Manpages - openat2.2

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 a structure of the following form:

  struct open_how {
      u64 flags;    /* O_* flags */
      u64 mode;     /* Mode for O_{CREAT,TMPFILE} */
      u64 resolve;  /* RESOLVE_* flags */
      /* ... */
  };

Any future extensions to openat2*() will be implemented as new fields appended to the above 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.

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 runtime. 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).