Re: Expected getdents behaviour

[Anton Altaparmakov <aia21@cam.ac.uk>]

On Thu, 2005-09-15 at 16:45 +0200, Miklos Szeredi wrote:
> > Oops.  I forgot to answer your question.  Yes, the filesystem needs to
> > consider the offset value in the second readdir to still be valid.  You
> > cannot keep rewinding back to zero every time you make a modification or
> > you would keep returning entries you have already returned and never
> > make any progress if e.g. some user does this in a loop at the same
> > time:
> > 
> > while 1; do
> > 	touch blah
> > 	rm blah
> > done
> > 
> > It would be a very effective DOS that any user can trigger.
> > 
> > Bonnie++'s code is just complete crap...  It is the author's fault that
> > it will not work on filesystems where the directory entries are not in
> > fixed locations...
> 
> Why is it crap?  'rm -rf' does exactly the same.

Then that is broken, too.  But have you ever looked?  I just did and you
are partially wrong.  It especially handles the case correctly by doing
a "rewinddir()" on the directory before doing another readdir() after it
has done an unlike.  However you are right in that it only enables this
code at ./configure time so on most Linux systems will not have it
enabled in which case it will indeed be broken on filesystems like ntfs.

> It's up to the filesystem to keep track of the file position in the
> directory, regardless of any deletions or additions.

That is completely untrue.  It is up to the user.  The file position in
a directory is completely up to the caller and in particular when a
directory is modified the file position in that directory is not changed
by the filesystem.  Doing this would in fact be impossible since the
creation/deletion in the kernel has no access to all the "struct file"s
with which a directory has been opened.

Best regards,

        Anton
-- 
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer / IRC: #ntfs on irc.freenode.net
WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/