termux-api.c

#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 <arpa/inet.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/un.h>
#include <time.h>
#include <unistd.h>

#ifdef __ANDROID__
#include <android/api-level.h>
#endif

#include "termux-api.h"

#define TERMUX_API_PACKAGE_VERSION "0.59.1"

#ifndef PREFIX
# define PREFIX "/data/data/com.termux/files/usr"
#endif

#define LISTEN_SOCKET_ADDRESS "com.termux.api://listen"

/* passes the arguments to the plugin via the unix socket, falling
 * back to exec_am_broadcast() if that doesn't work
 */
_Noreturn void contact_plugin(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);

    // ignore SIGPIPE, so am will be called when the connection is closed unexpectedly
    struct sigaction sigpipe_action = {
        .sa_handler = SIG_IGN,
        .sa_flags = 0
    };
    sigaction(SIGPIPE, &sigpipe_action, NULL);

    // Try to connect over the listen socket first if running on Android `< 14`.
    // On Android `>= 14`, if termux-api app process was started previously
    // and it started the socket server, but later Android froze the
    // process, the socket will still be connectable, but no response
    // will be received until the app process is unfrozen agin and
    // `read()` call below will hang indefinitely until that happens,
    // so use legacy `am broadcast` command, which will also unfreeze
    // the app process to deliver the intent.
    // - https://github.com/termux/termux-api/issues/638#issuecomment-1813233924
    int listenfd = -1;
    #ifdef __ANDROID__
        if (android_get_device_api_level() < 34) {
            listenfd = socket(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC, 0);
        }
    #endif
    if (listenfd != -1) {
        struct sockaddr_un listen_addr = { .sun_family = AF_UNIX };
        memcpy(listen_addr.sun_path+1, LISTEN_SOCKET_ADDRESS, strlen(LISTEN_SOCKET_ADDRESS));
        if (connect(listenfd, (struct sockaddr*) &listen_addr, sizeof(sa_family_t) + strlen(LISTEN_SOCKET_ADDRESS) + 1) == 0) {
            socklen_t optlen = sizeof(struct ucred);
            // check the uid to see if the socket is actually provided by the plugin
            struct ucred cred;
            if (getsockopt(listenfd, SOL_SOCKET, SO_PEERCRED, &cred, &optlen) == 0 && cred.uid == getuid()) {

                const char insock_str[] = "--es socket_input \"";
                const char outsock_str[] = "--es socket_output \"";
                const char method_str[] = "--es api_method \"";

                int len = 0;
                len += sizeof(insock_str)-1 + strlen(output_address_string)+2;
                len += sizeof(outsock_str)-1 + strlen(input_address_string)+2;
                len += sizeof(method_str)-1 + strlen(argv[1])+2;
                for (int i = 2; i<argc; i++) {
                    len += strlen(argv[i])+1;
                    if (strcmp(argv[i], "--es") == 0 || strcmp(argv[i], "-e") == 0 || strcmp(argv[i], "--esa") == 0) {
                        len += 2; // the string extra has to be enclosed in "
                    }
                    for (int a = 0; a<strlen(argv[i]); a++) {
                        if (argv[i][a] == '"') {
                            len += 1; // " has to be escaped, so one character more.
                            /* This assumes " is only present in
                            string extra arguments, but that is
                            probably an acceptable assumption to
                            make */
                        }
                    }
                }

                char* buffer = malloc(len);

                int offset = 0;
                memcpy(buffer+offset, insock_str, sizeof(insock_str)-1);
                offset += sizeof(insock_str)-1;

                memcpy(buffer+offset, output_address_string, strlen(output_address_string));
                offset += strlen(output_address_string);

                buffer[offset] = '"';
                offset++;
                buffer[offset] = ' ';
                offset++;

                memcpy(buffer+offset, outsock_str, sizeof(outsock_str)-1);
                offset += sizeof(outsock_str)-1;

                memcpy(buffer+offset, input_address_string, strlen(input_address_string));
                offset += strlen(input_address_string);

                buffer[offset] = '"';
                offset++;
                buffer[offset] = ' ';
                offset++;

                memcpy(buffer+offset, method_str, sizeof(method_str)-1);
                offset += sizeof(method_str)-1;

                memcpy(buffer+offset, argv[1], strlen(argv[1]));
                offset += strlen(argv[1]);

                buffer[offset] = '"';
                offset++;
                buffer[offset] = ' ';
                offset++;

                for (int i = 2; i<argc; i++) {
                    if (strcmp(argv[i], "--es") == 0 || strcmp(argv[i], "-e") == 0 || strcmp(argv[i], "--esa") == 0) {
                        memcpy(buffer+offset, argv[i], strlen(argv[i]));
                        offset += strlen(argv[i]);
                        buffer[offset] = ' ';
                        offset++;
                        i++;
                        if (i < argc) {
                            memcpy(buffer+offset, argv[i], strlen(argv[i]));
                            offset += strlen(argv[i]);
                            buffer[offset] = ' ';
                            offset++;
                        }
                        i++;
                        if (i < argc) {
                            buffer[offset] = '"';
                            offset++;
                            for (int a = 0; a<strlen(argv[i]); a++) {
                                if (argv[i][a] == '"') {
                                    buffer[offset] = '\\';
                                    offset++;
                                    buffer[offset] = '"';
                                    offset++;
                                } else {
                                    buffer[offset] = argv[i][a];
                                    offset++;
                                }
                            }
                            buffer[offset] = '"';
                            offset++;
                            buffer[offset] = ' ';
                            offset++;
                        }
                    } else {
                        memcpy(buffer+offset, argv[i], strlen(argv[i]));
                        offset += strlen(argv[i]);
                        buffer[offset] = ' ';
                        offset++;
                    }
                }

                int netlen = htons(len);

                bool err = false;
                // transmit the size
                int totransmit = 2;
                void* transmit = &netlen;
                while (totransmit > 0) {
                    int ret = send(listenfd, transmit, totransmit, 0);
                    if (ret == -1) {
                        err = true;
                        break;
                    }
                    totransmit -= ret;
                }

