Re: v5 of seccomp filter c/r patches

[Tycho Andersen <tycho.andersen@canonical.com>]

On Fri, Oct 02, 2015 at 02:10:24PM -0700, Kees Cook wrote:
> On Fri, Oct 2, 2015 at 9:27 AM, Tycho Andersen
> <tycho.andersen@canonical.com> wrote:
> > Hi all,
> >
> > Here's v5 of the seccomp filter c/r set. The individual patch notes have
> > changes, but two highlights are:
> >
> > * This series is now based on http://patchwork.ozlabs.org/patch/525492/ and
> >   will need to be built with that patch applied. This gets rid of two incorrect
> >   patches in the previous series and is a nicer API.
> >
> > * I couldn't figure out a nice way to have SECCOMP_GET_FILTER_FD return the
> >   same struct file across calls, so we still need a kcmp command. I've narrowed
> >   the scope of the one being added to only compare seccomp fds.
> >
> > Thoughts welcome,
> 
> Hi, sorry I've been slow/busy. I'm finally reading through these threads.
> 
> Happy bit:
> - avoiding eBPF and just saving the original filters makes things much easier.
> 
> Sad bit:
> - inventing a new interface for seccompfds feels like massive overkill to me.
> 
> While Andy has big dreams, we're not presently doing seccompfd
> monitoring, etc. There's no driving user for that kind of interface,
> and accepting the maintenance burden of it only for CRIU seems unwise.
> 
> So, I'll go back to what I originally proposed at LSS (which it looks
> like we're half way there now):
> 
> - save the original filter (done!)
> - extract filters through a single special-purpose interface (looks
> like ptrace is the way to go: root-only, stopped process, etc)
> - compare filter content and issue TSYNCs to merge detected sibling
> threads, since merging things that weren't merged before creates no
> problems.
> 
> This means the parenting logic is heuristic, but it's entirely in
> userspace, so the complexity burden doesn't live in seccomp which we,
> by design, want to keep as simple as possible.

Ok, how about,

struct sock_filter insns[BPF_MAXINSNS];
insn_cnt = ptrace(PTRACE_SECCOMP_GET_FILTER, pid, insns, i);

when asking for the ith filter? It returns either the number of
instructions, -EINVAL if something was wrong (i, pid,
CONFIG_CHECKPOINT_RESTORE isn't enabled). While it would always
succeed now, if/when the underlying filter was not created from a bpf
classic filter, we can return -EMEDIUMTYPE? (Suggestions welcome, I
picked this mostly based on what sounds nice.)

Tycho