reader.c

/* Privileged helper tool to read data from /proc/mem */
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdint.h>
#include <sys/mman.h>
#include <fcntl.h>

#define PAGE_SIZE 4096

int main(int argc, char** argv) {
    if(argc < 4) {
        fprintf(stderr, "usage %s <address> <size> <file>", argv[0]);
        return -1;
    }
    
    uint64_t phys_addr = strtoull(argv[1], 0, 0);
    uint64_t size = strtoull(argv[2], 0, 0);
    char* filename = argv[3];

    int mem_fd = open("/dev/mem", O_RDWR | O_SYNC);
    uint8_t *map_base = mmap(NULL,
			(size + 2 * PAGE_SIZE) - (size % PAGE_SIZE),
			PROT_READ | PROT_WRITE,
			MAP_SHARED,
			mem_fd,
			phys_addr & 0x7ffffffff000ull);	// phys_addr should be page-aligned.	

    uint8_t* virt_addr = map_base + (phys_addr & 0xfff);
    printf("reading %lld bytes @ 0x%llx\n", size, phys_addr);
    
    FILE* f = fopen(filename, "wb");
    fwrite(virt_addr, 1, size, f);
    fclose(f);
    return 0;
}