linux-stable.git/arch/mips/kernel/syscalls, branch v5.9.8

all arch: remove system call sys_sysctl

2020-08-15T02:56:56+00:00

Since commit 61a47c1ad3a4dc ("sysctl: Remove the sysctl system call"),
sys_sysctl is actually unavailable: any input can only return an error.

We have been warning about people using the sysctl system call for years
and believe there are no more users.  Even if there are users of this
interface if they have not complained or fixed their code by now they
probably are not going to, so there is no point in warning them any
longer.

So completely remove sys_sysctl on all architectures.

[nixiaoming@huawei.com: s390: fix build error for sys_call_table_emu]
 Link: http://lkml.kernel.org/r/20200618141426.16884-1-nixiaoming@huawei.com

Signed-off-by: Xiaoming Ni 
Signed-off-by: Andrew Morton 
Acked-by: Will Deacon 		[arm/arm64]
Acked-by: "Eric W. Biederman" 
Cc: Aleksa Sarai 
Cc: Alexander Shishkin 
Cc: Al Viro 
Cc: Andi Kleen 
Cc: Andrew Morton 
Cc: Andy Lutomirski 
Cc: Arnaldo Carvalho de Melo 
Cc: Arnd Bergmann 
Cc: Benjamin Herrenschmidt 
Cc: Bin Meng 
Cc: Borislav Petkov 
Cc: Brian Gerst 
Cc: Catalin Marinas 
Cc: chenzefeng 
Cc: Christian Borntraeger 
Cc: Christian Brauner 
Cc: Chris Zankel 
Cc: David Howells 
Cc: David S. Miller 
Cc: Diego Elio Pettenò 
Cc: Dmitry Vyukov 
Cc: Dominik Brodowski 
Cc: Fenghua Yu 
Cc: Geert Uytterhoeven 
Cc: Heiko Carstens 
Cc: Helge Deller 
Cc: "H. Peter Anvin" 
Cc: Ingo Molnar 
Cc: Iurii Zaikin 
Cc: Ivan Kokshaysky 
Cc: James Bottomley 
Cc: Jens Axboe 
Cc: Jiri Olsa 
Cc: Kars de Jong 
Cc: Kees Cook 
Cc: Krzysztof Kozlowski 
Cc: Luis Chamberlain 
Cc: Marco Elver 
Cc: Mark Rutland 
Cc: Martin K. Petersen 
Cc: Masahiro Yamada 
Cc: Matt Turner 
Cc: Max Filippov 
Cc: Michael Ellerman 
Cc: Michal Simek 
Cc: Miklos Szeredi 
Cc: Minchan Kim 
Cc: Namhyung Kim 
Cc: Naveen N. Rao 
Cc: Nick Piggin 
Cc: Oleg Nesterov 
Cc: Olof Johansson 
Cc: Paul Burton 
Cc: "Paul E. McKenney" 
Cc: Paul Mackerras 
Cc: Peter Zijlstra (Intel) 
Cc: Randy Dunlap 
Cc: Ravi Bangoria 
Cc: Richard Henderson 
Cc: Rich Felker 
Cc: Russell King 
Cc: Sami Tolvanen 
Cc: Sargun Dhillon 
Cc: Stephen Rothwell 
Cc: Sudeep Holla 
Cc: Sven Schnelle 
Cc: Thiago Jung Bauermann 
Cc: Thomas Bogendoerfer 
Cc: Thomas Gleixner 
Cc: Tony Luck 
Cc: Vasily Gorbik 
Cc: Vlastimil Babka 
Cc: Yoshinori Sato 
Cc: Zhou Yanjie 
Link: http://lkml.kernel.org/r/20200616030734.87257-1-nixiaoming@huawei.com
Signed-off-by: Linus Torvalds

Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next

2020-08-06T03:13:21+00:00

Pull networking updates from David Miller:

 1) Support 6Ghz band in ath11k driver, from Rajkumar Manoharan.

 2) Support UDP segmentation in code TSO code, from Eric Dumazet.

 3) Allow flashing different flash images in cxgb4 driver, from Vishal
    Kulkarni.

 4) Add drop frames counter and flow status to tc flower offloading,
    from Po Liu.

 5) Support n-tuple filters in cxgb4, from Vishal Kulkarni.

 6) Various new indirect call avoidance, from Eric Dumazet and Brian
    Vazquez.

