Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF

[Will Drewry]

On Fri, Jan 13, 2012 at 11:31 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> On 01/12, Will Drewry wrote:
>>
>> On Thu, Jan 12, 2012 at 11:23 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>> > On 01/12, Will Drewry wrote:
>> >>
>> >> On Thu, Jan 12, 2012 at 10:22 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>> >> >> +      */
>> >> >> +     regs = seccomp_get_regs(regs_tmp, &regs_size);
>> >> >
>> >> > Stupid question. I am sure you know what are you doing ;) and I know
>> >> > nothing about !x86 arches.
>> >> >
>> >> > But could you explain why it is designed to use user_regs_struct ?
>> >> > Why we can't simply use task_pt_regs() and avoid the (costly) regsets?
>> >>
>> >> So on x86 32, it would work since user_regs_struct == task_pt_regs
>> >> (iirc), but on x86-64
>> >> and others, that's not true.
>> >
>> > Yes sure, I meant that userpace should use pt_regs too.
>> >
>> >> If it would be appropriate to expose pt_regs to userspace, then I'd
>> >> happily do so :)
>> >
>> > Ah, so that was the reason. But it is already exported? At least I see
>> > the "#ifndef __KERNEL__" definition in arch/x86/include/asm/ptrace.h.
>> >
>> > Once again, I am not arguing, just trying to understand. And I do not
>> > know if this definition is part of abi.
>>
>> I don't either :/  My original idea was to operate on task_pt_regs(current),
>> but I noticed that PTRACE_GETREGS/SETREGS only uses the
>> user_regs_struct. So I went that route.
>
> Well, I don't know where user_regs_struct come from initially. But
> probably it is needed to allow to access the "artificial" things like
> fs_base. Or perhaps this struct mimics the layout in the coredump.

Not sure - added Roland whose name was on many of the files :)

I just noticed that ptrace ABI allows pt_regs access using the register
macros (PTRACE_PEEKUSR) and user_regs_struct access (PTRACE_GETREGS).

But I think the latter is guaranteed to have a certain layout while the macros
for PEEKUSR can do post-processing fixup.  (Which could be done in the
bpf evaluator load_pointer() helper if needed.)

>> I'd love for pt_regs to be fair game to cut down on the copying!
>
> Me too. I see no point in using user_regs_struct.

I'll rev the change to use pt_regs and drop all the helper code.  If
no one says otherwise, that certainly seems ideal from a performance
perspective, and I see pt_regs exported to userland along with ptrace
abi register offset macros.


Thanks!
will
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