[PATCH] linux: sysconf: limit _SC_MAX_ARG to 6 MiB [BZ #25305]

[Florian Weimer]

* Adhemerval Zanella:

> It is unfortunate that Linux did not provide a dynamic way to obtain
> this value, specially since it has changed over versions.  So we will
> need to continue using heuristics. 

We can do a binary search using a test binary, perhaps the dynamic
linker itself.  We can probe a few known values first to speed things
up, I guess.

It's hideous, but given that the kernel doesn't support us here, what
else can we do?

Thanks,
Florian