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     int errsv = errno;
  92     free(path);
  93     errno = errsv;
  94     return false;
  95   }
  96
  97   /* Get the dependency information. */
  98   for (size_t i = 0; i < nr_dependencies; i++)
  99     if (!get_dependency(path, dependency_names[i], p->dependencies[i]))
 100       return false;
 101
 102   /* Launch the script. */
 103   p->context = launch_script(path);
 104
 105   free(path);
 106
 107   return true;
 108 }
 109
 110 static void destroy_script(struct plugin *p)
 111 {
 112   struct script_context *context = p->context;
 113
 114   if (context != NULL) {
 115     /* Close the pipe. */
 116     (void) close(context->fd);
 117
 118     /* Kill the child process. */
 119     if (!wait_for_death(context->pid, 0, 500000L))
 120       ensure_death(context->pid);
 121
 122     free(context);
 123   }
 124 }
 125
 126 /* Invoke the hook by writing it on a single line to the scripts stdin. */
 127 static bool call_script_hook(struct plugin *s, const char *hook_name)
 128 {
 129   struct script_context *context = s->context;
 130   char *data;
 131   ssize_t data_length;
 132   ssize_t length;
 133   struct sigaction act, oldact;
 134
 135   /* Format the line. */
 136   data_length = asprintf(&data, "%s\n", hook_name);
 137
 138   if (data_length < 0)
 139     fatal_error("memory allocation failed");
 140
 141   /* When writing to a pipe when the read end is closed the kernel invariably
 142    * sends SIGPIPE.   Ignore it. */
 143   (void) sigemptyset(&(act.sa_mask));
 144   act.sa_flags = SA_RESTART;
 145   act.sa_handler = SIG_IGN;
 146   (void) sigaction(SIGPIPE, &act, &oldact);
 147
 148   /* Send hook name and a newline through the pipe. */
 149   length = write(context->fd, data, data_length);
 150
 151   /* Restore the previous SIGPIPE handler. */
 152   (void) sigaction(SIGPIPE, &oldact, NULL);
 153
 154   /* Scripts fail silently. */
 155   return (length == data_length);
 156 }
 157
 158 static struct script_context *launch_script(const char *path)
 159 {
 160   struct script_context *script = ensure_malloc(sizeof *script);
 161   const char *argv[] = { path, "hooks", NULL };
 162   struct child_process child = {
 163     .path = path,
 164     .argv = argv,
 165     .stdin_fd = REDIRECT_PIPE,
 166     .stdout_fd = REDIRECT_DEV_NULL,
 167     .stderr_fd = REDIRECT_DEV_NULL,
 168     .function = NULL,
 169   };
 170
 171   if (!create_child(&child)) {
 172     int errsv = errno;
 173     free(script);
 174     errno = errsv;
 175     return NULL;
 176   }
 177
 178   script->fd = child.stdin_fd;
 179   script->pid = child.pid;
 180
 181   return script;
 182 }
 183
 184 static char *read_dependency(const char *path, const char *dependency_name);
 185 static bool parse_dependency(char *data, struct list *dependency_list);
 186
 187 /* Get the dependency from the script. */
 188 static bool get_dependency(const char *path, const char *dependency_name,
 189     struct list *dependency_list)
 190 {
 191   /* Read the dependency data. */
 192   char *data;
 193   errno = 0;
 194   data = read_dependency(path, dependency_name);
 195
 196   if (data == NULL)  {
 197     return errno != 0;
 198   } else {
 199     /* Parse the dependency data. */
 200     bool result = parse_dependency(data, dependency_list);
 201     int errsv = errno;
 202     free(data);
 203     errno = errsv;
 204     return result;
 205   }
 206 }
 207
 208 /* Read the dependency data by starting the script with the name of the
 209  * dependency as a single command line argument.  The script should then print
 210  * the dependencies to its stdout one on per line. */
 211 static char *read_dependency(const char *path, const char *dependency_name)
 212 {
