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DevHeads IoT Integration Server•13mo ago
Marvee Amasi

Why Does movl (%eax), %edx Crash After Using Custom malloc_ Implementation in Assembly?

I have implemented a custom memory allocation function using sys_mmap for memory allocation. Here’s my code for the malloc_ function:

malloc_:
   pushq   %rbp
   movq    %rsp, %rbp

   mov     %rdi, %rcx        # store size
   movl    $9, %eax          # system call 9: sys_mmap
   movq    $0, %rdi          # start address
   movq    %rcx, %rsi        # size
   movl    $3, %edx          # page flags (PROT_READ | PROT_WRITE)
   mov     $34, %r10         # mem flags (MAP_PRIVATE | MAP_ANONYMOUS)
   movl    $-1, %r8d         # file descriptor
   movl    $0, %r9d          # offset
   syscall

   cmp     $0, %rax
   jg      .L1.malloc_exit_  # jump if allocation was successful
   mov     $0, %rax          # set return value to 0 if failed
   .L1.malloc_exit_:
   popq    %rbp
   retq


In my main function, I use malloc_ to allocate memory like this:

.globl main
main:
    pushq   %rbp
    movq    %rsp, %rbp

    mov     $512, %rdi        # size to allocate
    call    malloc_
    cmp     $0, %rax          # check if allocation failed
    je      exit
    // movl (%eax), %edx      // <---- causes a crash
    mov     (%rax), %rdx      // <---- works fine

exit:
    # Exit code here

The issue I don’t understand is why the line movl (%eax), %edx causes a crash, but using mov (%rax), %rdx works perfectly fine.

If I use the system provided malloc function instead, both lines work without issue. What could be causing this behavior with my custom malloc_ function?
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