# Pre12 Batch 03: POSIX ACL mode helper

## Change

- In `src/xattr.c`, `erofs_acl_from_mode` now obtains the owner, group, and
  other permissions with `acl_posix1e_mode_to_perm`.
- The helper tags are `ACL_USER_OBJ`, `ACL_GROUP_OBJ`, and `ACL_OTHER`, matching
  the adjacent `ae_tag` assignments.
- No other function lines, ACL entry ordering, `acl_cnt`, IDs, default ACL
  handling, fallback behavior, flags, errno behavior, or control flow changed.

## Helper confirmation

- The local FreeBSD header `sys/sys/acl.h` declares
  `acl_posix1e_mode_to_perm(acl_tag_t tag, mode_t mode)` for kernel consumers.
- The local FreeBSD implementation in `sys/kern/subr_acl_posix1e.c` maps
  `ACL_USER_OBJ` from `S_IRWXU`, `ACL_GROUP_OBJ` from `S_IRWXG`, and
  `ACL_OTHER` from `S_IRWXO`, returning the corresponding POSIX ACL read,
  write, and execute permission bits.
- `src/xattr.c` already includes `<sys/acl.h>`; no include change was needed.

## Exhaustive equivalence

A temporary standalone C program reproduced the local FreeBSD helper logic and
compared it with the three replaced expressions for every permission mode from
`0000` through `0777`:

```text
equivalent=1536/1536 modes=512 tags=3 mismatches=0
temporary_cleanup=ok
```

This covers 512 modes for each of the three tags. The temporary source and
executable were deleted and are not part of the repository.

## Static validation

- The `erofs_acl_from_mode` function is line-for-line unchanged except for the
  three `ae_perm` expressions.
- The source diff changes only those three expressions; this document is the
  only added file.
- `git diff --check` and `git diff --cached --check` completed without errors.
- Before commit, both unstaged status and staged status contained only
  `src/xattr.c` and `docs/pre12-batch-03.md` under `repo-pre-12`.

## Tests not run

Per the Batch 03 execution constraints, no project build, QEMU run, smoke test,
or feature test was run.