为什么 macOS 上的动态链接库似乎禁用了 ASLR？

Question

If I'm not mistaken, the following code should print out different addresses every time it's run. However, it always displays the same address for isupper (for example).

Code:

#include <stdio.h>
#include <dlfcn.h>

int main(int argc, char *argv[]) {
   printf("isspace @ %p\n", dlsym(RTLD_DEFAULT, "isspace"));
   return 0;
}

Output:

$ ./libc-simple 
isspace @ 0x7fff76b63035
$ ./libc-simple 
isspace @ 0x7fff76b63035
$ ./libc-simple 
isspace @ 0x7fff76b63035
$ ./libc-simple 
isspace @ 0x7fff76b63035

(I'm running macOS 10.14.6, but I tested the same code on another computer running macOS 10.15 with the same result.)

What am I missing?

Answer 1

As an optimization, macOS uses a shared mapping for a lot of the system libraries. They are loaded once at boot and used by all processes. For a given boot, the address is constant across all such processes. However, the address is randomized each boot for security.