 7) Fix BPF verifier failures on 32-bit pointer arithmetic, from
    Yonghong Song.

 8) Support querying and setting hardware address of a port function via
    devlink, use this in mlx5, from Parav Pandit.

 9) Support hw ipsec offload on bonding slaves, from Jarod Wilson.

10) Switch qca8k driver over to phylink, from Jonathan McDowell.

11) In bpftool, show list of processes holding BPF FD references to
    maps, programs, links, and btf objects. From Andrii Nakryiko.

12) Several conversions over to generic power management, from Vaibhav
    Gupta.

13) Add support for SO_KEEPALIVE et al. to bpf_setsockopt(), from Dmitry
    Yakunin.

14) Various https url conversions, from Alexander A. Klimov.

15) Timestamping and PHC support for mscc PHY driver, from Antoine
    Tenart.

16) Support bpf iterating over tcp and udp sockets, from Yonghong Song.

17) Support 5GBASE-T i40e NICs, from Aleksandr Loktionov.

18) Add kTLS RX HW offload support to mlx5e, from Tariq Toukan.

19) Fix the ->ndo_start_xmit() return type to be netdev_tx_t in several
    drivers. From Luc Van Oostenryck.

20) XDP support for xen-netfront, from Denis Kirjanov.

21) Support receive buffer autotuning in MPTCP, from Florian Westphal.

22) Support EF100 chip in sfc driver, from Edward Cree.

23) Add XDP support to mvpp2 driver, from Matteo Croce.

24) Support MPTCP in sock_diag, from Paolo Abeni.

25) Commonize UDP tunnel offloading code by creating udp_tunnel_nic
    infrastructure, from Jakub Kicinski.

26) Several pci_ --> dma_ API conversions, from Christophe JAILLET.

27) Add FLOW_ACTION_POLICE support to mlxsw, from Ido Schimmel.

28) Add SK_LOOKUP bpf program type, from Jakub Sitnicki.

29) Refactor a lot of networking socket option handling code in order to
    avoid set_fs() calls, from Christoph Hellwig.

30) Add rfc4884 support to icmp code, from Willem de Bruijn.

31) Support TBF offload in dpaa2-eth driver, from Ioana Ciornei.

32) Support XDP_REDIRECT in qede driver, from Alexander Lobakin.

33) Support PCI relaxed ordering in mlx5 driver, from Aya Levin.

34) Support TCP syncookies in MPTCP, from Flowian Westphal.

35) Fix several tricky cases of PMTU handling wrt. briding, from Stefano
    Brivio.

* git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next: (2056 commits)
  net: thunderx: initialize VF's mailbox mutex before first usage
  usb: hso: remove bogus check for EINPROGRESS
  usb: hso: no complaint about kmalloc failure
  hso: fix bailout in error case of probe
  ip_tunnel_core: Fix build for archs without _HAVE_ARCH_IPV6_CSUM
  selftests/net: relax cpu affinity requirement in msg_zerocopy test
  mptcp: be careful on subflow creation
  selftests: rtnetlink: make kci_test_encap() return sub-test result
  selftests: rtnetlink: correct the final return value for the test
  net: dsa: sja1105: use detected device id instead of DT one on mismatch
  tipc: set ub->ifindex for local ipv6 address
  ipv6: add ipv6_dev_find()
  net: openvswitch: silence suspicious RCU usage warning
  Revert "vxlan: fix tos value before xmit"
  ptp: only allow phase values lower than 1 period
  farsync: switch from 'pci_' to 'dma_' API
  wan: wanxl: switch from 'pci_' to 'dma_' API
  hv_netvsc: do not use VF device if link is down
  dpaa2-eth: Fix passing zero to 'PTR_ERR' warning
  net: macb: Properly handle phylink on at91sam9x
  ...

net: remove compat_sys_{get,set}sockopt

2020-07-20T01:16:40+00:00

Now that the ->compat_{get,set}sockopt proto_ops methods are gone
there is no good reason left to keep the compat syscalls separate.

This fixes the odd use of unsigned int for the compat_setsockopt
optlen and the missing sock_use_custom_sol_socket.

It would also easily allow running the eBPF hooks for the compat
syscalls, but such a large change in behavior does not belong into
a consolidation patch like this one.

Signed-off-by: Christoph Hellwig 
Signed-off-by: David S. Miller

arch: wire-up close_range()

2020-06-16T22:07:38+00:00

This wires up the close_range() syscall into all arches at once.

