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

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Date: Sat, 29 Jan 2005 15:05:58 -0500
From: Lee Dilkie <lee_dilkie@...el.com>
To: bugtraq@...urityfocus.com
Subject: Re: SECURITY.NNOV: Multiple applications fd_set structure bitmap
 array index overflow

David LeBlanc wrote:

> 
>
>-----Original Message-----
>From: Damien Miller [mailto:djm@...drot.org] said:
>
>  
>
>>This effectively limits select to a maximum of FD_SETSIZE descriptors
>>    
>>
>on Windows. I don't think that this limitiation exists on other
>platforms.
>
>---------------------------
>
>Note the bit where it says:
>
>#ifndef FD_SETSIZE
>#define FD_SETSIZE      64
>#endif /* FD_SETSIZE */
>
>So to make FD_SETSIZE any arbitrarily large value up to whatever your
>system can handle, you just redefine FD_SETSIZE before you #include
>winsock.h.
>
>  
>
Something you can't do in linux, is enlarge FD_SETSIZE.

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.

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).

-lee