.)
Unknowingly opening magic links can be risky for
some applications. Examples of such risks include
the following:
• If the process opening a pathname is a
controlling process that currently has no
controlling terminal (see [credentials(7)](../man7/credentials.7.html)), 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 container.
Because of such risks, an application may prefer to
disable magic link resolution using the
**RESOLVE_NO_MAGICLINKS** 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
resolution. This option implies
**RESOLVE_NO_MAGICLINKS**.
If the trailing component (i.e., basename) of
_pathname_ is a symbolic link, _how.resolve_ contains
**RESOLVE_NO_SYMLINKS**, 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 indiscriminately—i.e., for
purposes not specifically related 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
distribution 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 directory if
_dirfd_ is specified as **AT_FDCWD**.
Applications that employ the **RESOLVE_NO_XDEV** flag
are encouraged 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 indiscriminately—i.e., for purposes not
specifically related 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 distribution
release) 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 performed 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.resolve_, an error is returned.RETURN VALUE top
On success, a new file descriptor is returned. On error, -1 is
returned, and _[errno](../man3/errno.3.html)_ is set to indicate the error.ERRORS top
The set of errors returned by **openat2**() includes all of the errors
returned by [openat(2)](../man2/openat.2.html), 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 escape (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
performed 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 openhow_.
**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 top
Linux.HISTORY top
Linux 5.6.
The semantics of **RESOLVE_BENEATH** were modeled after FreeBSD's
**O_BENEATH**.NOTES top
Extensibility In order to allow for future extensibility, openat2() requires the user-space application to specify the size of the openhow 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 increase.) 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](../man3/errno.3.html)_ is set to **E2BIG**. This provides forwards-
compatibility.
Because the definition of _struct openhow_ may change in the future
(with new fields being added when system headers are updated),
user-space applications should zero-fill _struct openhow_ to ensure
that recompiling the program with new headers will not result in
spurious errors 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)](../man3/memset.3.html) 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 top
[openat(2)](../man2/openat.2.html), [open_how(2type)](../man2/open%5Fhow.2type.html), [path_resolution(7)](../man7/path%5Fresolution.7.html), [symlink(7)](../man7/symlink.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 openat2(2)
Pages that refer to this page:io_uring_enter2(2), io_uring_enter(2), mount_setattr(2), open(2), open_how(2type), syscalls(2), io_uring_prep_openat2(3), io_uring_prep_openat2_direct(3), path_resolution(7), symlink(7)