Re: FD_SET and FORTIFY_SOURCE

[Florian Weimer <fweimer at redhat dot com>]

On 02/28/2013 02:41 PM, Rich Felker wrote:
> The problem is that a large amount of otherwise-correct software uses
> malloc and out-of-bounds FD_SET arguments to work with sets larger
> than FD_SETSIZE. I have written on the issue before that the "correct"
> way to do this is allocate an array of fd_set objects and use / and %
> to address the bit rather than passing an out-of-bounds argument to
> FD_SET (which invokes UB), but some important software like libevent
> (if I remember correctly) remains wrong in this regard.

The order of bits is unspecified.  The fd_set elements are typically 
words, not bytes, and the bit order of the two access patterns differs 
on big-endian machines.  (I believe this has been used as an argument 
that little-endian is the natural order, the Internet not withstanding.) 
 I can't believe that your kludge is actually more portable than just 
requiring poll support. 8-)

(Writing a poll emulation on top of select isn't that hard, if you need it.)

--
Florian Weimer / Red Hat Product Security Team