x86_64 assembly string manipulation causes segmentation fault - c

Question

I am trying to write a "strcat" function in assembly and can't get the values in the memory I pass it to change. My tests are crashing and I don't understand why. I can't seem to find any good documentation on x86_64 assembly in an easy to digest manner either.
global _ft_strcat
_ft_strcat:
push rbx
push rdx
mov rbx, rsi
mov rdx, rdi
parse:
cmp byte [rdx], 0
je concat
inc rdx
jmp parse
concat:
cmp BYTE[rbx], 0
je finish
mov dl, BYTE[rbx]
mov BYTE[rdx], dl
inc rdx
inc rbx
jmp concat
finish:
mov BYTE[rdx], 0
mov rax, rdi
pop rdx
pop rbx
ret
The above is the function I am trying to write and below is my test.
int main(void)
{
char buffer[50] = "Hello, ";
ft_strcat(buffer, "World!");
printf("%s\n", buffer);
return (0);
}
I left out things such as includes and my header because that is not relevant to the question from what I can see. I ran this through a debugger and noticed that at the end of my function, the string pointed to by the rdi register has not changed, but I do go through the loop in the concat label and it looks like the values being extracted from the string being pointed to by rsi is indeed being copied into the dl register.

Answer 1

Your pushes and pops don't match so your routine changes rbp and rbx contrary to the ABI requirement to preserve them.

Answer 2

My problem was my lack of understand how I was manipulating the rdx register's lowest 8-bits. By inserting my character into dl, its value updated the overall value of rdx, which meant that I was not actually concatenating the string I had, but I was writing into regions of memory I didn't know I was writing into.
The code now looks like this
global _ft_strcat
_ft_strcat:
push rbx
push rdx
push rcx
xor rcx, rcx
mov rbx, rsi
mov rdx, rdi
parse:
cmp byte [rdx], 0
je concat
inc rdx
jmp parse
concat:
cmp BYTE[rbx], 0
je finish
mov cl, BYTE[rbx]
mov BYTE[rdx], cl
inc rdx
inc rbx
jmp concat
finish:
mov BYTE[rdx], 0
pop rcx
pop rdx
pop rbx
mov rax, rdi
ret
You will notice the addition of the Rex register and the use of its lower 8 bits for copying bytes over.