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Subject[PATCH man-pages 1/2] openat2.2: document new openat2(2) syscall
Date
Rather than trying to merge the new syscall documentation into open.2
(which would probably result in the man-page being incomprehensible),
instead the new syscall gets its own dedicated page with links between
open(2) and openat2(2) to avoid duplicating information such as the list
of O_* flags or common errors.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
man2/open.2 | 17 ++
man2/openat2.2 | 435 +++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 452 insertions(+)
create mode 100644 man2/openat2.2

diff --git a/man2/open.2 b/man2/open.2
index b0f485b41589..2a721c991a20 100644
--- a/man2/open.2
+++ b/man2/open.2
@@ -65,6 +65,10 @@ open, openat, creat \- open and possibly create a file
.BI "int openat(int " dirfd ", const char *" pathname ", int " flags );
.BI "int openat(int " dirfd ", const char *" pathname ", int " flags \
", mode_t " mode );
+.PP
+/* Documented separately, in \fBopenat2\fP(2). */
+.BI "int openat2(int " dirfd ", const char *" pathname ", \
+const struct open_how *" how ", size_t " size ");
.fi
.PP
.in -4n
@@ -933,6 +937,15 @@ If
is absolute, then
.I dirfd
is ignored.
+.SS openat2(2)
+The
+.BR openat2 (2)
+system call is an extension of
+.BR openat (),
+with a superset of features. To avoid making this man page too long, the
+description of
+.BR openat2 (2)
+and its features is documented in a separate man page.
.SH RETURN VALUE
.BR open (),
.BR openat (),
@@ -1220,6 +1233,9 @@ SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.
.BR openat ():
POSIX.1-2008.
.PP
+.BR openat2 (2)
+is Linux-specific.
+.PP
The
.BR O_DIRECT ,
.BR O_NOATIME ,
@@ -1778,6 +1794,7 @@ is ignored).
.BR mknod (2),
.BR mmap (2),
.BR mount (2),
+.BR openat2 (2),
.BR open_by_handle_at (2),
.BR read (2),
.BR socket (2),
diff --git a/man2/openat2.2 b/man2/openat2.2
new file mode 100644
index 000000000000..8588f0018657
--- /dev/null
+++ b/man2/openat2.2
@@ -0,0 +1,435 @@
+.\" Copyright (C) 2019 Aleksa Sarai <cyphar@cyphar.com>
+.\"
+.\" %%%LICENSE_START(VERBATIM)
+.\" Permission is granted to make and distribute verbatim copies of this
+.\" manual provided the copyright notice and this permission notice are
+.\" preserved on all copies.
+.\"
+.\" Permission is granted to copy and distribute modified versions of this
+.\" manual under the conditions for verbatim copying, provided that the
+.\" entire resulting derived work is distributed under the terms of a
+.\" permission notice identical to this one.
+.\"
+.\" Since the Linux kernel and libraries are constantly changing, this
+.\" manual page may be incorrect or out-of-date. The author(s) assume no
+.\" responsibility for errors or omissions, or for damages resulting from
+.\" the use of the information contained herein. The author(s) may not
+.\" have taken the same level of care in the production of this manual,
+.\" which is licensed free of charge, as they might when working
+.\" professionally.
+.\"
+.\" Formatted or processed versions of this manual, if unaccompanied by
+.\" the source, must acknowledge the copyright and authors of this work.
+.\" %%%LICENSE_END
+.TH OPENAT2 2 2019-11-05 "Linux" "Linux Programmer's Manual"
+.SH NAME
+openat2 \- open and possibly create a file (extended)
+.SH SYNOPSIS
+.nf
+.B #include <sys/types.h>
+.B #include <sys/stat.h>
+.B #include <fcntl.h>
+.PP
+.BI "int openat2(int " dirfd ", const char *" pathname ", \
+struct open_how *" how ", size_t " size ");
+.fi
+.PP
+.IR Note :
+There is no glibc wrapper for this system call; see NOTES.
+.SH DESCRIPTION
+The
+.BR openat2 ()
+system call opens the file specified by
+.IR pathname .
+If the specified file does not exist, it may optionally (if
+.B O_CREAT
+is specified in
+.IR how.flags )
+be created by
+.BR openat2() .
+.PP
+As with
+.BR openat (2),
+if
+.I pathname
+is relative, then it is interpreted relative to the
+directory referred to by the file descriptor
+.I dirfd
+(or the current working directory of the calling process, if
+.I dirfd
+is the special value
+.BR AT_FDCWD .)
+If
+.I pathname
+is absolute, then
+.I dirfd
+is ignored (unless
+.I how.resolve
+contains
+.BR RESOLVE_IN_ROOT,
+in which case
+.I pathname
+is resolved relative to
+.IR dirfd .)
