frc_remote.c

#define _GNU_SOURCE

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>

#include <dlfcn.h>
#include <stdio.h>
//#include <poll.h>

//struct pollfd pfd = {-1};
int csock = -1;
__attribute__((constructor)) void constructor() {
        int sock;
        if((sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1) return;
        struct sockaddr_in addr = {0};
        addr.sin_port = htons(5810);
        int enable = 1;
        setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &enable, sizeof(enable));
        if(bind(sock, (struct sockaddr *)&addr, sizeof(addr)) == -1) {
                close(sock);
                return;
        }
        csock = sock;
}

__attribute__((destructor)) void destructor() {
        if(csock != -1) close(csock);
}
ssize_t (*r_recvfrom)(int sockfd, void *buf, size_t len, int flags,
        struct sockaddr *src_addr, socklen_t *addrlen) = NULL;

int (*r_bind)(int sockfd, const struct sockaddr *addr,
        socklen_t addrlen) = NULL;

int important_fd = -1;
struct ctrl_packet {
        uint8_t takeover;
        uint8_t value;
};

struct ctrl_packet ctp = {0};

ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,
        struct sockaddr *src_addr, socklen_t *addrlen) {
        if(r_recvfrom == NULL) r_recvfrom = dlsym(RTLD_NEXT, "recvfrom");
        ssize_t ret = r_recvfrom(sockfd, buf, len, flags, src_addr, addrlen);
        if(sockfd == important_fd && ret >= 6) {
                if(csock != -1) while(recv(csock, &ctp, sizeof(ctp), MSG_DONTWAIT) == sizeof(ctp));
                uint8_t *u8b = (uint8_t *)buf;
                u8b[3] = (u8b[3] & ~ctp.takeover) | (ctp.value & ctp.takeover);
        }
        return ret;
}

int bind(int sockfd, const struct sockaddr *addr,
        socklen_t addrlen) {
        struct sockaddr_in *sai = (struct sockaddr_in *)addr;
        if(sai->sin_port == htons(1110)){
                important_fd = sockfd;
                printf("frc_remote ready\n");
                //printf("important_fd %d\n", important_fd);
        }
        if(r_bind == NULL) r_bind = dlsym(RTLD_NEXT, "bind");
        return r_bind(sockfd, addr, addrlen);
}