# [2021 年暑期 Linux 核心](https://hackmd.io/@sysprog/linux2021-summer/) 第 1 週測驗題
###### tags: `linux2021`

:::info
目的: 檢驗學員對 ==[Linux 核心行程](https://hackmd.io/@sysprog/linux-process)==, ==[List Management API](https://www.kernel.org/doc/html/latest/core-api/kernel-api.html)==, ==[Linux 核心模組](https://hackmd.io/@sysprog/linux-kernel-module)== 的認知
:::

==[作答表單](https://docs.google.com/forms/d/e/1FAIpQLSdxUat6RS15EiHqRXXoqIc-Ueq99BoCqTSCFL55lX6sQVXReA/viewform)==

(僅在 Linux v5.4 測試) 考慮 `hideproc` 這個 Linux 核心模組可隱藏特定的 process ID，用法如下: (假設 `cron` 的 PID 為 644)

$ sudo insmod hideproc.ko 
$ pidof cron
$ echo "add 644" | sudo tee /dev/hideproc 
$ pidof cron # 無法見到 cron
$ echo "del 644" | sudo tee /dev/hideproc 
$ pidof cron # 再次見到 cron
$ sudo rmmod hideproc

> 你或許會好奇：為何不能採用 `sudo echo "add 644" > /dev/hideproc` 命令呢？請參見 [How do I use sudo to redirect output to a location I don't have permission to write to?](https://stackoverflow.com/questions/82256/how-do-i-use-sudo-to-redirect-output-to-a-location-i-dont-have-permission-to-wr)

以下是 `main.c` 的程式碼: (部分遮蔽)

#include <linux/cdev.h>
#include <linux/ftrace.h>
#include <linux/kallsyms.h>
#include <linux/list.h>
#include <linux/module.h>
#include <linux/proc_fs.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("National Cheng Kung University, Taiwan");

enum RETURN_CODE { SUCCESS };

struct ftrace_hook {
    const char *name;
    void *func, *orig;
    unsigned long address;
    struct ftrace_ops ops;
};

static int hook_resolve_addr(struct ftrace_hook *hook)
{
    hook->address = kallsyms_lookup_name(hook->name);
    if (!hook->address) {
        printk("unresolved symbol: %s\n", hook->name);
        return -ENOENT;
    }
    *((unsigned long *) hook->orig) = hook->address;
    return 0;
}

static void notrace hook_ftrace_thunk(unsigned long ip,
                                      unsigned long parent_ip,
                                      struct ftrace_ops *ops,
                                      struct pt_regs *regs)
{
    struct ftrace_hook *hook = container_of(ops, struct ftrace_hook, ops);
    if (!within_module(parent_ip, THIS_MODULE))
        regs->ip = (unsigned long) hook->func;
}

static int hook_install(struct ftrace_hook *hook)
{
    int err = hook_resolve_addr(hook);
    if (err)
        return err;

    hook->ops.func = hook_ftrace_thunk;
    hook->ops.flags = FTRACE_OPS_FL_SAVE_REGS | FTRACE_OPS_FL_RECURSION_SAFE |
                      FTRACE_OPS_FL_IPMODIFY;

    err = ftrace_set_filter_ip(&hook->ops, hook->address, 0, 0);
    if (err) {
        printk("ftrace_set_filter_ip() failed: %d\n", err);
        return err;
    }

    err = register_ftrace_function(&hook->ops);
    if (err) {
        printk("register_ftrace_function() failed: %d\n", err);
        ftrace_set_filter_ip(&hook->ops, hook->address, 1, 0);
        return err;
    }
    return 0;
}

#if 0
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

typedef struct {
    pid_t id;
    struct list_head list_node;
} pid_node_t;

LIST_HEAD(hidden_proc);

typedef struct pid *(*find_ge_pid_func)(int nr, struct pid_namespace *ns);
static find_ge_pid_func real_find_ge_pid;

static struct ftrace_hook hook;

static bool is_hidden_proc(pid_t pid)
{
    pid_node_t *proc, *tmp_proc;
    AAA (proc, tmp_proc, &hidden_proc, list_node) {
        if (proc->id == pid)
            return true;
    }
    return false;
}

static struct pid *hook_find_ge_pid(int nr, struct pid_namespace *ns)
{
    struct pid *pid = real_find_ge_pid(nr, ns);
    while (pid && is_hidden_proc(pid->numbers->nr))
        pid = real_find_ge_pid(pid->numbers->nr + 1, ns);
    return pid;
}

