How to build filters?

[Nico Williams <nico at cryptonector dot com>]

The ld(1) manpage says much too little about how to build a filter,
only documenting the -f, -F, and -z loadfltr options.  What's missing
is how to mark symbols as filters (or auxiliary filters).  I've
searched far and wide and not found instructions on how to do this.

If the GNU link-editor and RTLD supports filters the way the Solaris
ld(1) and RTLD does then surely it must just be a matter of
documentation.  I'm guessing there's a way to write ld(1) scripts to
mark specific symbols as filters, but it's not obvious to me.  If
there is indeed a way I'd volunteer to write documentation for it if
some kind soul would give me some hints (or, preferably, point me at
existing documentation that I just simply missed in my search), enough
for me to figure it out.

Thanks,

Nico
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