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 struct script_context
  52 {
  53   /* The pipe file descriptor that is connected to the script's stdin. */
  54   int fd;
  55   /* The PID of the script. */
  56   pid_t pid;
  57 };
  58
  59 /* Get the dependency from the script.  No error detection.  Aborts on fatal
  60  * errors. */
  61 static void get_dependency(const char *path, const char *dependency_name,
  62     struct list *dependency_list);
  63 /* Launch the script creating a new script_context.  No error detection aborts
  64  * on fatal errors. */
  65 static struct script_context *launch_script(const char *path);
  66
  67
  68 static void close_script(struct plugin *s);
  69 static bool call_script_hook(struct plugin *s, const char *hook_name);
  70
  71 struct plugin *open_script(const char *name, char **error)
  72 {
  73   char *path;
  74   struct plugin *s = __allocate_plugin(name);
  75
  76   if (asprintf(&path, "%s/%s", VLOCK_SCRIPT_DIR, name) < 0) {
  77     *error = strdup("filename too long");
  78     goto path_error;
  79   }
  80
  81   /* Test for access.  This must be done manually because vlock most likely
  82    * runs as a setuid executable and would otherwise override restrictions.
  83    *
  84    * Also there is currently no error detection in case exec() fails later.
  85    */
  86   if (access(path, X_OK) < 0) {
  87     (void) asprintf(error, "%s: %s", path, strerror(errno));
  88     goto access_error;
  89   }
  90
  91   /* Get the dependency information. */
  92   for (size_t i = 0; i < nr_dependencies; i++)
  93     get_dependency(path, dependency_names[i], s->dependencies[i]);
  94
  95   /* Launch the script. */
  96   s->context = launch_script(path);
  97   s->close = close_script;
  98   s->call_hook = call_script_hook;
  99
 100   free(path);
 101
 102   return s;
 103
 104 access_error:
 105   free(path);
 106
 107 path_error:
 108   __destroy_plugin(s);
 109   return NULL;
 110 }
 111
 112 static void close_script(struct plugin *s)
 113 {
 114   struct script_context *context = s->context;
 115
 116   /* Close the pipe. */
 117   (void) close(context->fd);
 118
 119   /* Kill the child process. */
 120   if (!wait_for_death(context->pid, 0, 500000L))
 121     ensure_death(context->pid);
 122
 123   free(context);
 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_PIPE,
 167     .stderr_fd = REDIRECT_PIPE,
 168     .function = NULL,
 169   };
 170
 171   create_child(&child);
 172   script->fd = child.stdin_fd;
 173   script->pid = child.pid;
 174
 175   return script;
 176 }
 177
 178 static char *read_dependency(const char *path, const char *dependency_name);
 179 static void parse_dependency(char *data, struct list *dependency_list);
 180
 181 /* Get the dependency from the script. */
 182 static void get_dependency(const char *path, const char *dependency_name,
 183     struct list *dependency_list)
 184 {
 185   /* Read the dependency data. */
 186   char *data = read_dependency(path, dependency_name);
 187
 188   if (data != NULL)  {
 189     /* Parse the dependency data. */
 190     parse_dependency(data, dependency_list);
 191     free(data);
 192   }
 193 }
 194
 195 /* Read the dependency data by starting the script with the name of the
 196  * dependency as a single command line argument.  The script should then print
 197  * the dependencies to its stdout one on per line. */
 198 static char *read_dependency(const char *path, const char *dependency_name)
 199 {
 200   char *error = NULL;
 201   int pipe_fds[2];
 202   pid_t pid;
 203   struct timeval timeout = (struct timeval){1, 0};
