Re: [PATCH 5/7] seccomp_filter: Document what seccomp_filter is andhow it works.

[Eric Paris]

On Wed, 2011-05-04 at 12:06 -0400, Steven Rostedt wrote:
> On Wed, 2011-05-04 at 11:54 -0400, Eric Paris wrote:
> 
> > As this is a deny by default interface which only allows you to further
> > restrict you couldn't add more than 1 syscall if you didn't have an
> > explict 'apply' action.
> > 
> > SECCOMP_FILTER_SET, __NR_fo, "a=0"
> > SECCOMP_FILTER_SET, __NR_read, "1" == EPERM
> > 
> > Maybe apply on set is fine after the first apply, but we definitely need
> > some way to do more than 1 set before the rules are applied....
> 
> So we could have SET be 'or' and APPLY be 'and'.
> 
> SECCOMP_FILTER_SET, __NR_foo, "a=0"
> SECCOMP_FILTER_SET, __NR_read, "1" == EPERM

When I said "== EPERM" I meant that the given prctl call would return
EPERM.  I'm going to pretend that you didn't type it.

> SECCOPM_FILTER_APPLY
> 
> SECCOMP_FILTER_SET, __NR_foo, "b=0"
> SECCOPM_FILTER_APPLY
> 
> Will end up being:
> 
> (foo: a == 0 || read: "1") && (foo: b == 0)
> 
> The second set/apply now removes the read option, and foo only works if
> a is 0 and b is 0.
> 
> This would also work for children, as they can only restrict (with
> 'and') and can not add more control.

I think we pretty much agree although I'm pretty that we will have 1
filter per syscall.  So the rules would really be (in your syntax)

Rule1: (foo: a == 0 && b == 0)
OR
Rule2: (read: "1")

Although logically the same, it's not just one huge rule.  I don't see
any need for any operation other than an &&.  Before the first "set" you
can add new syscalls.  After the first set you can only && onto existing
syscalls.  So the following set of operations:

SECCOMP_FILTER_SET, __NR_foo, "a=0"
SECCOMP_FILTER_SET, __NR_read, "1"
SECCOPM_FILTER_APPLY
 
SECCOMP_FILTER_SET, __NR_foo, "b=0"
SECCOMP_FILTER_APPLY

SECCOMP_FILTER_SET, __NR_write, "1"
SECCOMP_FILTER_APPLY

Would return EPERM for the __NR_write entry since it was a new syscall
after a set.  I think we agree on all this.

I do have a question on some syntax proposed a while back.  Given:
SECCOMP_FILTER_SET, __NR_foo, "a=0"
SECCOMP_FILTER_SET, __NR_foo, "b=0"
SECCOMP_FILTER_APPLY

I would think to keep the interface consistent that should result in
foo: (a=0) && (b=0)

But I think the proposal was that we should instead have just
foo: (b=0)

What's the logic behind having a second call overwrite uncommitted
changes?  I sorta feel like if I put it in there, I must have wanted it
in there   :)

-Eric

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