# 2021q3 Homework1 (quiz1)
contributed by < darnellyao >

>[題目連結](https://hackmd.io/@sysprog/linux2021-summer-quiz1)
>
解答:

AAA = list_for_each_entry_safe
BBB = list_for_each_entry_safe
CCC = list_add_tail(&proc->list_node, &hidden_proc)
DDD = list_del(&proc->list_node)

>[作業連結](https://hackmd.io/@sysprog/linux2021-summer-homework1)
## 前置作業
* 目標: 可成功編譯程式，並正確執行
### 開發環境

OS: ubuntu 16.04 (VMware Workstation 16 Player)
$ uname -r
4.15.0-142-generic

### 編譯遇到的問題
1. error: implicit declaration of function ‘kmalloc’ [-Werror=implicit-function-declaration]
2. error: implicit declaration of function ‘kfree’ [-Werror=implicit-function-declaration]
3. error: implicit declaration of function ‘copy_to_user’ [-Werror=implicit-function-declaration]
4. error: implicit declaration of function ‘copy_from_user’ [-Werror=implicit-function-declaration]

make -C /lib/modules/`uname -r`/build M=/home/darnell/linux2021-summer/hw1 modules
make[1]: Entering directory '/usr/src/linux-headers-4.15.0-142-generic'
  CC [M]  /home/darnell/linux2021-summer/hw1/main.o
/home/darnell/linux2021-summer/hw1/main.c: In function ‘hide_process’:
/home/darnell/linux2021-summer/hw1/main.c:123:24: error: implicit declaration of function ‘kmalloc’ [-Werror=implicit-function-declaration]
     pid_node_t *proc = kmalloc(sizeof(pid_node_t), GFP_KERNEL);
                        ^
/home/darnell/linux2021-summer/hw1/main.c:123:24: warning: initialization makes pointer from integer without a cast [-Wint-conversion]
/home/darnell/linux2021-summer/hw1/main.c: In function ‘unhide_process’:
/home/darnell/linux2021-summer/hw1/main.c:134:9: error: implicit declaration of function ‘kfree’ [-Werror=implicit-function-declaration]
         kfree(proc);
         ^
/home/darnell/linux2021-summer/hw1/main.c: In function ‘device_read’:
/home/darnell/linux2021-summer/hw1/main.c:165:9: error: implicit declaration of function ‘copy_to_user’ [-Werror=implicit-function-declaration]
         copy_to_user(buffer + *offset, message, strlen(message));
         ^
/home/darnell/linux2021-summer/hw1/main.c: In function ‘device_write’:
/home/darnell/linux2021-summer/hw1/main.c:183:13: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
     message = kmalloc(len + 1, GFP_KERNEL);
             ^
/home/darnell/linux2021-summer/hw1/main.c:185:5: error: implicit declaration of function ‘copy_from_user’ [-Werror=implicit-function-declaration]
     copy_from_user(message, buffer, len);
     ^
cc1: some warnings being treated as errors
scripts/Makefile.build:330: recipe for target '/home/darnell/linux2021-summer/hw1/main.o' failed
make[2]: *** [/home/darnell/linux2021-summer/hw1/main.o] Error 1
Makefile:1584: recipe for target '_module_/home/darnell/linux2021-summer/hw1' failed
make[1]: *** [_module_/home/darnell/linux2021-summer/hw1] Error 2
make[1]: Leaving directory '/usr/src/linux-headers-4.15.0-142-generic'
Makefile:9: recipe for target 'all' failed
make: *** [all] Error 2

### 問題解決方案
* 加入對應的標頭檔
* 問題 1 & 2

    #include <linux/slab.h> 

* 問題 3 & 4

    #include <linux/uaccess.h> 

* 重新編譯

$ make
make -C /lib/modules/`uname -r`/build M=/home/darnell/linux2021-summer/hw1 modules
make[1]: Entering directory '/usr/src/linux-headers-4.15.0-142-generic'
  CC [M]  /home/darnell/linux2021-summer/hw1/main.o
  LD [M]  /home/darnell/linux2021-summer/hw1/hideproc.o
  Building modules, stage 2.
  MODPOST 1 modules
  CC      /home/darnell/linux2021-summer/hw1/hideproc.mod.o
  LD [M]  /home/darnell/linux2021-summer/hw1/hideproc.ko
make[1]: Leaving directory '/usr/src/linux-headers-4.15.0-142-generic'

