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gcc -fstack-protector

The protected c function

This simple function will be compiled with and without stack protection:

void g(char*);

void f() {

   char x[10];
   g(x);

}

Github repository about-gcc, path: /options/f/stack/protector/func.c

The Makefile

This Makefile produces three assembly files from the c function, one without stack protection, one compiled with -fstack-protector and one compiled with -fstack-protector-strong:

all: no-protection.S  stack-protector.S stack-protector-strong.S

no-protection.S: func.c
	gcc -S -masm=intel $< -o $@

stack-protector.S: func.c
	gcc -S -masm=intel $< -o $@ -fstack-protector

stack-protector-strong.S: func.c
	gcc -S -masm=intel $< -o $@ -fstack-protector-strong

Github repository about-gcc, path: /options/f/stack/protector/Makefile

Produced difference

When compiled with -fstack-protector or -fstack-protector-strong, the compiler inserted the following code before the call to g:

sub	rsp, 32
mov	rax, QWORD PTR fs:40
mov	QWORD PTR -8[rbp], rax
xor	eax, eax
lea	rax, -18[rbp]

and this code after the call:

mov	rax, QWORD PTR -8[rbp]
sub	rax, QWORD PTR fs:40
je	.L2
call	__stack_chk_fail@PLT
L2:

I was unable to detect a difference between compiling with -fstack-protector and -fstack-protector-strong.

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