Replace functions by loading library symbols and writing a unconditional jump. (x86 only)

reljmp.c

/*
 * Copyright (C) 2021 Hunter Kohler <jhunterkohler@gmail.com>
 */
#include <string.h>
#include <unistd.h>
#include <sys/mman.h>
#include "reljump.h"

size_t page_size()
{
    static size_t ret;
    return ret ? ret : (ret = sysconf(_SC_PAGESIZE));
}

void *pageof(const void *addr)
{
    return (void *)((uintptr_t)addr & ~(page_size() - 1));
}

int replace_func(void *func, void *repl)
{
    struct reljmp op = { 0xE9, repl - (func + sizeof(op)) };

    void *page = pageof(func);
    int error = mprotect(page, sizeof(op), PROT_WRITE);
    if (error)
        return error;

    memcpy(func, &op, sizeof(op));

    error = mprotect(page, sizeof(op), PROT_READ | PROT_EXEC);
    return error;
}

reljmp.h

/*
 * Copyright (C) 2021 Hunter Kohler <jhunterkohler@gmail.com>
 */
#ifndef RELJUMP_H_
#define RELJUMP_H_

#include <stdint.h>

struct reljmp {
    uint8_t opcode;
    int32_t len;
} __attribute__((packed));

int replace_func(void *func, void *repl);

#endif