Suggested-by: Arnd Bergmann 
Signed-off-by: Christian Brauner 
Reviewed-by: Oleg Nesterov 
Acked-by: Arnd Bergmann 
Acked-by: Michael Ellerman  (powerpc)
Cc: Jann Horn 
Cc: David Howells 
Cc: Dmitry V. Levin 
Cc: Linus Torvalds 
Cc: Al Viro 
Cc: Florian Weimer 
Cc: linux-api@vger.kernel.org
Cc: linux-alpha@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-ia64@vger.kernel.org
Cc: linux-m68k@lists.linux-m68k.org
Cc: linux-mips@vger.kernel.org
Cc: linux-parisc@vger.kernel.org
Cc: linuxppc-dev@lists.ozlabs.org
Cc: linux-s390@vger.kernel.org
Cc: linux-sh@vger.kernel.org
Cc: sparclinux@vger.kernel.org
Cc: linux-xtensa@linux-xtensa.org
Cc: linux-arch@vger.kernel.org
Cc: x86@kernel.org

vfs: add faccessat2 syscall

2020-05-14T14:44:25+00:00

POSIX defines faccessat() as having a fourth "flags" argument, while the
linux syscall doesn't have it.  Glibc tries to emulate AT_EACCESS and
AT_SYMLINK_NOFOLLOW, but AT_EACCESS emulation is broken.

Add a new faccessat(2) syscall with the added flags argument and implement
both flags.

The value of AT_EACCESS is defined in glibc headers to be the same as
AT_REMOVEDIR.  Use this value for the kernel interface as well, together
with the explanatory comment.

Also add AT_EMPTY_PATH support, which is not documented by POSIX, but can
be useful and is trivial to implement.

Signed-off-by: Miklos Szeredi

asm-generic: fix unistd_32.h generation format

2020-04-07T17:43:42+00:00

Generated files are also checked by sparse that's why add newline to
remove sparse (C=1) warning.

The issue was found on Microblaze and reported like this:
./arch/microblaze/include/generated/uapi/asm/unistd_32.h:438:45: warning:
no newline at end of file

Mips and PowerPC have it already but let's align with style used by m68k.

Signed-off-by: Michal Simek 
Signed-off-by: Andrew Morton 
Reviewed-by: Stefan Asserhall 
Acked-by: Max Filippov  (xtensa)
Cc: Arnd Bergmann 
Cc: Max Filippov 
Cc: Benjamin Herrenschmidt 
Cc: Chris Zankel 
Cc: David S. Miller 
Cc: Fenghua Yu 
Cc: Helge Deller 
Cc: Ivan Kokshaysky 
Cc: James Bottomley 
Cc: Matt Turner 
Cc: Michael Ellerman 
Cc: Paul Burton 
Cc: Paul Mackerras 
Cc: Ralf Baechle 
Cc: Rich Felker 
Cc: Richard Henderson 
Cc: Tony Luck 
Cc: Yoshinori Sato 
Link: http://lkml.kernel.org/r/4d32ab4e1fb2edb691d2e1687e8fb303c09fd023.1581504803.git.michal.simek@xilinx.com
Signed-off-by: Linus Torvalds

Merge tag 'mips_5.6' of git://git.kernel.org/pub/scm/linux/kernel/git/mips/linux

2020-01-31T19:28:31+00:00

Pull MIPS changes from Paul Burton:
 "Nothing too big or scary in here:

   - Support mremap() for the VDSO, primarily to allow CRIU to restore
     the VDSO to its checkpointed location.

   - Restore the MIPS32 cBPF JIT, after having reverted the enablement
     of the eBPF JIT for MIPS32 systems in the 5.5 cycle.

   - Improve cop0 counter synchronization behaviour whilst onlining CPUs
     by running with interrupts disabled.

   - Better match FPU behaviour when emulating multiply-accumulate
     instructions on pre-r6 systems that implement IEEE754-2008 style
     MACs.

   - Loongson64 kernels now build using the MIPS64r2 ISA, allowing them
     to take advantage of instructions introduced by r2.

   - Support for the Ingenic X1000 SoC & the really nice little CU Neo
     development board that's using it.

   - Support for WMAC on GARDENA Smart Gateway devices.

