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   if (p->context == NULL) {
 106     int errsv = errno;
 107     free(path);
 108     errno = errsv;
 109     return false;
 110   } else {
 111     free(path);
 112     return true;
 113   }
 114 }
 115
 116 static void destroy_script(struct plugin *p)
 117 {
 118   struct script_context *context = p->context;
 119
 120   if (context != NULL) {
 121     /* Close the pipe. */
 122     (void) close(context->fd);
 123
 124     /* Kill the child process. */
 125     if (!wait_for_death(context->pid, 0, 500000L))
 126       ensure_death(context->pid);
 127
 128     free(context);
 129   }
 130 }
 131
 132 /* Invoke the hook by writing it on a single line to the scripts stdin. */
 133 static bool call_script_hook(struct plugin *s, const char *hook_name)
 134 {
 135   struct script_context *context = s->context;
 136   char *data;
 137   ssize_t data_length;
 138   ssize_t length;
 139   struct sigaction act, oldact;
 140
 141   /* Format the line. */
 142   data_length = asprintf(&data, "%s\n", hook_name);
 143
 144   if (data_length < 0)
 145     fatal_error("memory allocation failed");
 146
 147   /* When writing to a pipe when the read end is closed the kernel invariably
 148    * sends SIGPIPE.   Ignore it. */
 149   (void) sigemptyset(&(act.sa_mask));
 150   act.sa_flags = SA_RESTART;
 151   act.sa_handler = SIG_IGN;
 152   (void) sigaction(SIGPIPE, &act, &oldact);
 153
 154   /* Send hook name and a newline through the pipe. */
 155   length = write(context->fd, data, data_length);
 156
 157   /* Restore the previous SIGPIPE handler. */
 158   (void) sigaction(SIGPIPE, &oldact, NULL);
 159
 160   /* Scripts fail silently. */
 161   return (length == data_length);
 162 }
 163
 164 static struct script_context *launch_script(const char *path)
 165 {
 166   struct script_context *script = ensure_malloc(sizeof *script);
 167   const char *argv[] = { path, "hooks", NULL };
 168   struct child_process child = {
 169     .path = path,
 170     .argv = argv,
 171     .stdin_fd = REDIRECT_PIPE,
 172     .stdout_fd = REDIRECT_DEV_NULL,
 173     .stderr_fd = REDIRECT_DEV_NULL,
 174     .function = NULL,
 175   };
 176
 177   if (!create_child(&child)) {
 178     int errsv = errno;
 179     free(script);
 180     errno = errsv;
 181     return NULL;
 182   }
 183
 184   script->fd = child.stdin_fd;
 185   script->pid = child.pid;
 186
 187   return script;
 188 }
 189
 190 static char *read_dependency(const char *path, const char *dependency_name);
 191 static bool parse_dependency(char *data, struct list *dependency_list);
 192
 193 /* Get the dependency from the script. */
 194 static bool get_dependency(const char *path, const char *dependency_name,
 195     struct list *dependency_list)
 196 {
 197   /* Read the dependency data. */
 198   char *data;
 199   errno = 0;
 200   data = read_dependency(path, dependency_name);
 201
 202   if (data == NULL)  {
 203     return errno != 0;
 204   } else {
 205     /* Parse the dependency data. */
 206     bool result = parse_dependency(data, dependency_list);
 207     int errsv = errno;
 208     free(data);
 209     errno = errsv;
 210     return result;
 211   }
 212 }
 213
 214 /* Read the dependency data by starting the script with the name of the
 215  * dependency as a single command line argument.  The script should then print
 216  * the dependencies to its stdout one on per line. */
 217 static char *read_dependency(const char *path, const char *dependency_name)
 218 {
 219   char *error = NULL;
 220   int pipe_fds[2];
 221   pid_t pid;
 222   struct timeval timeout = (struct timeval){1, 0};
