Regression: commit da029c11e6b1 broke toybox xargs.

[Rob Landley <rob@landley.net>]

Toybox has been trying to figure out how big an xargs is allowed to be
for a while:


http://lists.landley.net/pipermail/toybox-landley.net/2017-October/009186.html

We're trying to avoid the case where you can run something from the
command line, but not through xargs. In theory this limit is
sysconf(_SC_ARG_MAX) which on bionic and glibc returns 1/4 RLIMIT_STACK
(in accordance with <strike>the prophecy</strike> fs/exec.c function
get_arg_page()), but that turns out to be too simple. There's also a
131071 byte limit on each _individual_ argument, which I think I've
tracked down to fs/exec.c function setup_arg_pages() doing:

        stack_expand = 131072UL; /* randomly 32*4k (or 2*64k) pages *

And then it worked under ubuntu 14.04 but not current kernels. Why?
Because the above commit from Kees Cook broke it, by taking this:

include/uapi/linux/resource.h:
/*
 * Limit the stack by to some sane default: root can always
 * increase this limit if needed..  8MB seems reasonable.
 */
#define _STK_LIM        (8*1024*1024)

And hardwiring in a random adjustment as a "640k ought to be enough for
anybody" constant on TOP of the existing RLIMIT_STACK/4 check. Without
even adjusting the "oh of course root can make this bigger, this is just
a default value" comment where it's #defined.

Look, if you want to cap RLIMIT_STACK for suid binaries, go for it. The
existing code will notice and adapt. But this new commit is crazy and
arbitrary and introduces more random version dependencies (how is
sysconf() supposed to know the value, an #if/else staircase based on
kernel version in every libc)?

Please revert it,

Rob