   - Lots of cleanup & refactoring of SGI IP27 (Origin 2*) support in
     preparation for introducing IP35 (Origin 3*) support.

   - Various Kconfig & Makefile cleanups"

* tag 'mips_5.6' of git://git.kernel.org/pub/scm/linux/kernel/git/mips/linux: (60 commits)
  MIPS: PCI: Add detection of IOC3 on IO7, IO8, IO9 and Fuel
  MIPS: Loongson64: Disable exec hazard
  MIPS: Loongson64: Bump ISA level to MIPSR2
  MIPS: Make DIEI support as a config option
  MIPS: OCTEON: octeon-irq: fix spelling mistake "to" -> "too"
  MIPS: asm: local: add barriers for Loongson
  MIPS: Loongson64: Select mac2008 only feature
  MIPS: Add MAC2008 Support
  Revert "MIPS: Add custom serial.h with BASE_BAUD override for generic kernel"
  MIPS: sort MIPS and MIPS_GENERIC Kconfig selects alphabetically (again)
  MIPS: make CPU_HAS_LOAD_STORE_LR opt-out
  MIPS: generic: don't unconditionally select PINCTRL
  MIPS: don't explicitly select LIBFDT in Kconfig
  MIPS: sync-r4k: do slave counter synchronization with disabled HW interrupts
  MIPS: SGI-IP30: Check for valid pointer before using it
  MIPS: syscalls: fix indentation of the 'SYSNR' message
  MIPS: boot: fix typo in 'vmlinux.lzma.its' target
  MIPS: fix indentation of the 'RELOCS' message
  dt-bindings: Document loongson vendor-prefix
  MIPS: CU1000-Neo: Refresh defconfig to support HWMON and WiFi.
  ...

Merge tag 'threads-v5.6' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux

2020-01-30T03:38:34+00:00

Pull thread management updates from Christian Brauner:
 "Sargun Dhillon over the last cycle has worked on the pidfd_getfd()
  syscall.

  This syscall allows for the retrieval of file descriptors of a process
  based on its pidfd. A task needs to have ptrace_may_access()
  permissions with PTRACE_MODE_ATTACH_REALCREDS (suggested by Oleg and
  Andy) on the target.

  One of the main use-cases is in combination with seccomp's user
  notification feature. As a reminder, seccomp's user notification
  feature was made available in v5.0. It allows a task to retrieve a
  file descriptor for its seccomp filter. The file descriptor is usually
  handed of to a more privileged supervising process. The supervisor can
  then listen for syscall events caught by the seccomp filter of the
  supervisee and perform actions in lieu of the supervisee, usually
  emulating syscalls. pidfd_getfd() is needed to expand its uses.

  There are currently two major users that wait on pidfd_getfd() and one
  future user:

   - Netflix, Sargun said, is working on a service mesh where users
     should be able to connect to a dns-based VIP. When a user connects
     to e.g. 1.2.3.4:80 that runs e.g. service "foo" they will be
     redirected to an envoy process. This service mesh uses seccomp user
     notifications and pidfd to intercept all connect calls and instead
     of connecting them to 1.2.3.4:80 connects them to e.g.
     127.0.0.1:8080.

   - LXD uses the seccomp notifier heavily to intercept and emulate
     mknod() and mount() syscalls for unprivileged containers/processes.
     With pidfd_getfd() more uses-cases e.g. bridging socket connections
     will be possible.

   - The patchset has also seen some interest from the browser corner.
     Right now, Firefox is using a SECCOMP_RET_TRAP sandbox managed by a
     broker process. In the future glibc will start blocking all signals
     during dlopen() rendering this type of sandbox impossible. Hence,
     in the future Firefox will switch to a seccomp-user-nofication
     based sandbox which also makes use of file descriptor retrieval.
     The thread for this can be found at
     https://sourceware.org/ml/libc-alpha/2019-12/msg00079.html

  With pidfd_getfd() it is e.g. possible to bridge socket connections
  for the supervisee (binding to a privileged port) and taking actions
  on file descriptors on behalf of the supervisee in general.

  Sargun's first version was using an ioctl on pidfds but various people
  pushed for it to be a proper syscall which he duely implemented as
  well over various review cycles. Selftests are of course included.
  I've also added instructions how to deal with merge conflicts below.

