bugtraq - Re: SECURITY.NNOV: Multiple applications fd_set structure bitmap array index overflow

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Message-ID: <200501301743.j0UHh8Ma003878@vaticaan.Holland.Sun.COM>
Date: Sun, 30 Jan 2005 18:43:08 +0100
From: Casper.Dik@....COM
To: Lee Dilkie <lee_dilkie@...el.com>
Cc: bugtraq@...urityfocus.com
Subject: Re: SECURITY.NNOV: Multiple applications fd_set structure bitmap array index overflow



>from linux/posix_types/h:
>#undef      __FD_SETSIZE
>#define     __FD_SETSIZE   1024
>
>Well, you *can* change it, but it requires a recompile of the kernel and 
>all userland programs that create an fd_set.

Which is generally true for most Unix systems; traditionallly you can
redefine FD_SETSIZE for a larger set but you're limited by kernel
support (or libc support, as the case may be)

>In this regard, windows did get it right. However, the earlier comment 
>on using the windows async sockets is spot on, if you want performance. 
>Windows fd_set's are structured more like unix poll() arrays (un-ordered 
>array of fd's) and are not very efficient if there are many sockets on 
>one set. Also, a linux fd_set limits the fd *value* to < 1024, not just 
>the number of fd's in the set. So it's possible to only want to put one 
>fd on a fd_set but be unable to do so if it's value is > FD_SETSIZE 
>(which can be done by increasing the maximum number of file handles a 
>process is permitted to open).

Which is why poll(2) is the superior interface; but event driven
machnisms are in the end the best.

Casper