Re: eBPF / seccomp globals?

From: Alexei Starovoitov
Date: Thu Sep 03 2015 - 23:17:33 EST

On Fri, Sep 04, 2015 at 01:01:20AM +0000, Michael Tirado wrote:
> Hiyall,
> I have created a seccomp white list filter for a program that launches
> other less trustworthy programs. It's working great so far, but I
> have run into a little roadblock. the launcher program needs to call
> execve as it's final step, but that may not be present in the white
> list. I am wondering if there is any way to use some sort of global
> variable that will be preserved between syscall filter calls so that I
> can allow only one execve, if not present in white list by
> incrementing a counter variable.
> I see that in Documentation/networking/filter.txt one of the registers
> is documented as being a pointer to struct sk_buff, in the seccomp
> context this is a pointer to struct seccomp_data instead, right? and
> the line about callee saved registers R6-R9 probably refers to them
> being saved across calls within that filter, and not calls between
> filters?

R6-R9 are the registered preserved across calls to helper functions
within single program. They are not preserved across invocations
of the same program. At the start of the program only R1 (pointer
to context) is valid.
The eBPF programs used for kprobes, sockets and TC can simulate
global state via maps. Like a map of one element can have some
'struct globals { ... }' as a value in such map. Then programs
can keep global state in there. If a key into such map is cpu_id,
then such state becomes per-cpu global. Other tricks possible too.
Unfortunately seccomp doesn't have access to eBPF yet
(only classic BPF is supported), but, I believe, Tycho is
working on adding eBPF to seccomp and criu of eBPF programs...

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