# 2020q1 Homework4 (khttpd)
contributed by < `nickyanggg` >
###### tags: `linux2020`

## [作業要求](https://hackmd.io/@sysprog/linux2020-khttpd#-%E4%BD%9C%E6%A5%AD%E8%A6%81%E6%B1%82)
- [自我檢查清單](https://hackmd.io/@sysprog/linux2020-khttpd#-%E8%87%AA%E6%88%91%E6%AA%A2%E6%9F%A5%E6%B8%85%E5%96%AE)
- 將 [fibdrv 作業](https://hackmd.io/@nickyanggg/fibdrv) 整合進來

## 實驗環境

$ uname -a
Linux nickyanggg-ubuntu18 5.3.0-42-generic #34~18.04.1-Ubuntu SMP Fri Feb 28 13:42:26 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

$ gcc --version
gcc (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0

## Kernel module 的參數傳遞
觀察 `main.c`，可發現預設的 port 為 8081，而透過掛載的時候加上 `port=1999` 可將參數傳入，推測是透過 `module_param`，於是就追進去看。

#define DEFAULT_PORT 8081

static ushort port = DEFAULT_PORT;
module_param(port, ushort, S_IRUGO);

static int __init khttpd_init(void)
{
    int err = open_listen_socket(port, backlog, &listen_socket);
    if (err < 0) {
        pr_err("can't open listen socket\n");
        return err;
    }
    param.listen_socket = listen_socket;
    http_server = kthread_run(http_server_daemon, &param, KBUILD_MODNAME);
    if (IS_ERR(http_server)) {
        pr_err("can't start http server daemon\n");
        close_listen_socket(listen_socket);
        return PTR_ERR(http_server);
    }
    return 0;
}

參考 [Linux drivers module param](https://b8807053.pixnet.net/blog/post/339227792-linux-driver-modules-param)，發現 `module_param` 為 [<linux/moduleparam.h>](https://github.com/torvalds/linux/blob/master/include/linux/moduleparam.h) 中所定義的巨集，透過它可以在啟動系統或模組裝載時指定參數值。
- `name`：使用者看到的參數名，也是模組內接受參數的變數。
- `type`：參數的類型。
- `perm`：sysfs 中相應檔案的存取權限。

/**
 * module_param - typesafe helper for a module/cmdline parameter
 * @name: the variable to alter, and exposed parameter name.
 * @type: the type of the parameter
 * @perm: visibility in sysfs.
 *
 * @name becomes the module parameter, or (prefixed by KBUILD_MODNAME and a
 * ".") the kernel commandline parameter.  Note that - is changed to _, so
 * the user can use "foo-bar=1" even for variable "foo_bar".
 *
 * @perm is 0 if the the variable is not to appear in sysfs, or 0444
 * for world-readable, 0644 for root-writable, etc.  Note that if it
 * is writable, you may need to use kernel_param_lock() around
 * accesses (esp. charp, which can be kfreed when it changes).
 *
 * The @type is simply pasted to refer to a param_ops_##type and a
 * param_check_##type: for convenience many standard types are provided but
 * you can create your own by defining those variables.
 *
 * Standard types are:
 *	byte, short, ushort, int, uint, long, ulong
 *	charp: a character pointer
 *	bool: a bool, values 0/1, y/n, Y/N.
 *	invbool: the above, only sense-reversed (N = true).
 */
#define module_param(name, type, perm)				\
	module_param_named(name, name, type, perm)

## kHTTPd web 伺服器
> 出處：[CS:APP 第 11 章](https://hackmd.io/s/ByPlLNaTG)

`khttpd_init` 中的 `open_listen_socket` 基本上做的事就是開一個 socket、bind 到一個 port 上並開始 listen，和圖中的 `open_listenfd` 做的事一樣。接著會透過 kernel thread 去執行 `http_server_daemon`，當中的 `kernel_accept` 相當於圖中的 `accept` 是用來等待 client 端的連線。在處理 client 的 request 時，會另外開一條 thread 去執行 `http_server_worker`，讓新開的 socket 去和 client 連線接收 request 封包。因此原本的 socket 只要負責 listen 即可，一旦有 request 則透過新建的 socket 去處理。

int http_server_daemon(void *arg)
{
    struct socket *socket;
    struct task_struct *worker;
    struct http_server_param *param = (struct http_server_param *) arg;

    allow_signal(SIGKILL);
    allow_signal(SIGTERM);

    while (!kthread_should_stop()) {
        int err = kernel_accept(param->listen_socket, &socket, 0);
        if (err < 0) {
            if (signal_pending(current))
                break;
            pr_err("kernel_accept() error: %d\n", err);
            continue;
        }
        worker = kthread_run(http_server_worker, socket, KBUILD_MODNAME);
        if (IS_ERR(worker)) {
            pr_err("can't create more worker process\n");
            continue;
        }
    }
    return 0;
}

