assembly function segfault at return (nasm x64)

My asm function segfaults at return.

Here is the function prototype : void ft_cat(int fd);

Basically it get a fd from a C main and act like cat shell command.

I get no problem if I removed read and write parts so maybe the problem is around syscalls. I just can’t tell. I already spent hours and hours looking for it.

Any clue ?

%define MACH_SYSCALL(nb) 0x2000000 | nb
%define READ 3
%define WRITE 4
%define LSEEK 19
%define STDOUT 1
%define SEEK_CUR 1

section .text
    global _ft_cat

_ft_cat:
    push rbp            ; save base pointer
    mov rbp, rsp        ; place base pointer on stack
    sub rsp, 16         ; align stack to keep 16 bytes for buffering
    push rdi            ; save function parameter (int fd)

read:
    mov rdi, [rsp]      ; 1st param - get fd from stack
    mov rsi, rbp        ; 2nd param - buffer
    mov rdx, 16         ; 3rd param - buffer size
    mov rax, MACH_SYSCALL(READ)
    syscall
    cmp rax, 0          ; if read return <= 0 jump to end
    jng end

write:
    push rax            ; save read return
    mov rdi, STDOUT     ; 1st param
    mov rsi, rbp        ; 2nd param - buffer
    mov rdx, rax        ; 3rd param - read return
    mov rax, MACH_SYSCALL(WRITE)
    syscall
    pop rax
    cmp rax, 16         ; if read return < 16 then it is finished
    je read

end:
    mov rsp, rbp        ; restore stack and base pointers
    pop rbp             ;
    ret                 ; return >> segfault

Answer

Your allocated buffer is 16 bytes below ebp but you pass ebp to the read syscall so you overwrite the saved rbp, the return address and other things in the caller’s frame. You want lea rsi, [rbp-16] instead.

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Answer