  There's also a small fix coming from the kernel mentee project to
  correctly annotate struct sighand_struct with __rcu to fix various
  sparse warnings. We've received a few more such fixes and even though
  they are mostly trivial I've decided to postpone them until after -rc1
  since they came in rather late and I don't want to risk introducing
  build warnings.

  Finally, there's a new prctl() command PR_{G,S}ET_IO_FLUSHER which is
  needed to avoid allocation recursions triggerable by storage drivers
  that have userspace parts that run in the IO path (e.g. dm-multipath,
  iscsi, etc). These allocation recursions deadlock the device.

  The new prctl() allows such privileged userspace components to avoid
  allocation recursions by setting the PF_MEMALLOC_NOIO and
  PF_LESS_THROTTLE flags. The patch carries the necessary acks from the
  relevant maintainers and is routed here as part of prctl()
  thread-management."

* tag 'threads-v5.6' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux:
  prctl: PR_{G,S}ET_IO_FLUSHER to support controlling memory reclaim
  sched.h: Annotate sighand_struct with __rcu
  test: Add test for pidfd getfd
  arch: wire up pidfd_getfd syscall
  pid: Implement pidfd_getfd syscall
  vfs, fdtable: Add fget_task helper

MIPS: syscalls: fix indentation of the 'SYSNR' message

2020-01-20T23:39:34+00:00

It also lacks a whitespace (copy'n'paste error?) and also messes up the
output:

  SYSHDR  arch/mips/include/generated/uapi/asm/unistd_n32.h
  SYSHDR  arch/mips/include/generated/uapi/asm/unistd_n64.h
  SYSHDR  arch/mips/include/generated/uapi/asm/unistd_o32.h
  SYSNR  arch/mips/include/generated/uapi/asm/unistd_nr_n32.h
  SYSNR  arch/mips/include/generated/uapi/asm/unistd_nr_n64.h
  SYSNR  arch/mips/include/generated/uapi/asm/unistd_nr_o32.h
  WRAP    arch/mips/include/generated/uapi/asm/bpf_perf_event.h
  WRAP    arch/mips/include/generated/uapi/asm/ipcbuf.h

After:

  SYSHDR  arch/mips/include/generated/uapi/asm/unistd_n32.h
  SYSHDR  arch/mips/include/generated/uapi/asm/unistd_n64.h
  SYSHDR  arch/mips/include/generated/uapi/asm/unistd_o32.h
  SYSNR   arch/mips/include/generated/uapi/asm/unistd_nr_n32.h
  SYSNR   arch/mips/include/generated/uapi/asm/unistd_nr_n64.h
  SYSNR   arch/mips/include/generated/uapi/asm/unistd_nr_o32.h
  WRAP    arch/mips/include/generated/uapi/asm/bpf_perf_event.h
  WRAP    arch/mips/include/generated/uapi/asm/ipcbuf.h

Present since day 0 of syscall table generation introduction for MIPS.

Fixes: 9bcbf97c6293 ("mips: add system call table generation support")
Cc:  # v5.0+
Signed-off-by: Alexander Lobakin 
Signed-off-by: Paul Burton 
Cc: Ralf Baechle 
Cc: James Hogan 
Cc: Masahiro Yamada 
Cc: Rob Herring 
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org

open: introduce openat2(2) syscall

2020-01-18T14:19:18+00:00

/* Background. */
For a very long time, extending openat(2) with new features has been
incredibly frustrating. This stems from the fact that openat(2) is
possibly the most famous counter-example to the mantra "don't silently
accept garbage from userspace" -- it doesn't check whether unknown flags
are present[1].

This means that (generally) the addition of new flags to openat(2) has
been fraught with backwards-compatibility issues (O_TMPFILE has to be
defined as __O_TMPFILE|O_DIRECTORY|[O_RDWR or O_WRONLY] to ensure old
kernels gave errors, since it's insecure to silently ignore the
flag[2]). All new security-related flags therefore have a tough road to
being added to openat(2).

Userspace also has a hard time figuring out whether a particular flag is
supported on a particular kernel. While it is now possible with
contemporary kernels (thanks to [3]), older kernels will expose unknown
flag bits through fcntl(F_GETFL). Giving a clear -EINVAL during
openat(2) time matches modern syscall designs and is far more
fool-proof.

In addition, the newly-added path resolution restriction LOOKUP flags
(which we would like to expose to user-space) don't feel related to the
pre-existing O_* flag set -- they affect all components of path lookup.
We'd therefore like to add a new flag argument.

