What does junk the cache mean?
 

I am trying to see if
2.6.42 is vulnerable on my test server,
can you confirm you got the same:
[tester@test tmp]$ ./h00lyshit file

preparing
trying to exploit file

failed: Permission denied

They state:
** if y0u dont have one, make big file (~100MB) in /tmp with dd
** and try to junk the cache e.g. cat /usr/lib/* >/dev/null

What do they mean by
"and try to junk the cache e.g. cat /usr/lib/* >/dev/null" ?

maybe so ur kernel is not vulnerable..i got the same on a not affected kernel. on affected kernel i get Exec format error.

Hmm, from my knowledge, cat /usr/lib/* should show the contents of every file in /usr/lib, and >/dev/null sends everything printed to stdout (aka printed on screen and not an error) to /dev/null, that is, doesn't show it any more.
Basicly, I can't see any reason to use that command except to see which files you haven't got access to, or something like that. And, also, from what I know, /usr/lib has nothing to do with cache, but I am still new to this, so, anyone correct me if I am wrong.
Also, who are "they" and what's the deal with a 100Mb file in /tmp?

"they" would be the person(s) who wrote the exploit...

i believe the 100MB file is only needed if you don't have any other big file to run the exploit against...

Is it possible that they're just trying to clear the disc cache by reading arbitrary data?