## 整合 [fibdrv 作業](https://hackmd.io/OWwxp6sWRbKnbR-LZO0WOg) 進 kHTTPd
- include [<bignum.h>](https://github.com/nickyanggg/khttpd/blob/master/bignum.h)
- 將原本寫死的 `Hello world` 以及字串長度改寫成 `%d`、`%s`。
- 將 `http_server_response` 回傳成功的部分改寫成呼叫 `fib_url`。
- `fib_url` 會檢查是否為正確格式，並且呼叫 `fib_sequence` (fast doubling) 來計算應當回傳的數字。
- 因為 `bignum` 實作的方式，目前只會去計算 fib(4500) 以內的數。
- `fib_sequence` 的回傳內容會是反向的，所以要將它轉向。
- malloc 的空間要多加 `length * 2` 是因為除了 `%s` (fib 所得到的結果) 的長度之外，還要再加上 `%d` 的長度，那這邊就直接取一個確定大等於它的數。
- 最後 response 完之後要釋放回傳內容所用到的空間。

#define HTTP_RESPONSE_200_DUMMY                               \
    ""                                                        \
    "HTTP/1.1 200 OK" CRLF "Server: " KBUILD_MODNAME CRLF     \
    "Content-Type: text/plain" CRLF "Content-Length: %d" CRLF \
    "Connection: Close" CRLF CRLF "%s" CRLF
#define HTTP_RESPONSE_200_KEEPALIVE_DUMMY                     \
    ""                                                        \
    "HTTP/1.1 200 OK" CRLF "Server: " KBUILD_MODNAME CRLF     \
    "Content-Type: text/plain" CRLF "Content-Length: %d" CRLF \
    "Connection: Keep-Alive" CRLF CRLF "%s" CRLF
    
static char *fib_url(char *url, int keep_alive)
{
    char *response = keep_alive ? HTTP_RESPONSE_200_KEEPALIVE_DUMMY
                                : HTTP_RESPONSE_200_DUMMY;
    char *ret, *token, *tmp = url, result[MAX_DIGIT];
    int size, length, i;
    strsep(&tmp, "/");
    token = strsep(&tmp, "/");
    if (!strcmp(token, "fib")) {
        token = strsep(&tmp, "/");
        long num;
        kstrtol(token, 10, &num);
        // bignum max digit = 1000, fib(4500) digit = 941
        if (num > 4500 || num < 0)
            return NULL;
        char *rev_fib = fib_sequence(num)->decimal;
        length = strlen(rev_fib);
        for (i = 0; i < length; i++)
            result[i] = rev_fib[length - 1 - i];
        result[length] = '\0';
    } else
        return NULL;

    size = strlen(response) + length * 2;
    ret = kmalloc(size, GFP_KERNEL);
    snprintf(ret, size, response, length, result);
    return ret;
}

static int http_server_response(struct http_request *request, int keep_alive)
{
    char *response;

    pr_info("requested_url = %s\n", request->request_url);
    if (request->method != HTTP_GET)
        response = keep_alive ? HTTP_RESPONSE_501_KEEPALIVE : HTTP_RESPONSE_501;
    else
        response = fib_url(request->request_url, keep_alive);
    http_server_send(request->socket, response, strlen(response));
    kfree(response);
    return 0;
}

參考[黃鈺盛](https://gist.github.com/wilson6405/c8b86f71ce680efee26850298c9c237c)所提供的驗證程式，並改寫 `test.sh` 來驗證 $fib(1)$ ~ $fib(4500)$。

#!/usr/bin/env bash

KHTTPD_MOD=khttpd.ko

if [ "$EUID" -eq 0 ]
  then echo "Don't run this script as root"
  exit
fi

# load kHTTPd
sudo rmmod -f khttpd 2>/dev/null
sleep 1
sudo insmod $KHTTPD_MOD

# check fibonacci 0 ~ 4500
for i in {1..4500};
do
    python scripts/test.py -p 8081 -i $i
done

# epilogue
sudo rmmod khttpd
echo "Complete"

執行結果皆正確。

$ bash scripts/test.sh
Pass
Pass
Pass

...

Pass
Pass
Pass