Nerd Fortress

Recently at work we had a couple customers mention to us that while backing up files on Linux, symlinks and FIFO (names pipe) files were not being skipped. Trying to backup a FIFO is what we call a bad idea. Very, very bad. One customer reported that most of his filesystems were XFS. Sure enough, after some digging around and testing, I happened to discover this most useful bit of information:

XFS does not support dirent::d_type

What the?! In other words, a line such as this will always be false:

de->d_type == DT_FIFO

Jungle Disk uses readdir_r() to list files and check their types. Uh oh. That means that on XFS partitions, everything looks like a regular file! You can see this for yourself using code like this:

#include <iostream>
#include <cstdlib>
#include <dirent.h>
#include <sys/stat.h>

using namespace std;

int main(int argc, char *argv[])
{

  dirent holdde;
  dirent *de;
  struct stat st;

  cout << "DT_BLK = " << DT_BLK << endl;
  cout << "DT_CHR = " << DT_CHR << endl;
  cout << "DT_DIR = " << DT_DIR << endl;
  cout << "DT_FIFO = " << DT_FIFO << endl;
  cout << "DT_LNK = " << DT_LNK << endl;
  cout << "DT_REG = " << DT_REG << endl;
  cout << "DT_SOCK = " << DT_SOCK << endl;
  cout << "DT_UNKNOWN = " << DT_LNK << endl;

  DIR *dir = opendir("/home/vmuser/xfs");
  while (readdir_r(dir, &holdde, &de) == 0 && de)
  {
	  cout << de->d_name << " [" << (int)de->d_type << "] " << endl;
  }
  closedir(dir);

  lstat("/home/vmuser/xfs/TESTFIFO", &st);
  if (S_ISFIFO(st.st_mode))
	  cout << "XFS FIFO detected: " << st.st_mode << endl; 

  lstat("/home/vmuser/ext3/TESTFIFO", &st);
  if ((st.st_mode & S_IFIFO) != 0)
	  cout << "EXT3 FIFO detected: " << st.st_mode << endl; 

  return EXIT_SUCCESS;
}

So what’s the solution? Good old, reliable stat(). That function does work with XFS on Linux. Unfortunately, this means you will have to make an extra call for each file after listing the dir.