src/script.c

   1 /* script.c -- script routines for vlock, the VT locking program for linux
   2  *
   3  * This program is copyright (C) 2007 Frank Benkstein, and is free
   4  * software which is freely distributable under the terms of the
   5  * GNU General Public License version 2, included as the file COPYING in this
   6  * distribution.  It is NOT public domain software, and any
   7  * redistribution not permitted by the GNU General Public License is
   8  * expressly forbidden without prior written permission from
   9  * the author.
  10  *
  11  */
  12
  13 /* Scripts are executables that are run as unprivileged child processes of
  14  * vlock.  They communicate with vlock through stdin and stdout.
  15  *
  16  * When dependencies are retrieved they are launched once for each dependency
  17  * and should print the names of the plugins they depend on on stdout one per
  18  * line.  The dependency requested is given as a single command line argument.
  19  *
  20  * In hook mode the script is called once with "hooks" as a single command line
  21  * argument.  It should not exit until its stdin closes.  The hook that should
  22  * be executed is written to its stdin on a single line.
  23  *
  24  * Currently there is no way for a script to communicate errors or even success
  25  * to vlock.  If it exits it will linger as a zombie until the plugin is
  26  * destroyed.
  27  */
  28
  29 #if !defined(__FreeBSD__) && !defined(_GNU_SOURCE)
  30 #define _GNU_SOURCE
  31 #endif
  32
  33 #include <stdio.h>
  34 #include <string.h>
  35 #include <stdlib.h>
  36 #include <unistd.h>
  37 #include <limits.h>
  38 #include <sys/select.h>
  39 #include <fcntl.h>
  40 #include <signal.h>
  41 #include <errno.h>
  42 #include <sys/time.h>
  43 #include <time.h>
  44
  45 #include "list.h"
  46 #include "process.h"
  47 #include "util.h"
  48
  49 #include "plugin.h"
  50
  51 static bool init_script(struct plugin *p);
  52 static void destroy_script(struct plugin *p);
  53 static bool call_script_hook(struct plugin *p, const char *hook_name);
  54
  55 struct plugin_type *script = &(struct plugin_type){
  56   .init = init_script,
  57   .destroy = destroy_script,
  58   .call_hook = call_script_hook,
  59 };
  60
  61 struct script_context
  62 {
  63   /* The pipe file descriptor that is connected to the script's stdin. */
  64   int fd;
  65   /* The PID of the script. */
  66   pid_t pid;
  67 };
  68
  69 /* Get the dependency from the script. */
  70 static bool get_dependency(const char *path, const char *dependency_name,
  71     struct list *dependency_list);
  72 /* Launch the script creating a new script_context.  No error detection aborts
  73  * on fatal errors. */
  74 static struct script_context *launch_script(const char *path);
  75
  76 bool init_script(struct plugin *p)
  77 {
  78   char *path;
  79
  80   if (asprintf(&path, "%s/%s", VLOCK_SCRIPT_DIR, p->name) < 0) {
  81     errno = ENOMEM;
  82     return false;
  83   }
  84
  85   /* Test for access.  This must be done manually because vlock most likely
  86    * runs as a setuid executable and would otherwise override restrictions.
  87    *
  88    * Also there is currently no error detection in case exec() fails later.
  89    */
  90   if (access(path, X_OK) < 0) {
  91     free(path);
  92     return false;
  93   }
  94
  95   /* Get the dependency information. */
  96   for (size_t i = 0; i < nr_dependencies; i++)
  97     if (!get_dependency(path, dependency_names[i], p->dependencies[i]))
  98       return false;
  99
 100   /* Launch the script. */
 101   p->context = launch_script(path);
 102
 103   free(path);
 104
 105   return true;
 106 }
 107
 108 static void destroy_script(struct plugin *p)
 109 {
 110   struct script_context *context = p->context;
 111
 112   if (context != NULL) {
 113     /* Close the pipe. */
 114     (void) close(context->fd);
 115
 116     /* Kill the child process. */
 117     if (!wait_for_death(context->pid, 0, 500000L))
 118       ensure_death(context->pid);
 119
 120     free(context);
 121   }
 122 }
 123
 124 /* Invoke the hook by writing it on a single line to the scripts stdin. */
 125 static bool call_script_hook(struct plugin *s, const char *hook_name)
 126 {
 127   struct script_context *context = s->context;
 128   char *data;
 129   ssize_t data_length;
 130   ssize_t length;
 131   struct sigaction act, oldact;
 132
 133   /* Format the line. */
 134   data_length = asprintf(&data, "%s\n", hook_name);
 135
 136   if (data_length < 0)
 137     fatal_error("memory allocation failed");
 138
 139   /* When writing to a pipe when the read end is closed the kernel invariably
 140    * sends SIGPIPE.   Ignore it. */
 141   (void) sigemptyset(&(act.sa_mask));
 142   act.sa_flags = SA_RESTART;
 143   act.sa_handler = SIG_IGN;
 144   (void) sigaction(SIGPIPE, &act, &oldact);
 145
 146   /* Send hook name and a newline through the pipe. */
 147   length = write(context->fd, data, data_length);
 148
 149   /* Restore the previous SIGPIPE handler. */
 150   (void) sigaction(SIGPIPE, &oldact, NULL);
 151
 152   /* Scripts fail silently. */
 153   return (length == data_length);
 154 }
 155
 156 static struct script_context *launch_script(const char *path)
 157 {
 158   struct script_context *script = ensure_malloc(sizeof *script);
