[RFC][PATCH v3 00/10] common implementation of dirent file types

[Phillip Potter <phil@philpotter.co.uk>]

This cleanup series is a respin of Amir Goldstein's work, created
in late 2016. It removes several instances of duplicated code. Most
of the duplication dates back to git pre-historic era.

The controversial aspect of this cleanup is that it uses common
code to handle file system specific on-disk format bits. We
handled this aspect with care by placing compile-time checks.
All 7 file systems use a single byte to store dirent file type
on-disk and all of them use the same conversion routines from
i_mode to 4bits DT_* constants to 3bits on-disk FT_* constants.

Patch 1 places a common implementation in fs_types.c with some useful
conversion helpers. The corresponding header fs_types.h contains
related macros and definitions, as well as helper declarations.

Patches 2-3 make use of the helpers in ufs and hfsplus
without any on-disk implications.

Patches 4-10 replace the specific implementations in ext2, exofs,
ext4, ocfs2, f2fs, nilfs and btrfs with the common implementation.
The ext2 implementation appears to have been copied as a basis of
the others, and in particular, for code that tries to access the
ext2_type_by_mode array there is a bug that this patch series
fixes. Essentially, it is defined with size S_IFMT >> S_SHIFT,
so 15. This means that it is possible with a malformed inode to
get an index of 15, as the array is always accessed with:
ext2_type_by_mode[(mode & S_IFMT)>>S_SHIFT];

These patches also now include compile-time checks to ensure that
the file system specific on-disk bits are equivalent to the common
implementation FT_* bits. These compile-time checks are only
included once per file system, as my reasoning is that regardless
of their location, the build will fail/succeed anyway.

In addition, where needed (for patches which no longer apply),
I've rebased them to apply to the newest 4.19 kernel sources.
Each patch is independent of the others, except for the
common implementation itself which they all depend on.

General motivations for the patch (other than the bugfix above) are
the fact that it reduces the amount of code for most filesystems
(except ext4):
ufs: -27 lines
hfsplus: -12 lines
ext2: -9 lines
exofs: -16 lines
ext4: +1 line
ocfs2: -9 lines
f2fs: -7 lines
nilsfs2: -17 lines
btrfs: -2 lines

It also makes it easier to add hypothetical future filesystems which
wish to reuse these POSIX-type definitions and on-disk formats, as they
can simply now use the common implementation.

I welcome feedback, and thank those who have already provided it.
Hopefully, the relevant maintainers will see fit to merge this work.

v3:
- Moved compile-time checks to better locations, and shortened comments,
  as well as formatting to 80 columns
- Renamed header to fs_types.h, and placed comment regarding DT_* constants
  relation to POSIX and glibc dirent.h
- Changed "should" to "must" in comments mentioning the need for fs specific
  values to match the common values in fs_types.h
- Split functions and lookup tables to separate C file (fs/fs_types.c)
- Added kernel-doc comments for all three utility functions, and removed
  therefore unnecessary lines from comment in fs_types.h
- Tweaked commit messages slightly to explain that file systems using
  POSIX file types now don't need to define their own conversion routines
- Renamed fs_dtype function to fs_ftype_to_dtype for consistency with other
  helper functions
- Tweaked fs_ftype_to_dtype to take unsigned int argument, to prevent
  out of bounds access of memory
- Added additional text to comment in fs_types.h explaining the definitions
  must never change
- Changed DT_* types in fs_types.h to explicit constants as they can't
  change anyway
- 80 line violation in the ext2 patch - left as is due to the fact the code
  would look worse if fixed, other patches tweaked slightly to keep within
  80 line limit (by moving last parameter in various if statements)

v2:
- Rebased against Linux 4.19 by Phillip Potter
- This version does not remove filesystem specific XXX_FT_* definitions,
  as these values are now used in compile-time checks added by Phillip
  Potter to make sure they remain the same as the generic FT_* values
- Removed xfs patch (a variant of original patch has already been applied)
- Added SPDX tag to new header file and included it in MAINTAINERS

v1:
- Initial implementation by Amir Goldstein:
  https://marc.info/?l=linux-fsdevel&m=148217829301701&w=2

Phillip Potter (10):
  fs: common implementation of file type conversions
  ufs: use fs_umode_to_dtype() helper
  hfsplus: use fs_umode_to_dtype() helper
  ext2: use common file type conversion
  exofs: use common file type conversion
  ext4: use common file type conversion
  ocfs2: use common file type conversion
  f2fs: use common file type conversion
  nilfs2: use common file type conversion
  btrfs: use common file type conversion

 MAINTAINERS                        |   1 +
 fs/Makefile                        |   3 +-
 fs/btrfs/btrfs_inode.h             |   2 -
 fs/btrfs/delayed-inode.c           |   2 +-
 fs/btrfs/inode.c                   |  32 +++++----
 fs/exofs/dir.c                     |  48 +++++--------
 fs/ext2/dir.c                      |  49 ++++++--------
 fs/ext4/ext4.h                     |  33 ++++-----
 fs/f2fs/dir.c                      |  41 +++++------
 fs/f2fs/inline.c                   |   2 +-
 fs/fs_types.c                      | 105 +++++++++++++++++++++++++++++
 fs/hfsplus/dir.c                   |  16 +----
 fs/nilfs2/dir.c                    |  52 +++++---------
 fs/ocfs2/dir.c                     |  18 +----
 fs/ocfs2/ocfs2_fs.h                |  27 ++++----
 fs/ufs/util.h                      |  29 +-------
 include/linux/f2fs_fs.h            |   8 ++-
 include/linux/fs.h                 |  17 +----
 include/linux/fs_types.h           |  73 ++++++++++++++++++++
 include/uapi/linux/btrfs_tree.h    |   2 +
 include/uapi/linux/nilfs2_ondisk.h |   1 +
 21 files changed, 314 insertions(+), 247 deletions(-)
 create mode 100644 fs/fs_types.c
 create mode 100644 include/linux/fs_types.h

--
2.17.2