funcaddr.c

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <inttypes.h>

void* get_function_addr_elf_pie(const char* func_name, char* err_msg) {
    uintptr_t base_address = 0;
    FILE* maps = fopen("/proc/self/maps", "r");
    if (maps == NULL) {
        sprintf(err_msg, "Cannot open /proc/self/maps");
        return NULL;
    }

    char line[256];
    while (fgets(line, sizeof(line), maps) != NULL) {
        char perms[5];
        sscanf(line, "%" PRIxPTR "-%*x %4s", &base_address, perms);
        if (perms[2] == 'x') {
            break;
        }
    }

    fclose(maps);

    if (base_address == 0) {
        sprintf(err_msg, "Cannot find base address");
        return NULL;
    }

    FILE* offsets = fopen("maps.off.txt", "r");
    if (offsets == NULL) {
        sprintf(err_msg, "Cannot open offsets file");
        return NULL;
    }

    uintptr_t offset = 0;
    char name[256] = "";
    while (fscanf(offsets, "%lx %*s %255s", &offset, name) == 2) {
        if (strcmp(name, func_name) == 0) {
            break;
        }
    }

    fclose(offsets);

    if (strcmp(name, func_name) != 0) {
        sprintf(err_msg, "Cannot find function %s", func_name);
        return NULL;
    }

    void* func_addr = (void*)(base_address + offset - 0x1000);
    return func_addr;
}