* 載入 hideproc.ko 並執行

$ sudo insmod hideproc.ko 
$ pidof systemd
1502
$ echo "add 1502" | sudo tee /dev/hideproc 
$ pidof systemd # 無法見到 systemd
$ echo "del 1502" | sudo tee /dev/hideproc 
$ pidof systemd # 再次見到 systemd
1502
$ sudo rmmod hideproc

* kernel message

$ dmesg
...
[   81.314599] @ _hideproc_init
[   93.189709] @ _hideproc_exit

### 原始程式碼存在的問題
* 執行完 rmmod 後系統當機，將在4.中改進並實作測試

## 1.解釋上述程式碼運作原理，包含 ftrace 的使用
## 2.本程式僅在 Linux v5.4 測試，若你用的核心較新，請試著找出替代方案
>2020 年的變更 Unexporting kallsyms_lookup_name()
Access to kallsyms on Linux 5.7+

## 3.本核心模組只能隱藏單一 PID，請擴充為允許其 PPID 也跟著隱藏，或允許給定一組 PID 列表，而非僅有單一 PID
## 4.指出程式碼可改進的地方，並動手實作
### 1. 執行完 rmmod 後系統當機
* 主因: 移除模組時未釋放系統資源
* 程式碼修改

1. 打開 hook_remove 條件編譯

#if 1
void hook_remove(struct ftrace_hook *hook)
{
    int err = unregister_ftrace_function(&hook->ops);
    if (err)
        printk("unregister_ftrace_function() failed: %d\n", err);
    err = ftrace_set_filter_ip(&hook->ops, hook->address, 1, 0); 
    if (err)
        printk("ftrace_set_filter_ip() failed: %d\n", err);
}
#endif

2. 修補 _hideproc_exit 程式

static int _hideproc_init(void)
{
    int err, dev_major;
    dev_t dev;
    printk(KERN_INFO "@ %s\n", __func__);
    err = alloc_chrdev_region(&dev, 0, MINOR_VERSION, DEVICE_NAME);
    dev_major = MAJOR(dev);

    hideproc_class = class_create(THIS_MODULE, DEVICE_NAME);

    cdev_init(&cdev, &fops);
    cdev_add(&cdev, MKDEV(dev_major, MINOR_VERSION), 1); 
    device_create(hideproc_class, NULL, MKDEV(dev_major, MINOR_VERSION), NULL,
                  DEVICE_NAME);

    init_hook();

    return 0;
}

將 _hideproc_init 索取的系統資源依序釋放
init_hook() -> hook_remove()
device_create() -> device_destroy()
cdev_add() -> cdev_del()
class_create() -> class_destroy()
alloc_chrdev_region() -> unregister_chrdev_region()

static void _hideproc_exit(void)
{
    dev_t dev;
    printk(KERN_INFO "@ %s\n", __func__);
    hook_remove(&hook);
    device_destroy(hideproc_class , dev);
    cdev_del(&cdev);
    class_destroy(hideproc_class);
    unregister_chrdev_region(&dev, MINOR_VERSION);
}

* 執行結果: 程式可正確執行且系統不會當機

$ sudo insmod hideproc.ko 
$ pidof cron
897
$ echo "add 897" | sudo tee /dev/hideproc 
$ pidof cron # 無法見到 cron
$ echo "del 897" | sudo tee /dev/hideproc 
$ pidof cron # 再次見到 cron
897
$ sudo rmmod hideproc
$ dmesg
...
[ 2686.958441] @ _hideproc_init
[ 2796.544799] @ _hideproc_exit