+.PP
+The
+.BR openat2 ()
+system call is an extension of
+.BR openat (2)
+and provides a superset of its functionality.
+Rather than taking a single
+.I flag
+argument, an extensible structure (\fIhow\fP) is passed instead to allow for
+future extensions.
+.I size
+must be set to
+.IR "sizeof(struct open_how)" ,
+to facilitate future extensions (see the "Extensibility" section of the
+.B NOTES
+for more detail on how extensions are handled.)
+
+.SS The open_how structure
+The following structure indicates how
+.I pathname
+should be opened, and acts as a superset of the
+.IR flag " and " mode
+arguments to
+.BR openat (2).
+.PP
+.in +4n
+.EX
+struct open_how {
+ __aligned_u64 flags; /* O_* flags. */
+ __u16 mode; /* Mode for O_{CREAT,TMPFILE}. */
+ __u16 __padding[3]; /* Must be zeroed. */
+ __aligned_u64 resolve; /* RESOLVE_* flags. */
+};
+.EE
+.in
+.PP
+Any future extensions to
+.BR openat2 ()
+will be implemented as new fields appended to the above structure (or through
+reuse of pre-existing padding space), with the zero value of the new fields
+acting as though the extension were not present.
+.PP
+The meaning of each field is as follows:
+.RS
+
+.I flags
+.RS
+The file creation and status flags to use for this operation.
+All of the
+.B O_*
+flags defined for
+.BR openat (2)
+are valid
+.BR openat2 ()
+flag values.
+
+Unlike
+.BR openat (2),
+it is an error to provide
+.BR openat2 ()
+unknown or conflicting flags in
+.IR flags .
+.RE
+
+.IR mode
+.RS
+File mode for the new file, with identical semantics to the
+.I mode
+argument to
+.BR openat (2).
+However, unlike
+.BR openat (2),
+it is an error to provide
+.BR openat2 ()
+with a
+.I mode
+which contains bits other than
+.IR 0777 .
+
+It is an error to provide
+.BR openat2 ()
+a non-zero
+.IR mode " if " flags
+does not contain
+.BR O_CREAT " or " O_TMPFILE .
+.RE
+
+.I resolve
+.RS
+Change how the components of
+.I pathname
+will be resolved (see
+.BR 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
+.I resolve
+flags is given below.
+.TP
+.B RESOLVE_NO_XDEV
+Disallow traversal of mount points during path resolution (including all bind
+mounts).
+
+Users of this flag are encouraged to make its use configurable (unless it is
+used for a specific security purpose), as bind mounts are very widely used by
+end-users.
+Setting this flag indiscrimnately for all uses of
+.IR openat2 ()
+may result in spurious errors on previously-functional systems.
+.TP
+.B RESOLVE_NO_SYMLINKS
+Disallow resolution of symbolic links during path resolution.
+This option implies
+.BR RESOLVE_NO_MAGICLINKS .
+
+If the trailing component is a symbolic link, and
+.I flags
+contains both
+.BR O_PATH " and " O_NOFOLLOW ","
+then an
+.B O_PATH
+file descriptor referencing the symbolic link will be returned.
+
+Users of this 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 indiscrimnately for all uses of
+.IR openat2 ()
+may result in spurious errors on previously-functional systems.
+.TP
+.B RESOLVE_NO_MAGICLINKS
+Disallow all magic link resolution during path resolution.
+
+If the trailing component is a magic link, and
+.I flags
+contains both
+.BR O_PATH " and " O_NOFOLLOW ","
+then an
+.B O_PATH
+file descriptor referencing the magic link will be returned.
+
+Magic-links are symbolic link-like objects that are most notably found in
+.BR proc (5)
+(examples include
+.IR /proc/[pid]/exe " and " /proc/[pid]/fd/* .)
+Due to the potential danger of unknowingly opening these magic links, it may be
+preferable for users to disable their resolution entirely (see
+.BR symbolic link (7)
+for more details.)
+.TP
+.B 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
+.IR dirfd .
+This results in absolute symbolic links (and absolute values of
+.IR pathname )
+to be rejected.
+
+Currently, this flag also disables magic link resolution.
+However, this may change in the future.
+The caller should explicitly specify
+.B RESOLVE_NO_MAGICLINKS
+to ensure that magic links are not resolved.
+
+.TP
+.B RESOLVE_IN_ROOT
+Treat
+.I dirfd
+as the root directory while resolving
+.I pathname
+(as though the user called
+.BR chroot (2)
+with
+.IR dirfd
+as the argument.)
+Absolute symbolic links and ".." path components will be scoped to
+.IR dirfd .
+If
+.I pathname
+is an absolute path, it is also treated relative to
+.IR dirfd .