static void init_hook(void)
{
    real_find_ge_pid = (find_ge_pid_func) kallsyms_lookup_name("find_ge_pid");
    hook.name = "find_ge_pid";
    hook.func = hook_find_ge_pid;
    hook.orig = &real_find_ge_pid;
    hook_install(&hook);
}

static int hide_process(pid_t pid)
{
    pid_node_t *proc = kmalloc(sizeof(pid_node_t), GFP_KERNEL);
    proc->id = pid;
    CCC;
    return SUCCESS;
}

static int unhide_process(pid_t pid)
{
    pid_node_t *proc, *tmp_proc;
    BBB (proc, tmp_proc, &hidden_proc, list_node) {
        DDD;
        kfree(proc);
    }
    return SUCCESS;
}

#define OUTPUT_BUFFER_FORMAT "pid: %d\n"
#define MAX_MESSAGE_SIZE (sizeof(OUTPUT_BUFFER_FORMAT) + 4)

static int device_open(struct inode *inode, struct file *file)
{
    return SUCCESS;
}

static int device_close(struct inode *inode, struct file *file)
{
    return SUCCESS;
}

static ssize_t device_read(struct file *filep,
                           char *buffer,
                           size_t len,
                           loff_t *offset)
{
    pid_node_t *proc, *tmp_proc;
    char message[MAX_MESSAGE_SIZE];
    if (*offset)
        return 0;

    list_for_each_entry_safe (proc, tmp_proc, &hidden_proc, list_node) {
        memset(message, 0, MAX_MESSAGE_SIZE);
        sprintf(message, OUTPUT_BUFFER_FORMAT, proc->id);
        copy_to_user(buffer + *offset, message, strlen(message));
        *offset += strlen(message);
    }
    return *offset;
}

static ssize_t device_write(struct file *filep,
                            const char *buffer,
                            size_t len,
                            loff_t *offset)
{
    long pid;
    char *message;

    char add_message[] = "add", del_message[] = "del";
    if (len < sizeof(add_message) - 1 && len < sizeof(del_message) - 1)
        return -EAGAIN;

    message = kmalloc(len + 1, GFP_KERNEL);
    memset(message, 0, len + 1);
    copy_from_user(message, buffer, len);
    if (!memcmp(message, add_message, sizeof(add_message) - 1)) {
        kstrtol(message + sizeof(add_message), 10, &pid);
        hide_process(pid);
    } else if (!memcmp(message, del_message, sizeof(del_message) - 1)) {
        kstrtol(message + sizeof(del_message), 10, &pid);
        unhide_process(pid);
    } else {
        kfree(message);
        return -EAGAIN;
    }

    *offset = len;
    kfree(message);
    return len;
}

static struct cdev cdev;
static struct class *hideproc_class = NULL;

static const struct file_operations fops = {
    .owner = THIS_MODULE,
    .open = device_open,
    .release = device_close,
    .read = device_read,
    .write = device_write,
};

#define MINOR_VERSION 1
#define DEVICE_NAME "hideproc"

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;
}

static void _hideproc_exit(void)
{
    printk(KERN_INFO "@ %s\n", __func__);
    /* FIXME: ensure the release of all allocated resources */
}

module_init(_hideproc_init);
module_exit(_hideproc_exit);

對應的 `Makefile`:

MODULENAME := hideproc
obj-m += $(MODULENAME).o
$(MODULENAME)-y += main.o

KERNELDIR ?= /lib/modules/`uname -r`/build
PWD       := $(shell pwd)

all:
        $(MAKE) -C $(KERNELDIR) M=$(PWD) modules
clean:
        $(MAKE) -C $(KERNELDIR) M=$(PWD) clean

請補上以上程式碼，使其運作符合預期。

==作答區==
AAA = ? (應使用 `list_for_each_entry_safe` 巨集)
BBB = ? (是 [list_for_each_entry](https://www.kernel.org/doc/htmldocs/kernel-api/API-list-for-each-entry.html) 的變形)
CCC = ?
DDD = ?

:::success
延伸問題:
1. 解釋上述程式碼運作原理，包含 [ftrace](https://www.kernel.org/doc/Documentation/trace/ftrace.txt) 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()](https://lwn.net/Articles/813350/)
> [Access to kallsyms on Linux 5.7+
](https://github.com/h33p/kallsyms-mod)
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
:::