 159   const char *argv[] = { path, "hooks", NULL };
 160   struct child_process child = {
 161     .path = path,
 162     .argv = argv,
 163     .stdin_fd = REDIRECT_PIPE,
 164     .stdout_fd = REDIRECT_PIPE,
 165     .stderr_fd = REDIRECT_PIPE,
 166     .function = NULL,
 167   };
 168
 169   create_child(&child);
 170   script->fd = child.stdin_fd;
 171   script->pid = child.pid;
 172
 173   return script;
 174 }
 175
 176 static char *read_dependency(const char *path, const char *dependency_name);
 177 static bool parse_dependency(char *data, struct list *dependency_list);
 178
 179 /* Get the dependency from the script. */
 180 static bool get_dependency(const char *path, const char *dependency_name,
 181     struct list *dependency_list)
 182 {
 183   /* Read the dependency data. */
 184   char *data;
 185   errno = 0;
 186   data = read_dependency(path, dependency_name);
 187
 188
 189   if (data == NULL)  {
 190     return errno != 0;
 191   } else {
 192     /* Parse the dependency data. */
 193     bool result = parse_dependency(data, dependency_list);
 194     int errsv = errno;
 195     free(data);
 196     errno = errsv;
 197     return result;
 198   }
 199 }
 200
 201 /* Read the dependency data by starting the script with the name of the
 202  * dependency as a single command line argument.  The script should then print
 203  * the dependencies to its stdout one on per line. */
 204 static char *read_dependency(const char *path, const char *dependency_name)
 205 {
 206   char *error = NULL;
 207   int pipe_fds[2];
 208   pid_t pid;
 209   struct timeval timeout = (struct timeval){1, 0};
 210   char *data = ensure_calloc(1, sizeof *data);
 211   size_t data_length = 0;
 212
 213   /* Open a pipe for the script. */
 214   if (pipe(pipe_fds) < 0)
 215     fatal_error("pipe() failed");
 216
 217   pid = fork();
 218
 219   if (pid == 0) {
 220     /* Child. */
 221     int nullfd = open("/dev/null", O_RDWR);
 222
 223     if (nullfd < 0)
 224       _exit(1);
 225
 226     /* Redirect stdio. */
 227     (void) dup2(nullfd, STDIN_FILENO);
 228     (void) dup2(pipe_fds[1], STDOUT_FILENO);
 229     (void) dup2(nullfd, STDERR_FILENO);
 230
 231     /* Close all other file descriptors. */
 232     close_all_fds();
 233
 234     /* Drop privileges. */
 235     setgid(getgid());
 236     setuid(getuid());
 237
 238     (void) execl(path, path, dependency_name, NULL);
 239
 240     _exit(1);
 241   }
 242
 243   /* Close write end of the pipe. */
 244   (void) close(pipe_fds[1]);
 245
 246   if (pid < 0) {
 247     (void) close(pipe_fds[0]);
 248     free(data);
 249     fatal_error("fork() failed");
 250   }
 251
 252   /* Read the dependency from the child.  Reading fails if either the timeout
 253    * elapses or more that LINE_MAX bytes are read. */
 254   for (;;) {
 255     struct timeval t = timeout;
 256     struct timeval t1;
 257     struct timeval t2;
 258     char buffer[LINE_MAX];
 259     ssize_t length;
 260
 261     fd_set read_fds;
 262
 263     FD_ZERO(&read_fds);
 264     FD_SET(pipe_fds[0], &read_fds);
 265
 266     /* t1 is before select. */
 267     (void) gettimeofday(&t1, NULL);
 268
 269     if (select(pipe_fds[0]+1, &read_fds, NULL, NULL, &t) != 1) {
 270 timeout:
 271       asprintf(&error, "timeout while reading dependency '%s' from '%s", dependency_name, path);
 272       goto read_error;
 273     }
 274
 275     /* t2 is after select. */
 276     (void) gettimeofday(&t2, NULL);
 277
 278     /* Get the time that during select. */
 279     timersub(&t2, &t1, &t2);
 280
 281     /* This is very unlikely. */
 282     if (timercmp(&t2, &timeout, >))
 283       goto timeout;
 284
 285     /* Reduce the timeout. */
 286     timersub(&timeout, &t2, &timeout);
 287
 288     /* Read dependency data from the script. */
 289     length = read(pipe_fds[0], buffer, sizeof buffer - 1);
 290
 291     /* Did the script close its stdin or exit? */
 292     if (length <= 0)
 293       break;
 294
 295     if (data_length+length+1 > LINE_MAX) {
 296       asprintf(&error, "too much data while reading dependency '%s' from '%s'", dependency_name, path);
 297       goto read_error;
 298     }
 299
 300     /* Append the buffer to the data string. */
 301     data = ensure_realloc(data, data_length+length);
 302     strncpy(data+data_length, buffer, length);
 303     data_length += length;
 304   }
 305
 306   /* Terminate the data string. */
 307   data[data_length] = '\0';
 308
 309   /* Close the read end of the pipe. */
 310   (void) close(pipe_fds[0]);
 311   /* Kill the script. */
 312   if (!wait_for_death(pid, 0, 500000L))
 313     ensure_death(pid);
 314
 315   return data;
 316
 317 read_error:
 318   free(data);
 319   (void) close(pipe_fds[0]);
 320   if (!wait_for_death(pid, 0, 500000L))
 321     ensure_death(pid);
 322   fprintf(stderr, "%s\n", error);
 323   free(error);
 324   abort();
 325 }
 326
 327 static bool parse_dependency(char *data, struct list *dependency_list)
 328 {
 329   for (char *s = data, *saveptr;; s = NULL) {
 330     char *token = strtok_r(s, "\n", &saveptr);
 331
 332     if (token == NULL)
 333       break;
 334
 335     list_append(dependency_list, ensure_not_null(strdup(token), "could not copy string"));
 336   }
 337
 338   return true;
 339 }