libhed/access.c

   1 /* Low-level file access */
   2 /* $Id$ */
   3
   4 /*
   5  * hed - Hexadecimal editor
   6  * Copyright (C) 2004  Petr Baudis <pasky@ucw.cz>
   7  *
   8  * This program is free software; you can redistribute it and/or modify
   9  * it under the terms of version 2 of the GNU General Public License as
  10  * published by the Free Software Foundation.
  11  *
  12  * This program is distributed in the hope that it will be useful,
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15  * GNU General Public License for more details.
  16  *
  17  * You should have received a copy of the GNU General Public License
  18  * along with this program; if not, write to the Free Software
  19  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  20  */
  21
  22 /* The idea here is that an mmapped region may get beyond EOF, e.g. if
  23  * the file gets truncated by another process.  When accessing such
  24  * areas, the kernel generates a SIGBUS.  Our signal handler replaces
  25  * the faulty page with a new mapping.  The page is still tracked by
  26  * the cache, so it will be freed when the cache object is freed.
  27  */
  28
  29 #include <config.h>
  30
  31 #include <signal.h>
  32 #include <unistd.h>
  33
  34 #include "access.h"
  35 #include "cache.h"
  36
  37 #if HED_USES_MMAP
  38
  39 #include <sys/mman.h>
  40 #include <stdint.h>
  41
  42 unsigned int sys_page_shift;
  43 unsigned long sys_page_size;
  44
  45 #endif
  46
  47 #ifdef HED_CONFIG_MMAP
  48
  49 LIST_HEAD(fixup_files);
  50
  51 static struct hed_block_data *
  52 find_bdata(void *addr)
  53 {
  54         struct hed_file *file;
  55
  56         list_for_each_entry(file, &fixup_files, fixup_list) {
  57                 struct hed_cache *cache = file->cache;
  58                 int i;
  59                 for (i = 0; i < cache->nelem; ++i)
  60                         if (cache->revmap[i]->data == addr)
  61                                 return cache->revmap[i];
  62         }
  63         return NULL;
  64 }
  65
  66 static void
  67 sigbus_handler(int sig, struct siginfo *si, void *ctx)
  68 {
  69         void *page = (void*)(intptr_t)FILE_BLOCK_ROUND((intptr_t)si->si_addr);
  70         struct hed_block_data *bdata = find_bdata(page);
  71
  72         if (bdata) {
  73                 bdata->data = mmap(page, sys_page_size,
  74                                    PROT_READ | PROT_WRITE,
  75                                    MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED,
  76                                    -1, 0);
  77         } else {
  78                 /* Restart the instruction and fail */
  79                 signal(SIGBUS, SIG_DFL);
  80         }
  81 }
  82
  83 #endif  /* HED_CONFIG_MMAP */
  84
  85 #if defined(HED_CONFIG_MMAP) || defined(HED_CONFIG_SWAP_MMAP)
  86
  87 int
  88 file_access_init(void)
  89 {
  90         sys_page_size = sysconf(_SC_PAGESIZE);
  91         sys_page_shift = bitsize(sys_page_size);
  92         /* FILE_BLOCK_* macros only work with page sizes which
  93          * are a power of two. Abort if this is not the case. */
  94         assert(FILE_BLOCK_SIZE == FILE_BLOCK_ROUND(FILE_BLOCK_SIZE));
  95         assert(SWAP_BLOCK_SIZE >= FILE_BLOCK_SIZE);
  96
  97 #ifdef HED_CONFIG_MMAP
  98         struct sigaction sa;
  99
 100         sa.sa_flags = SA_SIGINFO;
 101         sa.sa_sigaction = sigbus_handler;
 102         sigemptyset(&sa.sa_mask);
 103
 104         return sigaction(SIGBUS, &sa, NULL);
 105 #else
 106         return 0;
 107 #endif
 108 }
 109
 110 #endif