On Wed, Jun 10, 2009 at 01:40:20PM +0800, Sucan Zhang wrote: > > why use cat command to edit a file , the blocks of the file will not change. If you do "cat > file" or "echo foo > file", the file is first getting truncated, and so _normally_ the block of the file which will get reallocated will be the same. It doesn't have to be, however. It also means that if you do: cat > file foo foo foo ^D and you crash before the ^D, the file will be lost. > but use vim command to edit a file, the blocks of the file will change? An carefully written editor such as vim will tend to do something like this: 1) write the new contest to file.new 2) fsync file.new to make sure the blocks are safely on disk 3) rename file.new to file, which will delete file and replace it with file.new That way if you crash in the middle of writing out the file, you don't lose the contents of the file. The bottom line is if you are really trying to do file-level security controls using block numbers, you need to reestablish the block numbers each time the filesystem has changed. As I mentioned, this also means you have to include the blocks for the inode table and the blocks containing the directory. It works only if the filesystem is static --- and you want to prevent anyone from changing directories and filenames. In general, it's really not a great way of doing partial security on filesystems. You may be better off using SELinux or Apparmor. - Ted _______________________________________________ Ext3-users mailing list Ext3-users@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/ext3-users
