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From: Daniel Phillips <phillips@bonn-fries.net>
To: Linus Torvalds <torvalds@transmeta.com>, <Andries.Brouwer@cwi.nl>
Subject: Re: [PATCH+discussion] symlink recursion
Date: Wed, 19 Jun 2002 16:44:48 +0200
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Cc: Alexander Viro <viro@math.psu.edu>, <linux-kernel@vger.kernel.org>
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On Wednesday 19 June 2002 01:57, Linus Torvalds wrote:
> I see no advantages to handling it by hand, since this isn't even a very
> deep recursion, and since even if you do the recursive part by hand by a
> linked list you still need to limit the depth _anyway_ to avoid DoS
> attacks.

It's the recursive trip through the filesystem that causes the problem.
Suddenly the stack space consumption becomes (N * greediest_filesystem) and
that has to be factored into all worst case calculations.  It's a huge hole
to blow out of this severely restricted resource, so reducing N to 1 is a big
deal.

Also, as a practical matter, it's much easier to lay down a rule for 
filesystem developers that reads: "thou shalt in no case use more than N 
bytes of stack on your longest path" than "in no case use more than N bytes 
of stack except on any symlink resolution path, in which case see the limit 
on recursive symlinks to know how to analyze that path".

-- 
Daniel
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