sysrq.c

   1 /* sysrq.c -- sysrq handling routines for vlock
   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 #include <stdio.h>
  14 #include <stdlib.h>
  15 #include <string.h>
  16 #include <errno.h>
  17 #include <unistd.h>
  18
  19 #define OLD_VALUE_LENGTH 8
  20
  21 static const char SYSRQ_PATH[] = "/proc/sys/kernel/sysrq";
  22 static const char SYSRQ_DISABLE_VALUE[] = "0\n";
  23
  24 static FILE *sysrq_file = NULL;
  25
  26 static char old_value[OLD_VALUE_LENGTH];
  27
  28 int disable_sysrq(void) {
  29   sysrq_file = fopen(SYSRQ_PATH, "r+");
  30
  31   if (!sysrq_file) {
  32     fprintf(stderr, "Warning: couldn't open '%s': %s\n", SYSRQ_PATH, strerror(errno));
  33     return 0;
  34   }
  35
  36   if (!fgets(old_value, OLD_VALUE_LENGTH, sysrq_file)) {
  37     fprintf(stderr, "Warning: couldn't get current sysrq setting, won't disable sysrq: %s\n", strerror(errno));
  38     goto error;
  39   }
  40
  41   rewind(sysrq_file);
  42
  43   if (
  44     !fputs(SYSRQ_DISABLE_VALUE, sysrq_file)
  45     || !(ftruncate(fileno(sysrq_file), ftello(sysrq_file)) == 0)
  46     || !(fflush(sysrq_file) == 0)
  47     ) {
  48     fprintf(stderr, "Warning: couldn't disable sysrq: %s\n", strerror(errno));
  49     goto error;
  50   }
  51
  52   rewind(sysrq_file);
  53
  54   return 1;
  55 error:
  56   fclose(sysrq_file);
  57   sysrq_file = NULL;
  58   return 0;
  59 }
  60
  61 void restore_sysrq(void) {
  62   if (!sysrq_file)
  63     return;
  64
  65   if (!fputs(old_value, sysrq_file)) {
  66     fprintf(stderr, "Warning: couldn't restore old sysrq setting: %s\n", strerror(errno));
  67   }
  68
  69   fclose(sysrq_file);
  70 }
  71
  72 void mask_sysrq(void) {
  73   if (disable_sysrq())
  74     atexit(restore_sysrq);
  75 }