my gcc version is 4.8.2 and operating system is ubuntu 14.04 (64 bit).I found that sometimes gcc auto generate the canary to do buffer overflow protection sometimes not, why?
case to generate canary: when SIZE is multiple of four
#include<stdio.h>#define SIZE 4int main(){ char s[SIZE]; scanf("%s", s); return 0;}asm after gcc -c -g -Wa,-a,-ad
... 4:a.c **** int main() 5:a.c **** { 13 .loc 1 5 0 14 .cfi_startproc 15 0000 55 pushq %rbp 16 .cfi_def_cfa_offset 16 17 .cfi_offset 6, -16 18 0001 4889E5 movq %rsp, %rbp 19 .cfi_def_cfa_register 6 20 0004 4883EC10 subq $16, %rsp 21 .loc 1 5 0 22 0008 64488B04 movq %fs:40, %rax 22 25280000 22 00 23 0011 488945F8 movq %rax, -8(%rbp) 24 0015 31C0 xorl %eax, %eax 6:a.c **** char s[SIZE]; 7:a.c **** scanf("%s", s);...case not to generate canary : not the multiple of four
#include<stdio.h>#define SIZE 2int main(){ char s[SIZE]; scanf("%s", s); return 0;}asm after gcc -c -g -Wa,-a,-ad
... 4:a.c **** int main() 5:a.c **** { 13 .loc 1 5 0 14 .cfi_startproc 15 0000 55 pushq %rbp 16 .cfi_def_cfa_offset 16 17 .cfi_offset 6, -16 18 0001 4889E5 movq %rsp, %rbp 19 .cfi_def_cfa_register 6 20 0004 4883EC10 subq $16, %rsp 6:a.c **** char s[SIZE]; 7:a.c **** scanf("%s", s);...