modules/nosysrq.c

   1 /* nosysrq.c -- SysRq protection plugin for vlock,
   2  *              the VT locking program for linux
   3  *
   4  * This program is copyright (C) 2007 Frank Benkstein, and is free
   5  * software which is freely distributable under the terms of the
   6  * GNU General Public License version 2, included as the file COPYING in this
   7  * distribution.  It is NOT public domain software, and any
   8  * redistribution not permitted by the GNU General Public License is
   9  * expressly forbidden without prior written permission from
  10  * the author.
  11  *
  12  */
  13
  14 #include <stdio.h>
  15 #include <stdlib.h>
  16 #include <string.h>
  17 #include <unistd.h>
  18 #include <errno.h>
  19
  20 #include "vlock_plugin.h"
  21
  22 const char *preceeds[] = { "new", "all", NULL };
  23 const char *depends[] = { "all", NULL };
  24
  25 #define SYSRQ_PATH "/proc/sys/kernel/sysrq"
  26 #define SYSRQ_DISABLE_VALUE "0\n"
  27
  28 struct sysrq_context {
  29   FILE *file;
  30   char value[32];
  31 };
  32
  33 /* Disable SysRq and save old value in context. */
  34 bool vlock_start(void **ctx_ptr)
  35 {
  36   struct sysrq_context *ctx;
  37
  38   /* Allocate the context. */
  39   if ((ctx = malloc(sizeof *ctx)) == NULL)
  40     return false;
  41
  42   /* XXX: add optional PAM check here */
  43
  44   /* Open the SysRq sysctl file for reading and writing. */
  45   if ((ctx->file = fopen(SYSRQ_PATH, "r+")) == NULL) {
  46     perror("vlock-nosysrq: could not open '" SYSRQ_PATH "'");
  47     if (errno == ENOENT)
  48       goto nothing_to_do;
  49     else
  50       goto err;
  51   }
  52
  53   /* Read the old value. */
  54   if (fgets(ctx->value, sizeof ctx->value, ctx->file) == NULL) {
  55     perror("vlock-nosysrq: could not read from '" SYSRQ_PATH "'");
  56     goto err;
  57   }
  58
  59   /* Check whether all data was read. */
  60   if (feof(ctx->file) != 0) {
  61     fprintf(stderr, "vlock-nosysrq: sysrq buffer to small: %zu\n",
  62             sizeof ctx->value);
  63     goto err;
  64   }
  65
  66   /* Check if SysRq was already disabled. */
  67   if (strcmp(SYSRQ_DISABLE_VALUE, ctx->value) == 0)
  68     goto nothing_to_do;
  69
  70   /* Disable SysRq. */
  71   if (fseek(ctx->file, 0, SEEK_SET) < 0
  72       || ftruncate(fileno(ctx->file), 0) < 0
  73       || fputs(SYSRQ_DISABLE_VALUE, ctx->file) < 0
  74       || fflush(ctx->file) < 0) {
  75     perror("vlock-nosysrq: could not write disable value to '" SYSRQ_PATH "'");
  76     goto err;
  77   }
  78
  79   *ctx_ptr = ctx;
  80   return true;
  81
  82 nothing_to_do:
  83   free(ctx);
  84   *ctx_ptr = NULL;
  85   return true;
  86
  87 err:
  88   errno = 0;
  89   free(ctx);
  90   return false;
  91 }
  92
  93
  94 /* Restore old SysRq value. */
  95 bool vlock_end(void **ctx_ptr)
  96 {
  97   struct sysrq_context *ctx = *ctx_ptr;
  98
  99   if (ctx == NULL)
 100     return true;
 101
 102   /* Restore SysRq. */
 103   if (fseek(ctx->file, 0, SEEK_SET) < 0
 104       || ftruncate(fileno(ctx->file), 0) < 0
 105       || fputs(ctx->value, ctx->file) < 0 || fflush(ctx->file) < 0)
 106     perror("vlock-nosysrq: could not write old value to '" SYSRQ_PATH "'");
 107
 108   free(ctx);
 109   return true;
 110 }