brightness.cpp

#include <cstdlib>
#include <iostream>
#include <sys/inotify.h>
#include <sys/mman.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/unistd.h>

#define TMP_FILE "/tmp/fake.brightness"
#define HIJACK_FILE "/sys/class/backlight/intel_backlight/brightness"

#define CHECK_ERR(func, ...) ({       \
    auto result = func(__VA_ARGS__);  \
    if (result == -1) {               \
        std::perror(#func);           \
        std::exit(EXIT_FAILURE);      \
    }                                 \
    result;                           \
})

#define PRINT_LOG(pattern, ...) printf(("[*] " pattern "\n"), ##__VA_ARGS__)


int open_overlay() {
    int fd;

    if (access(TMP_FILE, F_OK) == 0) {
        CHECK_ERR(chown, TMP_FILE, 0, 0);
        CHECK_ERR(chmod, TMP_FILE, 0644);
        fd = CHECK_ERR(open, TMP_FILE, O_RDONLY);
    } else {
        fd = CHECK_ERR(open, TMP_FILE, O_CREAT | O_RDONLY, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);
    }

    return fd;
}


int init_inotify() {
    int inotify_fd = CHECK_ERR(inotify_init);
    CHECK_ERR(inotify_add_watch, inotify_fd, TMP_FILE, IN_CLOSE_WRITE);
    return inotify_fd;
}


int mount_overlay() {
    struct stat buf_overlay { }, buf_hijack { };
    CHECK_ERR(stat, TMP_FILE, &buf_overlay);
    CHECK_ERR(stat, HIJACK_FILE, &buf_hijack);

    if (buf_overlay.st_dev == buf_hijack.st_dev && buf_overlay.st_ino == buf_hijack.st_ino) {
        PRINT_LOG("brightness hijack detected, umount old file first!");
        CHECK_ERR(umount, HIJACK_FILE);
    }

    int original_fd = open(HIJACK_FILE, O_WRONLY);

    CHECK_ERR(mount, TMP_FILE, HIJACK_FILE, "none", MS_BIND, nullptr);
    PRINT_LOG("bind mount: " TMP_FILE " -> " HIJACK_FILE);

    return original_fd;
}


int main() {
    if (geteuid() != 0) {
        PRINT_LOG("Please run as root!\n");
        return EXIT_FAILURE;
    }

    PRINT_LOG("current pid: %d", getpid());

    const int page_size = getpagesize();

    int overlay_fd = open_overlay();
    int inotify_fd = init_inotify();
    int original_fd = mount_overlay();

    void *addr = mmap(nullptr, page_size, PROT_READ, MAP_SHARED, overlay_fd, 0);

    close(overlay_fd);

    char buffer[4096] __attribute__((aligned(__alignof__(struct inotify_event))));
    for (;;) {
        ssize_t count = read(inotify_fd, buffer, sizeof(buffer));
        if (count == 0) {
            perror("inotify");
            break;
        }

        const inotify_event *event;
        for (char *ptr = buffer; ptr < buffer + count; ptr += sizeof(inotify_event) + event->len) {
            event = reinterpret_cast<const inotify_event *>(ptr);
            long level = std::strtol(static_cast<const char *>(addr), nullptr, 10);

            printf("brightness level: %ld\r", level);
            fflush(stdout);

            CHECK_ERR(lseek, original_fd, 0, SEEK_SET);
            CHECK_ERR(write, original_fd, addr, page_size);
        }
    }

    return EXIT_SUCCESS;
}