accept() 不会阻止程序执行

Question

我开始用 asm (NASM) 编写网络程序，从技术上讲，accept 函数会阻塞程序（被动套接字）。好吧，在我的程序中，我执行程序并且程序完成。我已经测试过将 backlog 设置为 1（监听功能），但这不是问题......会发生什么？

BITS 32

section .text
global _start
_start:
; Create the socket file descriptor
; int socket(int domain, int type, int protocol);

mov eax, 102  ; __NR_socketcall
mov ebx, 1    ; socketcall type (socket)

; socket parameters
push 0  ; IPPROTO_TCP
push 1  ; SOCK_STREAM
push 2  ; AF_INET

int 0x80  ; socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)

mov edx, eax  ; edx = socketfd


; Bind the socket with an address type
; int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen);

mov eax, 102  ; __NR_socketcall
mov ebx, 2    ; socketcall type (bind)

; build the sockaddr_in struct
push 0            ; INADDR_ANY
push WORD 0x0457  ; port 1111
push WORD 2       ; AF_INET
mov ecx, esp      ; struct ptr

; bind parameters
push 16   ; sockaddr struct size = sizeof(struct sockaddr) = 16
push ecx  ; sockaddr_in struct ptr
push edx  ; socket fd

int 0x80  ; bind(sockfd, {AF_INET, 11111, INADDR_ANY}, 16)


; Prepare to listen the incoming connection (passive socket)
; int listen(int sockfd, int backlog);

mov eax, 102  ; __NR_socketcall
mov ebx, 4    ; socketcall type (listen)

; listen parameters
push 0    ; nº connections in the waiting queue (0 = accept and we haven't queue)
push edx  ; socket fd

int 0x80  ; listen(sockfd, 0);


; Accept the incoming connection
; int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);

mov eax, 102  ; __NR_socketcall
mov ebx, 5    ; socketcall type (accept)

; accept parameters
push 0
push 0
push edx  ; socket fd

int 0x80  ; accept(sockfd, NULL, NULL)

; Exit
; int exit(int status)

mov eax, 1 ; __NR_exit
mov ebx, 0 ; exit code

int 0x80

Answer 1

您在每次参数推送的最后一次推送之后缺少mov ecx, esp，以及htons'ing 端口号。您的代码的固定版本应如下所示：

BITS 32

section .text
global _start
_start:
; Create the socket file descriptor
; int socket(int domain, int type, int protocol);

mov eax, 102  ; __NR_socketcall
mov ebx, 1    ; socketcall type (socket)

; socket parameters
push 6  ; IPPROTO_TCP
push 1  ; SOCK_STREAM
push 2  ; AF_INET
mov ecx, esp ; <<== uargs* in ecx

int 0x80  ; socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)

mov edx, eax  ; edx = socketfd


; Bind the socket with an address type
; int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen);

mov eax, 102  ; __NR_socketcall
mov ebx, 2    ; socketcall type (bind)

; build the sockaddr_in struct
push 0            ; INADDR_ANY
push WORD 0x5704  ; port 1111 == htons(1111)
push WORD 2       ; AF_INET
mov ecx, esp      ; struct ptr

; bind parameters
push 16   ; sockaddr struct size = sizeof(struct sockaddr) = 16
push ecx  ; sockaddr_in struct ptr
push edx  ; socket fd
mov ecx, esp      ; <<== uargs* in ecx

int 0x80  ; bind(sockfd, {AF_INET, 11111, INADDR_ANY}, 16)


; Prepare to listen the incoming connection (passive socket)
; int listen(int sockfd, int backlog);

mov eax, 102  ; __NR_socketcall
mov ebx, 4    ; socketcall type (listen)

; listen parameters
push 0    ; nº connections in the waiting queue (0 = accept and we haven't queue)
push edx  ; socket fd
mov ecx, esp      ; <<== uargs* in ecx

int 0x80  ; listen(sockfd, 0);


; Accept the incoming connection
; int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);

mov eax, 102  ; __NR_socketcall
mov ebx, 5    ; socketcall type (accept)

; accept parameters
push 0
push 0
push edx  ; socket fd
mov ecx, esp      ; struct ptr

int 0x80  ; accept(sockfd, NULL, NULL)

; Exit
; int exit(int status)

mov eax, 1 ; __NR_exit
mov ebx, 0 ; exit code

int 0x80

在这种情况下，您在程序上执行strace 以验证您看到系统调用中处理的参数是否正确非常重要。

如果我们strace您的原始程序，我们会得到：

socket(PF_UNSPEC, 0, 0)                 = -1 EFAULT (Bad address)
bind(1459879938, NULL, 2)               = -1 EBADF (Bad file descriptor)
listen(1459879938, 0)                   = -1 EBADF (Bad file descriptor)
accept(1459879938, 0, 0x2)              = -1 EBADF (Bad file descriptor)

所有这些看起来都很糟糕。

如果您查看compat_sys_socketcall 的来源，它会显示：

asmlinkage long compat_sys_socketcall(int call, u32 __user *args)

这意味着 EBX 是调用，ECX 指向参数的其余部分。