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docs/pre12-batch-03.md

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+# 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.