Re: Expected getdents behaviour

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

On Thu, 15 Sep 2005, Bryan Henderson wrote:
> Since others have opined differently, I'd like to add my voice to Ted's 
> and say his explanation is exactly my understanding too; I couldn't have 
> said it any better, right down to the part of cursing our reality.
> 
> We hate to admit that this is the requirement of directory positions, 
> because it's so hard to implement, but it's what is expected.  When I 
> explain the directory positioning requirement (which actually shows up in 
> lots of places in software design besides filesystem directories), I 
> always say the ideal is to think of the directory position as a white card 
> within a deck of playing cards.  When you say "tell me about the next 
> card," you get the card directly on top of the white card and that card 
> then moves under the white card.  People can insert and delete cards all 
> around the white card, but the sequence is still well defined.
> 
> In actuality, it's acceptable to fall short of that ideal and ignore some 
> or all insertions or deletions that happened after a pass started, and 
> many implementations do.  But what you'd have a really hard time getting 
> accepted is returning the same entry twice or skipping an entry that was 
> always there, as someone steps through the directory.

That may be so but AFAICS it is impossible to implement without causing a 
user triggerable OOM/DOS when you have a filesystem where directory 
entries are static but are a B tree for example.  The problem comes from 
the fact that there is no way to tell for how long that white card is 
valid to use your analogy.  You would have to create one the first time 
you see a readdir() from a particular file descriptor on the directory.  
But when are you free to get rid of it?  There is no way to tell.  When 
the directory is closed I hear you say, but what if it never gets closed 
because the user is malicious?

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/