Re: [PATCH v10 05/11] seccomp: add system call filtering using BPF

[Will Drewry]

On Wed, Feb 22, 2012 at 1:53 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> On 02/22/2012 11:47 AM, Will Drewry wrote:
>>>
>>> I highly disagree with every filter having to check the mode: Filters that
>>> don't check the arch on e.g. x86 are buggy, so they have to check it, even
>>> if it's a 32-bit or 64-bit only system, the filters can't know that and
>>> needs to check the arch at every syscall entry. All other info in the data
>>> depends on the arch, because of this there isn't much code to share between
>>> the two archs, so you can as well have one filter for each arch.
>>>
>>> Alternative approach: Tell the arch at filter install time and only run the
>>> filters with the same arch as the current system call. If no filters are run,
>>> deny the systemcall.
>>
>> This was roughly how I first implemented compat and non-compat
>> support.  It causes some implicit behavior across inheritance that is
>> not nice though.
>>
>
> This is trivially doable at the BPF level, right?  Just make this the
> first instruction in the program (either deny or jump to a separate
> program branch)... and then there is still "one program" without any
> weird inheritance issues?

Exactly, and that's what the patch does now (after your feedback :)

ld arch
je arch, 1, 0
ret SECCOMP_RET_KILL
<rest of bpf program>

At this point, I don't think it makes sense to do it a different way
than just in the BPF program even if it does mean leaving out the
check could leave the program open to compat-style bugs.  At least a
shared library and/or good practices should be able to catch that
error.

thanks!
will
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