kernel: core_hook: provide a better reboot handler (#523)

* Revert "feat: try manual reboot hook (#521)"

This reverts commit 1853d9decf.

* kernel: core_hook: provide a better reboot handler

I propose that you pass cmd and arg as reference.
this is so we can have much more extendable use of that pointer

kernel: core_hook: provide sys_reboot handler
- 2e2727d56c

kernel: kp_ksud: add sys_reboot kp hook
- 03285886b0

I'm proposing passing arg as reference to arg pointer and also pass int cmd
we can use it to pass numbers atleast.
for advanced usage, we can use it as a delimiter so we can pass a pointer to array.

example pass a char *array[] which decays to a char ** and then use cmd as the number of array members.
we can pass the pointer of the first member of the array and use cmd as the delimiter (count) of members.

for simpler usecase, heres some that I added.

kernel: core_hook: expose  umount list on sys_reboot interface
- 352de41e4b

kernel: core_hook: expose nuke_ext4_sysfs to sys_reboot interface
- 83fc684ccb

ksud: add cmd for add-try-umount, wipe-umount-list and nuke-ext4-sysfs
- a4eab4b8c3

more usage demos
https://github.com/backslashxx/lkm_template/tree/write-pointer-on-pointer
https://github.com/backslashxx/lkm_template/tree/pointer-reuse

I actually proposed sys_reboot upstream because of this pointer that is very usable.

Signed-off-by: backslashxx <118538522+backslashxx@users.noreply.github.com>

---------

Signed-off-by: backslashxx <118538522+backslashxx@users.noreply.github.com>

Website/docs/zh/guide/how-to-integrate.md

@@ -63,36 +63,6 @@ curl -LSs "https://raw.githubusercontent.com/SukiSU-Ultra/SukiSU-Ultra/main/kern
 
 请参考此文档 [https://github.com/~ (non-GKI 内核集成)](https://github.com/tiann/KernelSU/blob/main/website/docs/guide/how-to-integrate-for-non-gki.md#manually-modify-the-kernel-source) 和 [https://github.com/~ (GKI 内核构建)](https://kernelsu.org/zh_CN/guide/how-to-build.html) 进行手动集成。虽然第一个链接的标题是“适用于 non-GKI”，但它也适用于 GKI。两者都可以正常工作。
 
-并且手动修改 kernel/reboot.c, 进行手动 reboot hook
-
-```diff[reboot.c]
-diff --git a/kernel/reboot.c b/kernel/reboot.c
-index 8f08af3a7d04..3809b8aa6213 100644
---- a/kernel/reboot.c
-+++ b/kernel/reboot.c
-@@ -302,6 +302,9 @@ EXPORT_SYMBOL_GPL(kernel_power_off);
-
- DEFINE_MUTEX(system_transition_mutex);
-
-+#ifdef CONFIG_KSU
-+extern void ksu_handle_reboot(int magic1, int magic2, void __user * arg);
-+#endif
- /*
-  * Reboot system call: for obvious reasons only root may call it,
-  * and even root needs to set up some magic numbers in the registers
-@@ -317,6 +320,10 @@ SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
-        char buffer[256];
-        int ret = 0;
-
-+#ifdef CONFIG_KSU
-+       ksu_handle_reboot(magic1, magic2, arg);
-+#endif
-+
-        /* We only trust the superuser with rebooting the system. */
-        if (!ns_capable(pid_ns->user_ns, CAP_SYS_BOOT))
-                return -EPERM;
-```
-
 还有另一种集成方法，但是仍在开发中。
 
 <!-- 这是 backslashxx 的syscall manual hook，但目前无法使用。 -->

kernel/core_hook.c

@@ -653,7 +653,34 @@ int ksu_handle_setuid(struct cred *new, const struct cred *old)
     return 0;
 }
 
-extern void ksu_handle_reboot(int magic1, int magic2, void __user * arg); // supercalls.c
+// downstream: make sure to pass arg as reference, this can allow us to extend things.
+int ksu_handle_sys_reboot(int magic1, int magic2, unsigned int cmd, void __user **arg)
+{
+
+    if (magic1 != KSU_INSTALL_MAGIC1)
+        return 0;
+
+#ifdef CONFIG_KSU_DEBUG
+    pr_info("sys_reboot: intercepted call! magic: 0x%x id: %d\n", magic1, magic2);
+#endif
+
+    // Check if this is a request to install KSU fd
+    if (magic2 == KSU_INSTALL_MAGIC2) {
+        int fd = ksu_install_fd();
+        pr_info("[%d] install ksu fd: %d\n", current->pid, fd);
+
+        // downstream: dereference all arg usage!
+        if (copy_to_user((void __user *)*arg, &fd, sizeof(fd))) {
+            pr_err("install ksu fd reply err\n");
+        }
+
+        return 0;
+    }
+
+    // extensions
+
+    return 0;
+}
 
 // Init functons - kprobe hooks
 
@@ -663,13 +690,10 @@ static int reboot_handler_pre(struct kprobe *p, struct pt_regs *regs)
     struct pt_regs *real_regs = PT_REAL_REGS(regs);
     int magic1 = (int)PT_REGS_PARM1(real_regs);
     int magic2 = (int)PT_REGS_PARM2(real_regs);
-    unsigned long arg4;
+    int cmd = (int)PT_REGS_PARM3(real_regs);
+    void __user **arg = (void __user **)&PT_REGS_SYSCALL_PARM4(real_regs);
 
-    // Check if this is a request to install KSU fd
+    return ksu_handle_sys_reboot(magic1, magic2, cmd, arg);
-    arg4 = (unsigned long)PT_REGS_SYSCALL_PARM4(real_regs);
-    ksu_handle_reboot(magic1, magic2, (void __user *) arg4);
-
-    return 0;
 }
 
 static struct kprobe reboot_kp = {

kernel/supercalls.c

@@ -69,17 +69,6 @@ static void init_uid_scanner(void)
     }
 }
 
-void ksu_handle_reboot(int magic1, int magic2, void __user * arg) {
-    if (magic1 == KSU_INSTALL_MAGIC1 && magic2 == KSU_INSTALL_MAGIC2) {
-        int fd = ksu_install_fd();
-        pr_info("[%d] install ksu fd: %d\n", current->pid, fd);
-
-        if (copy_to_user(arg, &fd, sizeof(fd))) {
-            pr_err("install ksu fd reply err\n");
-        }
-    }
-}
-
 static int do_grant_root(void __user *arg)
 {
     // Check if current UID is allowed