termux-api.c

// termux-api.c - helper binary for calling termux api classes
// Usage: termux-api ${API_METHOD} ${ADDITIONAL_FLAGS}
//        This executes
//          am broadcast com.termux.api/.TermuxApiReceiver --es socket_input ${INPUT_SOCKET} 
//                                                        --es socket_output ${OUTPUT_SOCKET}
//                                                        --es ${API_METHOD}
//                                                        ${ADDITIONAL_FLAGS}
//        where ${INPUT_SOCKET} and ${OUTPUT_SOCKET} are addresses to linux abstract namespace sockets,
//        used to pass on stdin to the java implementation and pass back output from java to stdout.
#define _POSIX_SOURCE
#define _GNU_SOURCE
#include <fcntl.h>
#include <pthread.h>
#include <signal.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/un.h>
#include <time.h>
#include <unistd.h>

// Function which execs "am broadcast ..".
_Noreturn void exec_am_broadcast(int argc, char** argv, char* input_address_string, char* output_address_string)
{
    // Redirect stdout to /dev/null (but leave stderr open):
    close(STDOUT_FILENO);
    open("/dev/null", O_RDONLY);
    // Close stdin:
    close(STDIN_FILENO);

    int const extra_args = 15; // Including ending NULL.
    char** child_argv = malloc((sizeof(char*)) * (argc + extra_args));

    child_argv[0] = "am";
    child_argv[1] = "broadcast";
    child_argv[2] = "--user";
    child_argv[3] = "0";
    child_argv[4] = "-n";
    child_argv[5] = "com.termux.api/.TermuxApiReceiver";
    child_argv[6] = "--es";
    // Input/output are reversed for the java process (our output is its input):
    child_argv[7] = "socket_input";
    child_argv[8] = output_address_string;
    child_argv[9] = "--es";
    child_argv[10] = "socket_output";
    child_argv[11] = input_address_string;
    child_argv[12] = "--es";
    child_argv[13] = "api_method";
    child_argv[14] = argv[1];

    // Copy the remaining arguments -2 for first binary and second api name:
    memcpy(child_argv + extra_args, argv + 2, (argc-1) * sizeof(char*));

    // End with NULL:
    child_argv[argc + extra_args] = NULL;

    // Use an a executable taking care of PATH and LD_LIBRARY_PATH:
    execv("/data/data/com.termux/files/usr/bin/am", child_argv);

    perror("execv(\"/data/data/com.termux/files/usr/bin/am\")");
    exit(1);
}

_Noreturn void exec_callback(int fd)
{
    char *fds;
    if(asprintf(&fds, "%d", fd) == -1) { perror("asprintf"); }
    execl("/data/data/com.termux/files/usr/libexec/termux-callback", "termux-callback", fds, NULL);
    perror("execl(\"/data/data/com.termux/files/usr/libexec/termux-callback\")");
    exit(1);
}

void generate_uuid(char* str) {
    sprintf(str, "%x%x-%x-%x-%x-%x%x%x",
            arc4random(), arc4random(),                 // Generates a 64-bit Hex number
            (uint32_t) getpid(),                        // Generates a 32-bit Hex number
            ((arc4random() & 0x0fff) | 0x4000),         // Generates a 32-bit Hex number of the form 4xxx (4 indicates the UUID version)
            arc4random() % 0x3fff + 0x8000,             // Generates a 32-bit Hex number in the range [0x8000, 0xbfff]
            arc4random(), arc4random(), arc4random());  // Generates a 96-bit Hex number
}

// Thread function which reads from stdin and writes to socket.
void* transmit_stdin_to_socket(void* arg) {
    int output_server_socket = *((int*) arg);
    struct sockaddr_un remote_addr;
    socklen_t addrlen = sizeof(remote_addr);
    int output_client_socket = accept(output_server_socket, (struct sockaddr*) &remote_addr, &addrlen);

    ssize_t len;
    char buffer[1024];
    while (len = read(STDIN_FILENO, &buffer, sizeof(buffer)), len > 0) {
        if (write(output_client_socket, buffer, len) < 0) break;
    }
    // Close output socket on end of input:
    close(output_client_socket);
    return NULL;
}

