These docs are under active development and cover the v0.20 Kobicha security model.
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§13.2

Kernel Patches

KACS requires kernel patches beyond the LSM module itself. All patches are conditional on KACS being active and compile out when KACS is not configured.

§13.2.1 FACS handle-model patches

# Site Purpose
1 ksys_pwrite64() Deny positioned write on append-only fd.
2 ksys_pwritev() / pwritev2() Deny positioned/vectored write + RWF_NOAPPEND on append-only fd.
3 io_write() in io_uring Deny positioned write via io_uring on append-only fd.
4 aio_write() in fs/aio.c Deny positioned write via AIO on append-only fd.
5 do_faccessat() Skip credential swap to real_cred; use effective token.
6 do_handle_open() Gate open_by_handle_at() behind SeChangeNotifyPrivilege.
7 fchmod() / fchown() / futimens() New security_file_setattr hook with struct file *.
8 vfs_fallocate() Deny mutation modes on append-only fd.
9 vfs_fstat() / vfs_statx_fd() New security_file_getattr hook with struct file *.
10 fgetxattr() New security_file_getxattr hook with struct file *.
11 fsetxattr() / fremovexattr() New security_file_setxattr hook with struct file *.

§13.2.2 Kernel interface and derooting patches

# Site Purpose
12 do_dentry_open() Accept KACS desired_access, set f_mode from granted mask.
13 pidfd_getfd() Add PTRACE_MODE_GETFD to distinguish fd extraction from ptrace attach.
14 current_fsuid() Return projected UID from KACS token instead of cred->fsuid.
15 execveat / do_open_execat Check FILE_EXECUTE on the original fd's granted mask.
16 fchdir() / vfs_fchdir() New security_file_fchdir hook; check FILE_TRAVERSE in granted mask.

§13.2.3 Hook coordination

Patches 7, 9, 10, and 11 add file-based hooks that fire while struct file * is available. The subsequent dentry-based hook detects that the file hook already decided and becomes a no-op. Coordination uses a per-task marker:

Field Description
inode Which object was decided.
op_class SETATTR, GETATTR, SETXATTR, or GETXATTR.

The marker is scoped to exactly one syscall invocation, one inode, one operation class. It is cleared unconditionally at the end of each dentry-based hook.