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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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openat2(2) System Calls Manual openat2(2)
openat2 - open and possibly create a file (extended)
Standard C library (libc, -lc)
#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).
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 specified in how.flags) be created.
As with openat(2), 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 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 initialization. (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_resolution(7) 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) to (temporarily) modify
its root directory (to the directory referred to by
dirfd). However, unlike chroot(2) (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); examples include
/proc/pid/exe and /proc/pid/fd/*. (See symlink(7)
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)), 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.
On success, a new file descriptor is returned. On error, -1 is
returned, and errno is set to indicate the error.
The set of errors returned by openat2() includes all of the
errors returned 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 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
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.
Linux.
Linux 5.6.
The semantics of RESOLVE_BENEATH were modeled after FreeBSD's
O_BENEATH.
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 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 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 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) 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).
openat(2), open_how(2type), path_resolution(7), symlink(7)
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user-space interface documentation) project. Information about
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This page was obtained from the tarball man-pages-6.9.1.tar.gz
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2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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Linux man-pages 6.9.1 2024-05-02 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)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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