OPENAT2_REGULAR flag support for openat2

Hi,

I came upon this "Ability to only open regular files" uapi feature suggestion
from https://uapi-group.org/kernel-features/#ability-to-only-o...
and thought it would be something I could do as a first patch and get to
know the kernel code a bit better.

The following filesystems have been tested by building and booting the kernel
x86 bzImage in a Fedora 43 VM in QEMU. I have tested with OPENAT2_REGULAR that
regular files can be successfully opened and non-regular files (directory, fifo etc)
return -EFTYPE.
- btrfs
- NFS (loopback)
- SMB (loopback)

Changes in v6:
- OPENAT2_REGULAR stripped from file->f_flags in do_dentry_open so that it doesn't leak in fcntl(fd, F_GETFL)
- BUILD_BUG_ON updated to use VALID_OPENAT2_FLAGS instead of VALID_OPEN_FLAGS in build_open_flags and in fcntl_init
- v5 is at: https://lore.kernel.org/linux-fsdevel/20260307140726.7021...

Changes in v5:
- EFTYPE is already used in BSDs mentioned in commit message
- consistently return -EFTYPE in all filesystems
- v4 is at: https://lore.kernel.org/linux-fsdevel/20260221145915.8174...

Changes in v4:
- changed O_REGULAR to OPENAT2_REGULAR
- OPENAT2_REGULAR does not affect O_PATH
- atomic_open codepaths updated to work properly for OPENAT2_REGULAR
- commit message includes the uapi-group URL
- v3 is at: https://lore.kernel.org/linux-fsdevel/20260127180109.6669...

Changes in v3:
- included motivation about O_REGULAR flag in commit message e.g., programs not wanting to be tricked into opening device nodes
- fixed commit message wrongly referencing ENOTREGULAR instead of ENOTREG
- fixed the O_REGULAR flag in arch/parisc/include/uapi/asm/fcntl.h from 060000000 to 0100000000
- added 2 commits converting arch/{mips,sparc}/include/uapi/asm/fcntl.h O_* macros from hex to octal
- v2 is at: https://lore.kernel.org/linux-fsdevel/20260126154156.5572...

Changes in v2:
- rename ENOTREGULAR to ENOTREG
- define ENOTREG in uapi/asm-generic/errno.h (instead of errno-base.h) and in arch/*/include/uapi/asm/errno.h files
- override O_REGULAR in arch/{alpha,sparc,parisc}/include/uapi/asm/fcntl.h due to clash with include/uapi/asm-generic/fcntl.h
- I have kept the kselftest but now that O_REGULAR and ENOTREG can have different value on different architectures I am not sure if it's right
- v1 is at: https://lore.kernel.org/linux-fsdevel/20260125141518.5949...

Thanks.

Regards,
Dorjoy

Dorjoy Chowdhury (4):
  openat2: new OPENAT2_REGULAR flag support
  kselftest/openat2: test for OPENAT2_REGULAR flag
  sparc/fcntl.h: convert O_* flag macros from hex to octal
  mips/fcntl.h: convert O_* flag macros from hex to octal

 arch/alpha/include/uapi/asm/errno.h           |  2 +
 arch/alpha/include/uapi/asm/fcntl.h           |  1 +
 arch/mips/include/uapi/asm/errno.h            |  2 +
 arch/mips/include/uapi/asm/fcntl.h            | 22 +++++------
 arch/parisc/include/uapi/asm/errno.h          |  2 +
 arch/parisc/include/uapi/asm/fcntl.h          |  1 +
 arch/sparc/include/uapi/asm/errno.h           |  2 +
 arch/sparc/include/uapi/asm/fcntl.h           | 35 +++++++++---------
 fs/ceph/file.c                                |  4 ++
 fs/fcntl.c                                    |  4 +-
 fs/gfs2/inode.c                               |  6 +++
 fs/namei.c                                    |  4 ++
 fs/nfs/dir.c                                  |  4 ++
 fs/open.c                                     |  8 ++--
 fs/smb/client/dir.c                           | 14 ++++++-
 include/linux/fcntl.h                         |  2 +
 include/uapi/asm-generic/errno.h              |  2 +
 include/uapi/asm-generic/fcntl.h              |  4 ++
 tools/arch/alpha/include/uapi/asm/errno.h     |  2 +
 tools/arch/mips/include/uapi/asm/errno.h      |  2 +
 tools/arch/parisc/include/uapi/asm/errno.h    |  2 +
 tools/arch/sparc/include/uapi/asm/errno.h     |  2 +
 tools/include/uapi/asm-generic/errno.h        |  2 +
 .../testing/selftests/openat2/openat2_test.c  | 37 ++++++++++++++++++-
 24 files changed, 131 insertions(+), 35 deletions(-)

-- 
2.53.0