+
+However, unlike
+.BR chroot (2)
+(which changes the filesystem root permanently for a process),
+.B RESOLVE_IN_ROOT
+allows a program to efficiently restrict path resolution for only certain
+operations.
+It also has several hardening features (such detecting escape attempts during
+.I ".."
+resolution) which
+.BR chroot (2)
+does not.
+
+Currently, this flag also disables magic link resolution.
+However, this may change in the future.
+The caller should explicitly specify
+.B RESOLVE_NO_MAGICLINKS
+to ensure that magic links are not resolved.
+.PP
+It is an error to provide
+.BR openat2 ()
+unknown flags in
+.IR resolve .
+.RE
+.RE
+
+.SH RETURN VALUE
+On success, a new file descriptor is returned.
+On error, -1 is returned, and
+.I errno
+is set appropriately.
+
+.SH ERRORS
+The set of errors returned by
+.BR openat2 ()
+includes all of the errors returned by
+.BR openat (2),
+as well as the following additional errors:
+.TP
+.B EINVAL
+An unknown flag or invalid value was specified in
+.IR how .
+.TP
+.B EINVAL
+.I mode
+is non-zero, but
+.I flags
+does not contain
+.BR O_CREAT " or " O_TMPFILE .
+.TP
+.B EINVAL
+.I size
+was smaller than any known version of
+.IR "struct open_how" .
+.TP
+.B E2BIG
+An extension was specified in
+.IR how ,
+which the current kernel does not support (see the "Extensibility" section of
+the
+.B NOTES
+for more detail on how extensions are handled.)
+.TP
+.B EAGAIN
+.I resolve
+contains either
+.BR 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.)
+Callers may choose to retry the
+.BR openat2 ()
+call.
+.TP
+.B EXDEV
+.I resolve
+contains either
+.BR RESOLVE_IN_ROOT " or " RESOLVE_BENEATH ,
+and an escape from the root during path resolution was detected.
+
+.TP
+.B EXDEV
+.I resolve
+contains
+.BR RESOLVE_NO_XDEV ,
+and a path component attempted to cross a mount point.
+
+.TP
+.B ELOOP
+.I resolve
+contains
+.BR RESOLVE_NO_SYMLINKS ,
+and one of the path components was a symbolic link (or magic link).
+.TP
+.B ELOOP
+.I resolve
+contains
+.BR RESOLVE_NO_MAGICLINKS ,
+and one of the path components was a magic link.
+
+.SH VERSIONS
+.BR openat2 ()
+first appeared in Linux 5.6.
+
+.SH CONFORMING TO
+This system call is Linux-specific.
+
+The semantics of
+.B RESOLVE_BENEATH
+were modelled after FreeBSD's
+.BR O_BENEATH .
+
+.SH NOTES
+Glibc does not provide a wrapper for this system call; call it using
+.BR syscall (2).
+
+.SS Extensibility
+In order to allow for
+.I struct open_how
+to be extended in future kernel revisions,
+.BR openat2 ()
+requires userspace to specify the size of
+.I struct open_how
+structure they are passing.
+By providing this information, it is possible for
+.BR openat2 ()
+to provide both forwards- and backwards-compatibility \(em with
+.I size
+acting as an implicit version number (because new extension fields will always
+be appended, the size will always increase.)
+This extensibility design is very similar to other system calls such as
+.BR perf_setattr "(2), " perf_event_open "(2), and " clone (3).
+
+If we let
+.I usize
+be the size of the structure according to userspace and
+.I ksize
+be the size of the structure which the kernel supports, then there are only
+three cases to consider:
+
+.RS
+.IP * 3
+If
+.IR ksize " equals " usize ,
+then there is no version mismatch and
+.I how
+can be used verbatim.
+.IP *
+If
+.IR ksize " is larger than " usize ,
+then there are some extensions the kernel supports which the userspace program
+is unaware of.
+Because all extensions must have their zero values be a no-op, the kernel
+treats all of the extension fields not set by userspace to have zero values.
+This provides backwards-compatibility.
+.IP *
+If
+.IR ksize " is smaller than " usize ,
+then there are some extensions which the userspace program is aware of but the
+kernel does not support.
+Because all extensions must have their zero values be a no-op, the kernel can
+safely ignore the unsupported extension fields if they are all-zero.
+If any unsupported extension fields are non-zero, then -1 is returned and
+.I errno
+is set to
+.BR E2BIG .
+This provides forwards-compatibility.
+.RE
+
+Therefore, most userspace programs will not need to have any special handling
+of extensions.
+However, if a userspace program wishes to determine what extensions the running
+kernel supports, they may conduct a binary search on
+.IR size
+(to find the largest value which doesn't produce an error of
+.BR E2BIG .)
+
+.SH SEE ALSO
+.BR openat (2),
+.BR path_resolution (7),
+.BR symlink (7)
--
2.24.0
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