libinstaller/setadv.c

   1 /* ----------------------------------------------------------------------- *
   2  *
   3  *   Copyright 2007-2008 H. Peter Anvin - All Rights Reserved
   4  *
   5  *   This program is free software; you can redistribute it and/or modify
   6  *   it under the terms of the GNU General Public License as published by
   7  *   the Free Software Foundation, Inc., 53 Temple Place Ste 330,
   8  *   Boston MA 02111-1307, USA; either version 2 of the License, or
   9  *   (at your option) any later version; incorporated herein by reference.
  10  *
  11  * ----------------------------------------------------------------------- */
  12
  13 /*
  14  * setadv.c
  15  *
  16  * (Over)write a data item in the auxilliary data vector.  To
  17  * delete an item, set its length to zero.
  18  *
  19  * Return 0 on success, -1 on error, and set errno.
  20  *
  21  */
  22
  23 #include <string.h>
  24 #include <errno.h>
  25 #include "syslxint.h"
  26
  27 unsigned char syslinux_adv[2*ADV_SIZE];
  28
  29 #define ADV_MAGIC1      0x5a2d2fa5      /* Head signature */
  30 #define ADV_MAGIC2      0xa3041767      /* Total checksum */
  31 #define ADV_MAGIC3      0xdd28bf64      /* Tail signature */
  32
  33 static void cleanup_adv(unsigned char *advbuf)
  34 {
  35   int i;
  36   uint32_t csum;
  37
  38   /* Make sure both copies agree, and update the checksum */
  39   set_32(advbuf, ADV_MAGIC1);
  40
  41   csum = ADV_MAGIC2;
  42   for (i = 8; i < ADV_SIZE-4; i += 4)
  43     csum -= get_32(advbuf+i);
  44
  45   set_32(advbuf+4, csum);
  46   set_32(advbuf+ADV_SIZE-4, ADV_MAGIC3);
  47
  48   memcpy(advbuf+ADV_SIZE, advbuf, ADV_SIZE);
  49 }
  50
  51 int syslinux_setadv(int tag, size_t size, const void *data)
  52 {
  53   uint8_t *p;
  54   size_t left;
  55   uint8_t advtmp[ADV_SIZE];
  56
  57   if ((unsigned)tag-1 > 254) {
  58     errno = EINVAL;
  59     return -1;                  /* Impossible tag value */
  60   }
  61
  62   if (size > 255) {
  63     errno = ENOSPC;             /* Max 255 bytes for a data item */
  64     return -1;
  65   }
  66
  67   left = ADV_LEN;
  68   p = advtmp;
  69   memcpy(p, syslinux_adv+2*4, left); /* Make working copy */
  70
  71   while (left >= 2) {
  72     uint8_t ptag = p[0];
  73     size_t  plen = p[1]+2;
  74
  75     if (ptag == ADV_END)
  76       break;
  77
  78     if (ptag == tag) {
  79       /* Found our tag.  Delete it. */
  80
  81       if (plen >= left) {
  82         /* Entire remainder is our tag */
  83         break;
  84       }
  85       memmove(p, p+plen, left-plen);
  86     } else {
  87       /* Not our tag */
  88       if (plen > left)
  89         break;                  /* Corrupt tag (overrun) - overwrite it */
  90
  91       left -= plen;
  92       p += plen;
  93     }
  94   }
  95
  96   /* Now (p, left) reflects the position to write in and how much space
  97      we have for our data. */
  98
  99   if (size) {
 100     if (left < size+2) {
 101       errno = ENOSPC;           /* Not enough space for data */
 102       return -1;
 103     }
 104
 105     *p++ = tag;
 106     *p++ = size;
 107     memcpy(p, data, size);
 108     p += size;
 109     left -= size+2;
 110   }
 111
 112   memset(p, 0, left);
 113
 114   /* If we got here, everything went OK, commit the write */
 115   memcpy(syslinux_adv+2*4, advtmp, ADV_LEN);
 116   cleanup_adv(syslinux_adv);
 117
 118   return 0;
 119 }
 120
 121 void syslinux_reset_adv(unsigned char *advbuf)
 122 {
 123   /* Create an all-zero ADV */
 124   memset(advbuf+2*4, 0, ADV_LEN);
 125   cleanup_adv(advbuf);
 126 }
 127
 128 static int adv_consistent(const unsigned char *p)
 129 {
 130   int i;
 131   uint32_t csum;
 132
 133   if (get_32(p) != ADV_MAGIC1 || get_32(p+ADV_SIZE-4) != ADV_MAGIC3)
 134     return 0;
 135
 136   csum = 0;
 137   for (i = 4; i < ADV_SIZE-4; i += 4)
 138     csum += get_32(p+i);
 139
 140   return csum == ADV_MAGIC2;
 141 }
 142
 143 /*
 144  * Verify that an in-memory ADV is consistent, making the copies consistent.
 145  * If neither copy is OK, return -1 and call syslinux_reset_adv().
 146  */
 147 int syslinux_validate_adv(unsigned char *advbuf)
 148 {
 149   if (adv_consistent(advbuf+0*ADV_SIZE)) {
 150     memcpy(advbuf+ADV_SIZE, advbuf, ADV_SIZE);
 151     return 0;
 152   } else if (adv_consistent(advbuf+1*ADV_SIZE)) {
 153     memcpy(advbuf, advbuf+ADV_SIZE, ADV_SIZE);
 154     return 0;
 155   } else {
 156     syslinux_reset_adv(advbuf);
 157     return -1;
 158   }
 159 }