在程序集中訪問malloc'd內存

Question

我試圖訪問內存我在程序集中使用了malloced，但是我只是一再地收到段錯誤。我在下面的代碼中做了什麼錯誤，我確定它很簡單，但我看不到它！

編輯：我使用的64位NASM裝配

; Allocate room for 8 integers 
mov r8, 8 
mov rdi, r8 
imul rdi, 8 ; Multiply by 8 (8 bytes per entry in 64bit) 
xor rax, rax 
call malloc 
add rsp, 8 
test rax, rax 
jz malloc_failure 
mov r8, rsp 

; r8 now = base of array 

; Set the first element to be 100 
mov r9, 0 
add r9, r8 
mov qword [r9], 100 

malloc_failure: 
deallocate_start: 
dealloc_1: 
mov rdi, r8 
xor rax, rax 
call free 
add rsp, 8 
deallocate_end: 
call os_return  ; return to operating system 

而段錯誤（不是很有趣...）

matrix05% ./arr5 
Segmentation fault 

Answer 1

mov r8, 8 
mov rdi, r8 
imul rdi, 8 
xor rax, rax 
call malloc 
add rsp, 8  ;; here we _add_ 8 bytes to the stack pointer 
       ;; this is equivalent to _popping_ off the stack 
       ;; remember, the x86 stack grows down! 
test rax, rax ;; rax is indeed where the return value is..... but: 
jz malloc_failure 
mov r8, rsp  ;; we overwrite r8 with the stack pointer (why??) 

; r8 now = base of array ;; no it's not 

mov r9, 0 
add r9, r8  ;; r9 = r8 = stack pointer 
mov qword [r9], 100 ;; we now write 100 to the current stack pointer. 
       ;; The stack pointer initially (on entry to the function) 
       ;; pointed to a return address; where exactly are you overwriting? 

malloc_failure: 
deallocate_start: 
dealloc_1: 
mov rdi, r8 
xor rax, rax 
call free 
add rsp, 8  ;; we pop from the stack pointer _again_. I do hope there's a sub rsp, 16 at the top... 
deallocate_end: 
call os_return  ; return to operating system (and probably crash because our stack is FUBAR'd)