mask_ptr.c

#define _GNU_SOURCE

#include <stdio.h>
#include <stdint.h>

#include <err.h>
#include <sys/mman.h>
#include <sys/random.h>
#include <unistd.h>

void* get_random_ptr() {
    void* res;
    if (getrandom(&res, sizeof(res), 0) != sizeof(res)) {
        err(1, "failed to get random bytes");
    }
    return res;
}

void* mask_ptr(void* x) {
  return (void*)((uintptr_t)x & 0x7ffffffff000ULL);
}

int main() {
    void* raw_addr = get_random_ptr();
    void* requested_addr = mask_ptr(raw_addr);
    printf("raw: %p\n", raw_addr);
    printf("req: %p\n", requested_addr);

    void* real_addr = mmap(
        requested_addr,
        1 << 16,
        PROT_READ | PROT_WRITE,
        MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED,
        -1,
        0);

    if (real_addr == MAP_FAILED) {
        err(1, "mmap() failed");
    }
    printf("got: %p\n", real_addr);

    return 0;
}

res_aarch64.txt

> lscpu | grep 'Model name'
Model name:                      Cortex-A53

> llvm-objdump --disassemble-symbols=mask_ptr --no-leading-addr ./mask_ptr
<mask_ptr>:
 00 8c 74 92            and     x0, x0, #0xfffffffff000
 c0 03 5f d6            ret

> gcc -O2 -std=c17 -o mask_ptr mask_ptr.c && ./mask_ptr
raw: 0xc286004d2cb683e
req: 0x6004d2cb6000
got: 0x6004d2cb6000

res_riscv64.txt

> rg 'uarch' /proc/cpuinfo | sort -u
uarch           : sifive,u74-mc

> llvm-objdump --disassemble-symbols=mask_ptr --no-leading-addr ./mask_ptr
<mask_ptr>:
 b7 07 00 e0            lui     a5, 917504
 c5 83                  srli    a5, a5, 17
 7d 8d                  and     a0, a0, a5
 82 80                  ret

user@starfive /home/user/mask_ptr> gcc -O2 -std=c17 -o mask_ptr mask_ptr.c && ./mask_ptr
raw: 0xdf3bc2042627c2f7
req: 0x42042627c000
mask_ptr: mmap() failed: Cannot allocate memory

res_x86_64.txt

> rg 'model name' /proc/cpuinfo | sort -u
model name      : 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz

> llvm-objdump --disassemble-symbols=mask_ptr --no-leading-addr --x86-asm-syntax=intel ./mask_ptr
<mask_ptr>:
 48 b8 00 f0 ff ff ff 7f 00 00          movabs  rax, 0x7ffffffff000
 48 21 f8                               and     rax, rdi
 c3                                     ret

> gcc -O2 -std=c17 -o mask_ptr mask_ptr.c && ./mask_ptr
raw: 0x595cd8dfb43916b7
req: 0x58dfb4391000
got: 0x58dfb4391000