 204   char *data = ensure_calloc(1, sizeof *data);
 205   size_t data_length = 0;
 206
 207   /* Open a pipe for the script. */
 208   if (pipe(pipe_fds) < 0)
 209     fatal_error("pipe() failed");
 210
 211   pid = fork();
 212
 213   if (pid == 0) {
 214     /* Child. */
 215     int nullfd = open("/dev/null", O_RDWR);
 216
 217     if (nullfd < 0)
 218       _exit(1);
 219
 220     /* Redirect stdio. */
 221     (void) dup2(nullfd, STDIN_FILENO);
 222     (void) dup2(pipe_fds[1], STDOUT_FILENO);
 223     (void) dup2(nullfd, STDERR_FILENO);
 224
 225     /* Close all other file descriptors. */
 226     close_all_fds();
 227
 228     /* Drop privileges. */
 229     setgid(getgid());
 230     setuid(getuid());
 231
 232     (void) execl(path, path, dependency_name, NULL);
 233
 234     _exit(1);
 235   }
 236
 237   /* Close write end of the pipe. */
 238   (void) close(pipe_fds[1]);
 239
 240   if (pid < 0) {
 241     (void) close(pipe_fds[0]);
 242     free(data);
 243     fatal_error("fork() failed");
 244   }
 245
 246   /* Read the dependency from the child.  Reading fails if either the timeout
 247    * elapses or more that LINE_MAX bytes are read. */
 248   for (;;) {
 249     struct timeval t = timeout;
 250     struct timeval t1;
 251     struct timeval t2;
 252     char buffer[LINE_MAX];
 253     ssize_t length;
 254
 255     fd_set read_fds;
 256
 257     FD_ZERO(&read_fds);
 258     FD_SET(pipe_fds[0], &read_fds);
 259
 260     /* t1 is before select. */
 261     (void) gettimeofday(&t1, NULL);
 262
 263     if (select(pipe_fds[0]+1, &read_fds, NULL, NULL, &t) != 1) {
 264 timeout:
 265       asprintf(&error, "timeout while reading dependency '%s' from '%s", dependency_name, path);
 266       goto read_error;
 267     }
 268
 269     /* t2 is after select. */
 270     (void) gettimeofday(&t2, NULL);
 271
 272     /* Get the time that during select. */
 273     timersub(&t2, &t1, &t2);
 274
 275     /* This is very unlikely. */
 276     if (timercmp(&t2, &timeout, >))
 277       goto timeout;
 278
 279     /* Reduce the timeout. */
 280     timersub(&timeout, &t2, &timeout);
 281
 282     /* Read dependency data from the script. */
 283     length = read(pipe_fds[0], buffer, sizeof buffer - 1);
 284
 285     /* Did the script close its stdin or exit? */
 286     if (length <= 0)
 287       break;
 288
 289     if (data_length+length+1 > LINE_MAX) {
 290       asprintf(&error, "too much data while reading dependency '%s' from '%s'", dependency_name, path);
 291       goto read_error;
 292     }
 293
 294     /* Append the buffer to the data string. */
 295     data = ensure_realloc(data, data_length+length);
 296     strncpy(data+data_length, buffer, length);
 297     data_length += length;
 298   }
 299
 300   /* Terminate the data string. */
 301   data[data_length] = '\0';
 302
 303   /* Close the read end of the pipe. */
 304   (void) close(pipe_fds[0]);
 305   /* Kill the script. */
 306   if (!wait_for_death(pid, 0, 500000L))
 307     ensure_death(pid);
 308
 309   return data;
 310
 311 read_error:
 312   free(data);
 313   (void) close(pipe_fds[0]);
 314   if (!wait_for_death(pid, 0, 500000L))
 315     ensure_death(pid);
 316   fprintf(stderr, "%s\n", error);
 317   free(error);
 318   abort();
 319 }
 320
 321 static void parse_dependency(char *data, struct list *dependency_list)
 322 {
 323   for (char *s = data, *saveptr;; s = NULL) {
 324     char *token = strtok_r(s, "\n", &saveptr);
 325
 326     if (token == NULL)
 327       break;
 328
 329     list_append(dependency_list, ensure_not_null(strdup(token), "could not copy string"));
 330   }
 331 }