assembly function segfault at return (nasm x64)

Question

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

That's what I thought but then why no problem when i remove read and write blocks ? What in them modifies stack alignement ? In read i dont where i touch the stack and in write i push then pop. — Rox Teddy
– Rox Teddy, Commented Mar 29, 2015 at 20:47

Jester · Accepted Answer · 2015-03-29 20:57:51Z

1

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.

answered Mar 29, 2015 at 20:57

Jester

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4 Comments

Jester Over a year ago

Actually both for the read and the write syscall where you set up mov rsi, rbp ; 2nd param - buffer.

Rox Teddy Over a year ago

Thank you it works. I don't really understand why yet but l'll try to figure out. How comes we need to save ebp at start but it keeps reseting at each syscall. And also if my MOV were bad how can I read all of my file and it crash only at return. Thank you again ;)

Jester Over a year ago

ebp does not reset. You allocated your buffer at ebp-16 so you need to pass that as address, otherwise you will overwrite the return address on the stack. That will fault at the ret because that will try to grab it from the stack and jump there.

Rox Teddy Over a year ago

alriiight. I get it now. Thanks a lot !

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