/*
 * wxdemo.c
 *
 * OpenBSD W^X demonstration.
 *
 * This program allocates an anonymous RWX page, writes machine code
 * into it, and executes that code.
 *
 * Build:
 *     cc -Wall -Wextra -O2 -Wl,-z,wxneeded wxdemo.c -o wxdemo
 *
 * On a normal filesystem:
 *     ./wxdemo
 *
 * On a filesystem mounted with wxallowed:
 *     ./wxdemo
 *
 * This example is for amd64.
 */

#include <sys/mman.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

int
main(void)
{
    /*
     * amd64のマシンコード
     * 呼出側に整数の42を返す
     *
     *     mov $42, %eax
     *     ret
     *
     * Machine code:
     *     b8 2a 00 00 00 c3
     */
    unsigned char code[] = {
        0xb8, 0x2a, 0x00, 0x00, 0x00,
        0xc3
    };

    size_t pagesize = (size_t)getpagesize();

    /* コード実行とデータの書き込み両方が
       可能なメモリ領域を取得 */
    void *mem = mmap(NULL, pagesize,
        PROT_READ | PROT_WRITE | PROT_EXEC,
        MAP_ANON | MAP_PRIVATE,
        -1, 0);

    if (mem == MAP_FAILED) {
        perror("mmap(RWX)");
        return 1;
    }

    printf("RWX mapping succeeded: %p\n", mem);

    /* 取得したメモリ領域にマシンコードを書き込み */
    memcpy(mem, code, sizeof(code));

    /* 書き込んだマシンコードを関数として定義 */
    int (*function)(void) = (int (*)(void))mem;

    /* 定義した関数を実行 */
    printf("Executing generated code...\n");
    int result = function();

    printf("Generated code returned: %d\n", result);

    if (munmap(mem, pagesize) == -1) {
        perror("munmap");
        return 1;
    }

    return 0;
}