Ubuntu Manpage: openat2 - open and possibly create a file (extended)

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 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.

RETURN VALUE

       On success, a new file descriptor is returned.  On error, -1 is returned, and errno is set
       to indicate the error.

ERRORS

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.

STANDARDS

       Linux.

HISTORY

       Linux 5.6.

       The semantics of RESOLVE_BENEATH were modeled after FreeBSD's O_BENEATH.

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  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).