 223   char *data = ensure_calloc(1, sizeof *data);
 224   size_t data_length = 0;
 225
 226   /* Open a pipe for the script. */
 227   if (pipe(pipe_fds) < 0)
 228     fatal_error("pipe() failed");
 229
 230   pid = fork();
 231
 232   if (pid == 0) {
 233     /* Child. */
 234     int nullfd = open("/dev/null", O_RDWR);
 235
 236     if (nullfd < 0)
 237       _exit(1);
 238
 239     /* Redirect stdio. */
 240     (void) dup2(nullfd, STDIN_FILENO);
 241     (void) dup2(pipe_fds[1], STDOUT_FILENO);
 242     (void) dup2(nullfd, STDERR_FILENO);
 243
 244     /* Close all other file descriptors. */
 245     close_all_fds();
 246
 247     /* Drop privileges. */
 248     setgid(getgid());
 249     setuid(getuid());
 250
 251     (void) execl(path, path, dependency_name, NULL);
 252
 253     _exit(1);
 254   }
 255
 256   /* Close write end of the pipe. */
 257   (void) close(pipe_fds[1]);
 258
 259   if (pid < 0) {
 260     (void) close(pipe_fds[0]);
 261     free(data);
 262     fatal_error("fork() failed");
 263   }
 264
 265   /* Read the dependency from the child.  Reading fails if either the timeout
 266    * elapses or more that LINE_MAX bytes are read. */
 267   for (;;) {
 268     struct timeval t = timeout;
 269     struct timeval t1;
 270     struct timeval t2;
 271     char buffer[LINE_MAX];
 272     ssize_t length;
 273
 274     fd_set read_fds;
 275
 276     FD_ZERO(&read_fds);
 277     FD_SET(pipe_fds[0], &read_fds);
 278
 279     /* t1 is before select. */
 280     (void) gettimeofday(&t1, NULL);
 281
 282     if (select(pipe_fds[0]+1, &read_fds, NULL, NULL, &t) != 1) {
 283 timeout:
 284       asprintf(&error, "timeout while reading dependency '%s' from '%s", dependency_name, path);
 285       goto read_error;
 286     }
 287
 288     /* t2 is after select. */
 289     (void) gettimeofday(&t2, NULL);
 290
 291     /* Get the time that during select. */
 292     timersub(&t2, &t1, &t2);
 293
 294     /* This is very unlikely. */
 295     if (timercmp(&t2, &timeout, >))
 296       goto timeout;
 297
 298     /* Reduce the timeout. */
 299     timersub(&timeout, &t2, &timeout);
 300
 301     /* Read dependency data from the script. */
 302     length = read(pipe_fds[0], buffer, sizeof buffer - 1);
 303
 304     /* Did the script close its stdin or exit? */
 305     if (length <= 0)
 306       break;
 307
 308     if (data_length+length+1 > LINE_MAX) {
 309       asprintf(&error, "too much data while reading dependency '%s' from '%s'", dependency_name, path);
 310       goto read_error;
 311     }
 312
 313     /* Append the buffer to the data string. */
 314     data = ensure_realloc(data, data_length+length);
 315     strncpy(data+data_length, buffer, length);
 316     data_length += length;
 317   }
 318
 319   /* Terminate the data string. */
 320   data[data_length] = '\0';
 321
 322   /* Close the read end of the pipe. */
 323   (void) close(pipe_fds[0]);
 324   /* Kill the script. */
 325   if (!wait_for_death(pid, 0, 500000L))
 326     ensure_death(pid);
 327
 328   return data;
 329
 330 read_error:
 331   free(data);
 332   (void) close(pipe_fds[0]);
 333   if (!wait_for_death(pid, 0, 500000L))
 334     ensure_death(pid);
 335   fprintf(stderr, "%s\n", error);
 336   free(error);
 337   abort();
 338 }
 339
 340 static bool parse_dependency(char *data, struct list *dependency_list)
 341 {
 342   for (char *s = data, *saveptr;; s = NULL) {
 343     char *token = strtok_r(s, "\n", &saveptr);
 344
 345     if (token == NULL)
 346       break;
 347
 348     list_append(dependency_list, ensure_not_null(strdup(token), "could not copy string"));
 349   }
 350
 351   return true;
 352 }