Bug#11756764 48726: MYSQLD KEEPS CRASHING WITH SIGSEGV

                    WITH MYISAM_USE_MMAP ENABLED

MySQL server can crash due to segmentation fault when
started with myisam_use_mmap.

The reason behind this being, while making a request to
unmap (munmap) the previously mapped memory (mmap), the
size passed was 7 bytes larger than the size requested at
the time of mapping. This can eventually unmap the adjacent
memory mapped block, belonging to some other memory-map pool.
Hence the subsequent call to mmap can map a region which was
still a valid memory mapped area.

Fixed by removing the extra 7-byte margin which was erroneously
added to the size, used for unmappping.

storage/myisam/mi_close.c

@@ -87,7 +87,12 @@ int mi_close(register MI_INFO *info)
     }
 #ifdef HAVE_MMAP
     if (share->file_map)
+    {
+      if (share->options & HA_OPTION_COMPRESS_RECORD)
         _mi_unmap_file(info);
+      else
+        mi_munmap_file(info);
+    }
 #endif
     if (share->decode_trees)
     {

storage/myisam/mi_packrec.c

@@ -1553,13 +1553,14 @@ my_bool _mi_memmap_file(MI_INFO *info)
 
 void _mi_unmap_file(MI_INFO *info)
 {
-  VOID(my_munmap((char*) info->s->file_map, 
-                 (size_t) info->s->mmaped_length + MEMMAP_EXTRA_MARGIN));
+  DBUG_ASSERT(info->s->options & HA_OPTION_COMPRESS_RECORD);
+
+  VOID(my_munmap((char*) info->s->file_map, (size_t) info->s->mmaped_length));
 
   if (myisam_mmap_size != SIZE_T_MAX)
   {
     pthread_mutex_lock(&THR_LOCK_myisam_mmap);
-    myisam_mmap_used-= info->s->mmaped_length + MEMMAP_EXTRA_MARGIN;
+    myisam_mmap_used-= info->s->mmaped_length;
     pthread_mutex_unlock(&THR_LOCK_myisam_mmap);
   }
 }