Small remote command execution

client.c

#include <stdio.h>
#include <stdlib.h>

#include "libwrapper.h"

int main(int argc, char *const argv[]) {

  int ctrl = prepare_socket(true);

  if (argc < 2) {
    fprintf(stderr, "Usage: %s command\n", argv[0]);
    exit(1);
  }

  if (write_args(ctrl, argv + 1)) {
    printf("Arguments sent.\n");
  }
}

libwrapper.c

#include "libwrapper.h"

#include <stdio.h>
#include <signal.h>
#include <stdbool.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <sys/wait.h>

#define BUF_LEN 1024

static const char *const sock_path = "socket";
static char buf[1024];

void spawn_new(pid_t* child, char *const argv[]) {
  int wstatus;
  int new_pid;
  if (argv[0] == NULL) {
    return;
  }
  /* Kill old child */ 
  if (*child > 0) {
    kill(*child, SIGTERM);
    waitpid(*child, &wstatus, 0);
  }
  /* Spawn new child */
  if ((new_pid = fork()) == -1) {
    fprintf(stderr, "Unable to fork.\n");
  } else if (new_pid > 0) {
    *child = new_pid;
  } else if (execv(argv[0], argv) == -1) {
    fprintf(stderr, "Unable to run '%s'.\n", argv[0]);
  }
}

int prepare_socket(bool is_client) {
  int ctrl = socket(AF_UNIX, SOCK_STREAM, 0);
  if (ctrl == -1) {
    fprintf(stderr, "Unable to create UNIX socket.\n");
    exit(1);
  }

  struct sockaddr_un addr;
  memset(&addr, 0, sizeof(addr));
  addr.sun_family = AF_UNIX;
  strncpy(addr.sun_path, sock_path, sizeof(addr.sun_path) - 1);
  if (is_client) {
    if (connect(ctrl, (struct sockaddr*)&addr, sizeof(addr)) == -1) {
      fprintf(stderr, "Unable to connect to UNIX socket.\n");
      exit(1);
    }
  } else {
    if (bind(ctrl, (struct sockaddr*)&addr, sizeof(addr)) == -1) {
      fprintf(stderr, "Unable to bind UNIX socket.\n");
      exit(1);
    }
    if (listen(ctrl, 10) == -1) {
      fprintf(stderr, "Unable to listen on UNIX socket.\n");
      exit(1);
    }
    printf("Listening.\n");
  }
  return ctrl;
}

char ** read_args(int fd) {
  int bytes_read = read(fd, buf, sizeof(buf));
  if (bytes_read == -1 || bytes_read > BUF_LEN) {
    fprintf(stderr, "Unable to read from socket.\n");
    return NULL;
  }
  int len = *(int*)buf;
  char** result = (char**) malloc((len + 1) * sizeof(char*));
  result[len] = NULL;
  char* src = buf + sizeof(int);
  for (int i = 0; i < len; i++) {
    int param_len = strnlen(src, buf + BUF_LEN - src);
    result[i] = strndup(src, buf + BUF_LEN - src);
    src += param_len + 1;
  }
  return result;
}

static int arrlen(char *const array[]) {
  int i = 0;
  char *const *ptr = array;
  while (*ptr != NULL) {
    i++;
    ptr++;
  }
  return i;
}

bool write_args(int fd, char *const argv[]) {
  int len = arrlen(argv);
  ((int*)buf)[0] = len;
  char* dst = buf + sizeof(int);
  char *const *param = argv;
  while (dst < buf + BUF_LEN && *param != NULL) {
    int param_len = strlen(*param);
    strncpy(dst, *param, buf + BUF_LEN - dst);
    dst += param_len + 1;
    param++;
  }
  if (dst >= buf + BUF_LEN) {
    fprintf(stderr, "Buffer full.\n");
    return false;
  }
  if (write(fd, buf, dst - buf) == -1) {
    fprintf(stderr, "Write failed.\n");
    return false;
  }
  return true;
}

libwrapper.h

#ifndef LIBWRAPPER_H
#define LIBWRAPPER_H
#include <stdbool.h>
#include <sys/types.h>

void spawn_new(pid_t* child, char *const argv[]);

int prepare_socket(bool is_client);

char** read_args(int fd);

bool write_args(int fd, char *const argv[]);
#endif

server.c

#include <stdio.h>
#include <unistd.h>
#include <sys/socket.h>
#include <sys/un.h>

#include "libwrapper.h"

int main(int argc, char *const argv[]) {

  int ctrl = prepare_socket(false);
  pid_t child = -1;

  struct sockaddr_un remote;
  socklen_t remote_len;
  int client_fd;
  while ((client_fd = accept(ctrl, (struct sockaddr *)&remote, &remote_len)) != -1) {
    char** new_args = read_args(client_fd);
    if (new_args != NULL) {
      spawn_new(&child, new_args);
    }
    close(client_fd);
  }
  perror(NULL);
  close(ctrl);
}