Re: [PATCH v8 5/9] seccomp: split mode set routines

[Andy Lutomirski]

On Fri, Jun 27, 2014 at 12:55 PM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> On 06/27, Andy Lutomirski wrote:
>>
>> On Fri, Jun 27, 2014 at 12:27 PM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>> > On 06/27, Kees Cook wrote:
>> >>
>> >> It looks like SMP ARM issues dsb for rmb, which seems a bit expensive.
>> >> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204g/CIHJFGFE.htm
>> >>
>> >> ...
>> >>
>> >> I really want to avoid adding anything to the secure_computing()
>> >> execution path. :(
>> >
>> > I must have missed something but I do not understand your concerns.
>> >
>> > __secure_computing() is not trivial, and we are going to execute the
>> > filters. Do you really think rmb() can add the noticeable difference?
>> >
>> > Not to mention that we can only get here if we take the slow syscall
>> > enter path due to TIF_SECCOMP...
>> >
>>
>> On my box, with my fancy multi-phase seccomp patches, the total
>> seccomp overhead for a very short filter is about 13ns.  Adding a full
>> barrier would add several ns, I think.
>
> I am just curious, does this 13ns overhead include the penalty from the
> slow syscall enter path triggered by TIF_SECCOMP ?

Yes, which is more or less the whole point of that patch series.  I
rewrote part of the TIF_SECCOMP-but-no-tracing case in assembly :)

I'm playing with rewriting it in C, but it's looking like it'll be a
bit more far-reaching than I was hoping.

--Andy
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