 213   char *error = NULL;
 214   int pipe_fds[2];
 215   pid_t pid;
 216   struct timeval timeout = (struct timeval){1, 0};
 217   char *data = ensure_calloc(1, sizeof *data);
 218   size_t data_length = 0;
 219
 220   /* Open a pipe for the script. */
 221   if (pipe(pipe_fds) < 0)
 222     fatal_error("pipe() failed");
 223
 224   pid = fork();
 225
 226   if (pid == 0) {
 227     /* Child. */
 228     int nullfd = open("/dev/null", O_RDWR);
 229
 230     if (nullfd < 0)
 231       _exit(1);
 232
 233     /* Redirect stdio. */
 234     (void) dup2(nullfd, STDIN_FILENO);
 235     (void) dup2(pipe_fds[1], STDOUT_FILENO);
 236     (void) dup2(nullfd, STDERR_FILENO);
 237
 238     /* Close all other file descriptors. */
 239     close_all_fds();
 240
 241     /* Drop privileges. */
 242     setgid(getgid());
 243     setuid(getuid());
 244
 245     (void) execl(path, path, dependency_name, NULL);
 246
 247     _exit(1);
 248   }
 249
 250   /* Close write end of the pipe. */
 251   (void) close(pipe_fds[1]);
 252
 253   if (pid < 0) {
 254     (void) close(pipe_fds[0]);
 255     free(data);
 256     fatal_error("fork() failed");
 257   }
 258
 259   /* Read the dependency from the child.  Reading fails if either the timeout
 260    * elapses or more that LINE_MAX bytes are read. */
 261   for (;;) {
 262     struct timeval t = timeout;
 263     struct timeval t1;
 264     struct timeval t2;
 265     char buffer[LINE_MAX];
 266     ssize_t length;
 267
 268     fd_set read_fds;
 269
 270     FD_ZERO(&read_fds);
 271     FD_SET(pipe_fds[0], &read_fds);
 272
 273     /* t1 is before select. */
 274     (void) gettimeofday(&t1, NULL);
 275
 276     if (select(pipe_fds[0]+1, &read_fds, NULL, NULL, &t) != 1) {
 277 timeout:
 278       asprintf(&error, "timeout while reading dependency '%s' from '%s", dependency_name, path);
 279       goto read_error;
 280     }
 281
 282     /* t2 is after select. */
 283     (void) gettimeofday(&t2, NULL);
 284
 285     /* Get the time that during select. */
 286     timersub(&t2, &t1, &t2);
 287
 288     /* This is very unlikely. */
 289     if (timercmp(&t2, &timeout, >))
 290       goto timeout;
 291
 292     /* Reduce the timeout. */
 293     timersub(&timeout, &t2, &timeout);
 294
 295     /* Read dependency data from the script. */
 296     length = read(pipe_fds[0], buffer, sizeof buffer - 1);
 297
 298     /* Did the script close its stdin or exit? */
 299     if (length <= 0)
 300       break;
 301
 302     if (data_length+length+1 > LINE_MAX) {
 303       asprintf(&error, "too much data while reading dependency '%s' from '%s'", dependency_name, path);
 304       goto read_error;
 305     }
 306
 307     /* Append the buffer to the data string. */
 308     data = ensure_realloc(data, data_length+length);
 309     strncpy(data+data_length, buffer, length);
 310     data_length += length;
 311   }
 312
 313   /* Terminate the data string. */
 314   data[data_length] = '\0';
 315
 316   /* Close the read end of the pipe. */
 317   (void) close(pipe_fds[0]);
 318   /* Kill the script. */
 319   if (!wait_for_death(pid, 0, 500000L))
 320     ensure_death(pid);
 321
 322   return data;
 323
 324 read_error:
 325   free(data);
 326   (void) close(pipe_fds[0]);
 327   if (!wait_for_death(pid, 0, 500000L))
 328     ensure_death(pid);
 329   fprintf(stderr, "%s\n", error);
 330   free(error);
 331   abort();
 332 }
 333
 334 static bool parse_dependency(char *data, struct list *dependency_list)
 335 {
 336   for (char *s = data, *saveptr;; s = NULL) {
 337     char *token = strtok_r(s, "\n", &saveptr);
 338
 339     if (token == NULL)
 340       break;
 341
 342     list_append(dependency_list, ensure_not_null(strdup(token), "could not copy string"));
 343   }
 344
 345   return true;
 346 }