Using Structures in NASM x64 Assembly
25 September, 2018 - 19:20
There isn't a lot of examples out there on using structures in assembly, I'll preface this post by letting you know ahead of time that you should know x64 assembly. The reason you may want to do this is, for example you want to perform this operation in C (stripped from a port bind code):
// Create sockaddr_in struct (man 7 ip)
struct sockaddr_in host_addr;
// AF_INET for IPv4
host_addr.sin_family = AF_INET;
// Set port number to 1234, set to network byte order by htons
host_addr.sin_port = htons(1234);
// Listen on any interface
host_addr.sin_addr.s_addr = inet_addr("192.168.1.16");
struct sockaddr_in host_addr;
// AF_INET for IPv4
host_addr.sin_family = AF_INET;
// Set port number to 1234, set to network byte order by htons
host_addr.sin_port = htons(1234);
// Listen on any interface
host_addr.sin_addr.s_addr = inet_addr("192.168.1.16");
and rather than doing it using conventional methods such as pushing values to the stack and using the stack pointer, I wanted a more natural approach by using actual C style structure. So normally, a C structure is defined as so:
struct Customer{
int id;
char name[64];
char address[64];
int balance;
};
struct Customer calvin;
calvin.name ="Calvin";
...
int id;
char name[64];
char address[64];
int balance;
};
struct Customer calvin;
calvin.name ="Calvin";
...
to do this in assembly, you simply write everything like so, I've gone ahead and included a method of accessing the parameters using a simple write syscall to print Calvin's name and address then exit:
segment .data
struc Customer
c_id resb 4
c_name resb 64
c_address resb 64
c_balance resb 4
endstruc
calvin istruc Customer
at c_id , dd 7
at c_name , db "Calvin" ,
at c_address , db " 12 Mockingbird Lane",
at c_balance , dd 12500
iend
segment .text
global _start
_start:
xor rax, rax
mov rax,1
xor rdi, rdi
mov rdi,1
xor rsi,rsi
mov rsi, calvin + c_name ;remember you are using the address of calvin and moving all the way up to the address of c_name.
mov rdx, 7
syscall
xor rax, rax
mov rax,1
xor rdi, rdi
mov rdi,1
xor rsi,rsi
mov rsi, calvin + c_address
mov rdx, 24
syscall
xor rax,rax
mov rax,60
xor rdi,rdi
syscall
struc Customer
c_id resb 4
c_name resb 64
c_address resb 64
c_balance resb 4
endstruc
calvin istruc Customer
at c_id , dd 7
at c_name , db "Calvin" ,
at c_address , db " 12 Mockingbird Lane",
at c_balance , dd 12500
iend
segment .text
global _start
_start:
xor rax, rax
mov rax,1
xor rdi, rdi
mov rdi,1
xor rsi,rsi
mov rsi, calvin + c_name ;remember you are using the address of calvin and moving all the way up to the address of c_name.
mov rdx, 7
syscall
xor rax, rax
mov rax,1
xor rdi, rdi
mov rdi,1
xor rsi,rsi
mov rsi, calvin + c_address
mov rdx, 24
syscall
xor rax,rax
mov rax,60
xor rdi,rdi
syscall
And that's pretty much it! 
Later on I may write a full guide detailing how to write shellcode using Linux, if anything, I'll post it here.