Magisk/jni/daemon/socket_trans.c

/* socket_trans.c - Functions to transfer data through socket
 */

#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <limits.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>

#include "magisk.h"
#include "utils.h"
#include "daemon.h"

/*
 * Receive a file descriptor from a Unix socket.
 * Contributed by @mkasick
 *
 * Returns the file descriptor on success, or -1 if a file
 * descriptor was not actually included in the message
 *
 * On error the function terminates by calling exit(-1)
 */
int recv_fd(int sockfd) {
    // Need to receive data from the message, otherwise don't care about it.
    char iovbuf;

    struct iovec iov = {
        .iov_base = &iovbuf,
        .iov_len  = 1,
    };

    char cmsgbuf[CMSG_SPACE(sizeof(int))];

    struct msghdr msg = {
        .msg_iov        = &iov,
        .msg_iovlen     = 1,
        .msg_control    = cmsgbuf,
        .msg_controllen = sizeof(cmsgbuf),
    };

    xrecvmsg(sockfd, &msg, MSG_WAITALL);

    // Was a control message actually sent?
    switch (msg.msg_controllen) {
    case 0:
        // No, so the file descriptor was closed and won't be used.
        return -1;
    case sizeof(cmsgbuf):
        // Yes, grab the file descriptor from it.
        break;
    default:
        goto error;
    }

    struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);

    if (cmsg             == NULL                  ||
        cmsg->cmsg_len   != CMSG_LEN(sizeof(int)) ||
        cmsg->cmsg_level != SOL_SOCKET            ||
        cmsg->cmsg_type  != SCM_RIGHTS) {
error:
        LOGE("unable to read fd");
        exit(-1);
    }

    return *(int *)CMSG_DATA(cmsg);
}

/*
 * Send a file descriptor through a Unix socket.
 * Contributed by @mkasick
 *
 * On error the function terminates by calling exit(-1)
 *
 * fd may be -1, in which case the dummy data is sent,
 * but no control message with the FD is sent.
 */
void send_fd(int sockfd, int fd) {
    // Need to send some data in the message, this will do.
    struct iovec iov = {
        .iov_base = "",
        .iov_len  = 1,
    };

    struct msghdr msg = {
        .msg_iov        = &iov,
        .msg_iovlen     = 1,
    };

    char cmsgbuf[CMSG_SPACE(sizeof(int))];

    if (fd != -1) {
        // Is the file descriptor actually open?
        if (fcntl(fd, F_GETFD) == -1) {
            if (errno != EBADF) {
                PLOGE("unable to send fd");
            }
            // It's closed, don't send a control message or sendmsg will EBADF.
        } else {
            // It's open, send the file descriptor in a control message.
            msg.msg_control    = cmsgbuf;
            msg.msg_controllen = sizeof(cmsgbuf);

            struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);

            cmsg->cmsg_len   = CMSG_LEN(sizeof(int));
            cmsg->cmsg_level = SOL_SOCKET;
            cmsg->cmsg_type  = SCM_RIGHTS;

            *(int *)CMSG_DATA(cmsg) = fd;
        }
    }

    xsendmsg(sockfd, &msg, 0);
}

int read_int(int fd) {
    int val;
    xxread(fd, &val, sizeof(int));
    return val;
}

void write_int(int fd, int val) {
    xwrite(fd, &val, sizeof(int));
}

char* read_string(int fd) {
    int len = read_int(fd);
    if (len > PATH_MAX || len < 0) {
        LOGE("invalid string length %d", len);
        exit(1);
    }
    char* val = xmalloc(sizeof(char) * (len + 1));
    xxread(fd, val, len);
    val[len] = '\0';
    return val;
}

void write_string(int fd, const char* val) {
    int len = strlen(val);
    write_int(fd, len);
    xwrite(fd, val, len);
}