Why Linux kernel LSM_HOOK macro is defined with many parameters? - c

Question

In Linux kernel, the LSM_HOOK usage is like:
LSM_HOOK(int, 0, binder_set_context_mgr, const struct cred *mgr)
LSM_HOOK(int, 0, binder_transaction, const struct cred *from,
const struct cred *to)
LSM_HOOK(int, 0, binder_transfer_binder, const struct cred *from,
const struct cred *to)
LSM_HOOK(int, 0, binder_transfer_file, const struct cred *from,
const struct cred *to, struct file *file)
LSM_HOOK(int, 0, ptrace_access_check, struct task_struct *child,
unsigned int mode)
LSM_HOOK(int, 0, ptrace_traceme, struct task_struct *parent)
LSM_HOOK(int, 0, capget, struct task_struct *target, kernel_cap_t *effective,
kernel_cap_t *inheritable, kernel_cap_t *permitted)
LSM_HOOK(int, 0, capset, struct cred *new, const struct cred *old,
const kernel_cap_t *effective, const kernel_cap_t *inheritable,
const kernel_cap_t *permitted)
LSM_HOOK(int, 0, capable, const struct cred *cred, struct user_namespace *ns,
int cap, unsigned int opts)
The LSM_HOOK is defined as:
struct security_hook_heads {
#define LSM_HOOK(RET, DEFAULT, NAME, ...) struct hlist_head NAME;
#include "lsm_hook_defs.h"
#undef LSM_HOOK
} __randomize_layout;
In this situation, parameters except for NAME are all discarded. I am curious why the macro expansion needs so many arguments like above. Thanks!

Answer 1

This is because the macro LSM_HOOK has different implementations in various subsystem or modules, as you can see, in the struct, it undef the macro immediately after usage, so in this particular module, the remaining arguments are not needed and ignored.
But let's look at another module, e.g. in bpf_lsm.h, it defines LSM_HOOK like this:
#define LSM_HOOK(RET, DEFAULT, NAME, ...) \
RET bpf_lsm_##NAME(__VA_ARGS__);
#include <linux/lsm_hook_defs.h>
#undef LSM_HOOK
where the extra arguments are passed into the bpf_lsm_... functions.
So this kind of macro provides extensibility and flexibility among different modules.