Assembly x86 brk() call use

by

others have pointed out a few things that are wrong with your code. I would like to add that you would not add 20 bits to the current breakpoint (or 20 bytes like add rbx, 20 actually does), you would simply add 5 bytes.

Also, your first syscall argument will not be in rbx, it will be in rdi. The 64-bit syscall ABI uses different system call numbers, different registers, and a different instruction (syscall instead of int 0x80) than the 32-bit ABI (which is still available in 64-bit processes). See also the x86 tag wiki for more ABI links.

Here’s how your code should look:

push rbp
mov rbp, rsp

;; sys_brk(0)
mov   rax, 12         ; 12 is SYS_brk (/usr/include/asm/unistd_64.h)
mov   rdi, 0          ; rdi for first syscall arg in the 64-bit ABI, not rbx
syscall               ; syscall, not int 0x80, for the 64-bit ABI

mov   qword [brk_firstLocation], rax

;; sys_brk(old_break + 5)
lea   rdi, [rax + 5]  ; add 5 bytes to the break point
mov   rax, 12
syscall               ; set the new breakpoint

At this point you can use brk_firstLocation as a pointer to whatever 5 byte struct you want to store on the heap. Here’s how you would put values in that memory space:

mov   rdi, [brk_firstLocation]   ; load the pointer from memory, if you didn't already have it in a register

mov   byte [rdi], 'A'            ; a char in the first byte
mov   [rdi+1], ecx               ; a 32-bit value in the last 4 bytes.

More Related Contents:

  1. What are the calling conventions for UNIX & Linux system calls (and user-space functions) on i386 and x86-64
  2. Where is the x86-64 System V ABI documented?
  3. What registers are preserved through a linux x86-64 function call
  4. glibc scanf Segmentation faults when called from a function that doesn’t align RSP
  5. Can’t call C standard library function on 64-bit Linux from assembly (yasm) code
  6. Using printf in assembly leads to empty output when piping, but works on the terminal
  7. Printing an integer as a string with AT&T syntax, with Linux system calls instead of printf
  8. How are the fs/gs registers used in Linux AMD64?
  9. Why do x86-64 Linux system calls modify RCX, and what does the value mean?
  10. Calling printf in x86_64 using GNU assembler
  11. Linux default behavior of executable .data section changed between 5.4 and 5.9?
  12. What does @plt mean here?
  13. Why is RCX not used for passing parameters to system calls, being replaced with R10? [duplicate]
  14. Number of executed Instructions different for Hello World program Nasm Assembly and C
  15. Segmentation fault on printf – NASM 64bit Linux
  16. Running 32 bit assembly code on a 64 bit Linux & 64 bit Processor : Explain the anomaly
  17. How to convert Linux 32-bit gcc inline assembly to 64-bit code? [closed]
  18. Linux x64: why does r10 come before r8 and r9 in syscalls?
  19. Writing a putchar in Assembly for x86_64 with 64 bit Linux?
  20. x86_64 Assembly Linux System Call Confusion
  21. Assembling 32-bit binaries on a 64-bit system (GNU toolchain)
  22. Nasm segmentation fault on RET in _start
  23. What are the return values of system calls in Assembly?
  24. How to make an executable ELF file in Linux using a hex editor?
  25. What’s the difference between “statically linked” and “not a dynamic executable” from Linux ldd?
  26. Compiler using local variables without adjusting RSP
  27. How to print a character in Linux x86 NASM?
  28. Is garbage allowed in high bits of parameter and return value registers in x86-64 SysV ABI?
  29. Trying to understand gcc’s complicated stack-alignment at the top of main that copies the return address
  30. How do i read single character input from keyboard using nasm (assembly) under ubuntu?

Leave a Comment