                // transmit the argument list
                if (! err) {
                    totransmit = len;
                    transmit = buffer;
                    while (totransmit > 0) {
                        int ret = send(listenfd, transmit, totransmit, 0);
                        if (ret == -1) {
                            err = true;
                            break;
                        }
                        totransmit -= ret;
                    }
                }

                if (! err) {
                    char readbuffer[100];
                    int ret;
                    bool first = true;
                    err = true;
                    while ((ret = read(listenfd, readbuffer, 99)) > 0) {
                        // if a single null byte is received as the first message, the call was successful
                        if (ret == 1 && readbuffer[0] == 0 && first) {
                            err = false;
                            break;
                        }
                        // otherwise it's an error message
                        readbuffer[ret] = '\0';
                        // printing out the error is good for debug purposes, but feel free to disable this
                        fprintf(stderr, "%s", readbuffer);
                        fflush(stderr);
                        first = false;
                    }
                }

                // if everything went well, there is no need to call am
                if (! err) {
                    exit(0);
                }
            }
        }
    }

    exec_am_broadcast(argc, argv, input_address_string, output_address_string);
}

// 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);

    const int child_pre_argc = 14;
    const int child_post_argc = argc - 1; // Except `argv[0]`.
    const int child_argc = child_pre_argc + child_post_argc;

    size_t child_argv_size = (sizeof(char*)) * (child_argc + 1); // Including trailing `NULL`.
    // Do not directly cast, otherwise can trigger null pointer dereference if `NULL` is returned.
    void* result = malloc(child_argv_size);
    if (result == NULL) {
        perror("malloc failed for am child args");
        exit(1);
    }

    char **child_argv = (char **) result;

    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";

    // Copy the remaining arguments except `argv[0]`, `argv[1]` should be `api_method` extra value:
    memcpy(child_argv + child_pre_argc, argv + 1, child_post_argc * sizeof(char*));

    // End with NULL:
    child_argv[child_argc] = NULL;

    // Use an a executable taking care of PATH and LD_LIBRARY_PATH:
    execv(PREFIX "/bin/am", child_argv);

    // We should not reach here, if we do, then free memory we malloc'ed
    free(child_argv);
    perror("execv(\"" PREFIX "/bin/am\")");
    exit(1);
}

_Noreturn void exec_callback(int fd)
{
    char *fds;
    if (asprintf(&fds, "%d", fd) == -1)
        perror("asprintf");

    /* TERMUX_EXPORT_FD and TERMUX_USB_FD are (currently) specific for
       termux-usb, so there's some room for improvement here (this
       function should be generic) */
    char errmsg[256];
    char *export_to_env = getenv("TERMUX_EXPORT_FD");
    if (export_to_env && strncmp(export_to_env, "true", 4) == 0) {
        if (setenv("TERMUX_USB_FD", fds, true) == -1)
            perror("setenv");
        execl(PREFIX "/libexec/termux-callback", "termux-callback", NULL);
        sprintf(errmsg, "execl(\"" PREFIX "/libexec/termux-callback\")");
    } else {
        execl(PREFIX "/libexec/termux-callback", "termux-callback", fds, NULL);
        sprintf(errmsg, "execl(\"" PREFIX "/libexec/termux-callback\", %s)", fds);
    }
    perror(errmsg);
    exit(1);
}

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

// 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 run_api_command(int argc, char **argv) {
    // If only `--version` argument is passed
    if (argc == 2 && strcmp(argv[1], "--version") == 0) {
        fprintf(stdout, "%s\n", TERMUX_API_PACKAGE_VERSION);
        fflush(stdout);
        exit(0);
    }

    // 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_addr_str[100];  // This program reads from it.
    char output_addr_str[100]; // This program writes to it.

    generate_uuid(input_addr_str);
    generate_uuid(output_addr_str);

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

    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;
    }

    int ret;
    ret = bind(input_server_socket, (struct sockaddr*) &input_addr,
               sizeof(sa_family_t) + strlen(input_addr_str) + 1);
    if (ret == -1) {
        perror("bind(input)");
        return ret;
    }

    ret = bind(output_server_socket, (struct sockaddr*) &output_addr,
               sizeof(sa_family_t) + strlen(output_addr_str) + 1);
    if (ret == -1) {
        perror("bind(output)");
        return ret;
    }

    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();
    if (fork_result == -1) {
        perror("fork()");
        return -1;
    } else if (fork_result == 0)
        contact_plugin(argc, argv, input_addr_str, output_addr_str);

    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);

    /* Device has been opened, time to actually get the fd */
    int fd = transmit_socket_to_stdout(input_client_socket);
    close(input_client_socket);
    return fd;
}