Adding a new syscall allows us to finally fix the flag-ignoring problem,
and we can make it extensible enough so that we will hopefully never
need an openat3(2).

/* Syscall Prototype. */
  /*
   * open_how is an extensible structure (similar in interface to
   * clone3(2) or sched_setattr(2)). The size parameter must be set to
   * sizeof(struct open_how), to allow for future extensions. All future
   * extensions will be appended to open_how, with their zero value
   * acting as a no-op default.
   */
  struct open_how { /* ... */ };

  int openat2(int dfd, const char *pathname,
              struct open_how *how, size_t size);

/* Description. */
The initial version of 'struct open_how' contains the following fields:

  flags
    Used to specify openat(2)-style flags. However, any unknown flag
    bits or otherwise incorrect flag combinations (like O_PATH|O_RDWR)
    will result in -EINVAL. In addition, this field is 64-bits wide to
    allow for more O_ flags than currently permitted with openat(2).

  mode
    The file mode for O_CREAT or O_TMPFILE.

    Must be set to zero if flags does not contain O_CREAT or O_TMPFILE.

  resolve
    Restrict path resolution (in contrast to O_* flags they affect all
    path components). The current set of flags are as follows (at the
    moment, all of the RESOLVE_ flags are implemented as just passing
    the corresponding LOOKUP_ flag).

    RESOLVE_NO_XDEV       => LOOKUP_NO_XDEV
    RESOLVE_NO_SYMLINKS   => LOOKUP_NO_SYMLINKS
    RESOLVE_NO_MAGICLINKS => LOOKUP_NO_MAGICLINKS
    RESOLVE_BENEATH       => LOOKUP_BENEATH
    RESOLVE_IN_ROOT       => LOOKUP_IN_ROOT

open_how does not contain an embedded size field, because it is of
little benefit (userspace can figure out the kernel open_how size at
runtime fairly easily without it). It also only contains u64s (even
though ->mode arguably should be a u16) to avoid having padding fields
which are never used in the future.

Note that as a result of the new how->flags handling, O_PATH|O_TMPFILE
is no longer permitted for openat(2). As far as I can tell, this has
always been a bug and appears to not be used by userspace (and I've not
seen any problems on my machines by disallowing it). If it turns out
this breaks something, we can special-case it and only permit it for
openat(2) but not openat2(2).

After input from Florian Weimer, the new open_how and flag definitions
are inside a separate header from uapi/linux/fcntl.h, to avoid problems
that glibc has with importing that header.

/* Testing. */
In a follow-up patch there are over 200 selftests which ensure that this
syscall has the correct semantics and will correctly handle several
attack scenarios.

In addition, I've written a userspace library[4] which provides
convenient wrappers around openat2(RESOLVE_IN_ROOT) (this is necessary
because no other syscalls support RESOLVE_IN_ROOT, and thus lots of care
must be taken when using RESOLVE_IN_ROOT'd file descriptors with other
syscalls). During the development of this patch, I've run numerous
verification tests using libpathrs (showing that the API is reasonably
usable by userspace).

/* Future Work. */
Additional RESOLVE_ flags have been suggested during the review period.
These can be easily implemented separately (such as blocking auto-mount
during resolution).

Furthermore, there are some other proposed changes to the openat(2)
interface (the most obvious example is magic-link hardening[5]) which
would be a good opportunity to add a way for userspace to restrict how
O_PATH file descriptors can be re-opened.

Another possible avenue of future work would be some kind of
CHECK_FIELDS[6] flag which causes the kernel to indicate to userspace
which openat2(2) flags and fields are supported by the current kernel
(to avoid userspace having to go through several guesses to figure it
out).

[1]: https://lwn.net/Articles/588444/
[2]: https://lore.kernel.org/lkml/CA+55aFyyxJL1LyXZeBsf2ypriraj5ut1XkNDsunRBqgVjZU_6Q@mail.gmail.com
[3]: commit 629e014bb834 ("fs: completely ignore unknown open flags")
[4]: https://sourceware.org/bugzilla/show_bug.cgi?id=17523
[5]: https://lore.kernel.org/lkml/20190930183316.10190-2-cyphar@cyphar.com/
[6]: https://youtu.be/ggD-eb3yPVs

Suggested-by: Christian Brauner 
Signed-off-by: Aleksa Sarai 
Signed-off-by: Al Viro