// Main thread function which reads from input socket and writes to stdout.
int transmit_socket_to_stdout(int input_socket_fd) {
    ssize_t len;
    char buffer[1024];
    char cbuf[256];
    struct iovec io = { .iov_base = buffer, .iov_len = sizeof(buffer) };
    struct msghdr msg = { 0 };
    int fd = -1;  // An optional file descriptor received through the socket
    msg.msg_iov = &io;
    msg.msg_iovlen = 1;
    msg.msg_control = cbuf;
    msg.msg_controllen = sizeof(cbuf);
    while ((len = recvmsg(input_socket_fd, &msg, 0)) > 0) {
        struct cmsghdr * cmsg = CMSG_FIRSTHDR(&msg);
        if (cmsg && cmsg->cmsg_len == CMSG_LEN(sizeof(int))) {
            if (cmsg->cmsg_type == SCM_RIGHTS) {
                fd = *((int *) CMSG_DATA(cmsg));
            }
        }
        // A file descriptor must be accompanied by a non-empty message,
        // so we use "@" when we don't want any output.
        if (fd != -1 && len == 1 && buffer[0] == '@') { len = 0; }
        write(STDOUT_FILENO, buffer, len);
        msg.msg_controllen = sizeof(cbuf);
    }
    if (len < 0) perror("recvmsg()");
    return fd;
}

int main(int argc, char** argv) {
    // Do not transform children into zombies when they terminate:
    struct sigaction sigchld_action = { .sa_handler = SIG_DFL, .sa_flags = SA_RESTART | SA_NOCLDSTOP | SA_NOCLDWAIT };
    sigaction(SIGCHLD, &sigchld_action, NULL);

    char input_address_string[100];  // This program reads from it.
    char output_address_string[100]; // This program writes to it.

    generate_uuid(input_address_string);
    generate_uuid(output_address_string);

    struct sockaddr_un input_address = { .sun_family = AF_UNIX };
    struct sockaddr_un output_address = { .sun_family = AF_UNIX };
    // Leave struct sockaddr_un.sun_path[0] as 0 and use the UUID string as abstract linux namespace:
    strncpy(&input_address.sun_path[1], input_address_string, strlen(input_address_string));
    strncpy(&output_address.sun_path[1], output_address_string, strlen(output_address_string));

    int input_server_socket = socket(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC, 0);
    if (input_server_socket == -1) { perror("socket()"); return 1; }
    int output_server_socket = socket(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC, 0);
    if (output_server_socket == -1) { perror("socket()"); return 1; }

    if (bind(input_server_socket, (struct sockaddr*) &input_address, sizeof(sa_family_t) + strlen(input_address_string) + 1) == -1) {
        perror("bind(input)");
        return 1;
    }
    if (bind(output_server_socket, (struct sockaddr*) &output_address, sizeof(sa_family_t) + strlen(output_address_string) + 1) == -1) {
        perror("bind(output)");
        return 1;
    }

    if (listen(input_server_socket, 1) == -1) { perror("listen()"); return 1; }
    if (listen(output_server_socket, 1) == -1) { perror("listen()"); return 1; }

    pid_t fork_result = fork();
    switch (fork_result) {
        case -1: perror("fork()"); return 1;
        case 0: exec_am_broadcast(argc, argv, input_address_string, output_address_string);
    }

    struct sockaddr_un remote_addr;
    socklen_t addrlen = sizeof(remote_addr);
    int input_client_socket = accept(input_server_socket, (struct sockaddr*) &remote_addr, &addrlen);

    pthread_t transmit_thread;
    pthread_create(&transmit_thread, NULL, transmit_stdin_to_socket, &output_server_socket);

    int fd = transmit_socket_to_stdout(input_client_socket);
    close(input_client_socket);
    if (fd != -1) { exec_callback(fd); }

    return 0;
}