[PATCH 2/7] seccomp: Refactor the filter callback and the API

Andy Lutomirski luto at amacapital.net
Wed Jul 16 13:56:20 PDT 2014

On Wed, Jul 16, 2014 at 1:12 PM, Kees Cook <keescook at chromium.org> wrote:
> On Tue, Jul 15, 2014 at 12:32 PM, Andy Lutomirski <luto at amacapital.net> wrote:
>> The reason I did this is to add a seccomp API that will be usable
>> for an x86 fast path.  The x86 entry code needs to use a rather
>> expensive slow path for a syscall that might be visible to things
>> like ptrace.  By splitting seccomp into two phases, we can check
>> whether we need the slow path and then use the fast path in if the
>> filter allows the syscall or just returns some errno.
>> As a side effect, I think the new code is much easier to understand
>> than the old code.
> I'd agree. The #idefs got a little weirder, but the actual code flow
> was much easier to read. I wonder if "phase1" and "phase2" should be
> renamed "pretrace" and "tracing" or something more meaningful? Or
> "fast" and "slow"?

Queue the bikeshedding :)

I like "phase1" and "phase2" because it makes it clear that phase1 has
to come first.  But I'd be amenable to counterarguments.

>> This has one user-visible effect: the audit record written for
>> SECCOMP_RET_TRACE is now a simple indication that SECCOMP_RET_TRACE
>> happened.  It used to depend in a complicated way on what the tracer
>> did.  I couldn't make much sense of it.
> I think this change is okay. The only way to get the audit record to
> report SIGSYS before was to have an additional signal come in and kill
> it while the tracer was working on it. Which is confusing too. I like
> this way better.

Thanks :)


> -Kees
> --
> Kees Cook
> Chrome OS Security

Andy Lutomirski
AMA Capital Management, LLC

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