Is shadow call stack in llvm 7 ok?

Hi

I try the shadow call stack in llvm.

clang -fsanitize=shadow-call-stack ./test.c

test.c

#include<stdio.h>

void A(){
printf(“A\n”);
}

int main(){
A();
return 0;
}

And then produe weird code

0000000000400570 :
400570: 4c 8b 14 24 mov r10,QWORD PTR [rsp]
400574: 4d 31 db xor r11,r11
400577: 65 49 83 03 08 add QWORD PTR gs:[r11],0x8
40057c: 65 4d 8b 1b mov r11,QWORD PTR gs:[r11]
400580: 65 4d 89 13 mov QWORD PTR gs:[r11],r10

at 0x400577 the program crashes

because r11 is 0 after 0x400574 .

I wonder that shadow call stack in llvm 7 is ok now?

Thank you.

Hi,

at 0x400577 the program crashes

because r11 is 0 after 0x400574 .

Looking at ShadowCallStack.cpp, the 0 is very intentional. So to use
this feature you'll need a runtime willing to give each thread a valid
shadow stack and set the base and of the gs register to point at it.

The documentation mentions that you'll probably have to write your own
runtime: https://clang.llvm.org/docs/ShadowCallStack.html. You didn't
mention which OS you were using, but these notes seem to cover what
would be needed to actually set GS on various platforms:
https://gist.github.com/MerryMage/f22e75d5128c07d77630ca01c4272937.

You'd have to come up with your own methods to make sure that happens
on each thread before any instrumented code runs.

Cheers.

Tim.