Subject: Re: [PATCH v7 net-next 1/3] filter: add Extended BPF interpreter and converter

From: Pavel Emelyanov <[email-masked]>
Date: Tue, 11 Mar 2014 22:19:06 +0400

On 03/11/2014 10:03 PM, Alexei Starovoitov wrote:
> On Tue, Mar 11, 2014 at 10:40 AM, Pavel Emelyanov <[email-masked]> wrote:
>> On 03/10/2014 02:00 AM, Daniel Borkmann wrote:
>>> On 03/09/2014 06:08 PM, Alexei Starovoitov wrote:
>>>> On Sun, Mar 9, 2014 at 5:29 AM, Daniel Borkmann <[email-masked]> wrote:
>>>>> On 03/09/2014 12:15 AM, Alexei Starovoitov wrote:
>>>>>>
>>>>>> Extended BPF extends old BPF in the following ways:
>>>>>> - from 2 to 10 registers
>>>>>>     Original BPF has two registers (A and X) and hidden frame pointer.
>>>>>>     Extended BPF has ten registers and read-only frame pointer.
>>>>>> - from 32-bit registers to 64-bit registers
>>>>>>     semantics of old 32-bit ALU operations are preserved via 32-bit
>>>>>>     subregisters
>>>>>> - if (cond) jump_true; else jump_false;
>>>>>>     old BPF insns are replaced with:
>>>>>>     if (cond) jump_true; /* else fallthrough */
>>>>>> - adds signed > and >= insns
>>>>>> - 16 4-byte stack slots for register spill-fill replaced with
>>>>>>     up to 512 bytes of multi-use stack space
>>>>>> - introduces bpf_call insn and register passing convention for zero
>>>>>>     overhead calls from/to other kernel functions (not part of this patch)
>>>>>> - adds arithmetic right shift insn
>>>>>> - adds swab32/swab64 insns
>>>>>> - adds atomic_add insn
>>>>>> - old tax/txa insns are replaced with 'mov dst,src' insn
>>>>>>
>>>>>> Extended BPF is designed to be JITed with one to one mapping, which
>>>>>> allows GCC/LLVM backends to generate optimized BPF code that performs
>>>>>> almost as fast as natively compiled code
>>>>>>
>>>>>> sk_convert_filter() remaps old style insns into extended:
>>>>>> 'sock_filter' instructions are remapped on the fly to
>>>>>> 'sock_filter_ext' extended instructions when
>>>>>> sysctl net.core.bpf_ext_enable=1
>>>>>>
>>>>>> Old filter comes through sk_attach_filter() or
>>>>>> sk_unattached_filter_create()
>>>>>>    if (bpf_ext_enable) {
>>>>>>       convert to new
>>>>>>       sk_chk_filter() - check old bpf
>>>>>>       use sk_run_filter_ext() - new interpreter
>>>>>>    } else {
>>>>>>       sk_chk_filter() - check old bpf
>>>>>>       if (bpf_jit_enable)
>>>>>>           use old jit
>>>>>>       else
>>>>>>           use sk_run_filter() - old interpreter
>>>>>>    }
>>>>>>
>>>>>> sk_run_filter_ext() interpreter is noticeably faster
>>>>>> than sk_run_filter() for two reasons:
>>>>>>
>>>>>> 1.fall-through jumps
>>>>>>     Old BPF jump instructions are forced to go either 'true' or 'false'
>>>>>>     branch which causes branch-miss penalty.
>>>>>>     Extended BPF jump instructions have one branch and fall-through,
>>>>>>     which fit CPU branch predictor logic better.
>>>>>>     'perf stat' shows drastic difference for branch-misses.
>>>>>>
>>>>>> 2.jump-threaded implementation of interpreter vs switch statement
>>>>>>     Instead of single tablejump at the top of 'switch' statement, GCC will
>>>>>>     generate multiple tablejump instructions, which helps CPU branch
>>>>>> predictor
>>>>>>
>>>>>> Performance of two BPF filters generated by libpcap was measured
>>>>>> on x86_64, i386 and arm32.
>>>>>>
>>>>>> fprog #1 is taken from Documentation/networking/filter.txt:
>>>>>> tcpdump -i eth0 port 22 -dd
>>>>>>
>>>>>> fprog #2 is taken from 'man tcpdump':
>>>>>> tcpdump -i eth0 'tcp port 22 and (((ip[2:2] - ((ip[0]&0xf)<<2)) -
>>>>>>      ((tcp[12]&0xf0)>>2)) != 0)' -dd
>>>>>>
>>>>>> Other libpcap programs have similar performance differences.
>>>>>>
>>>>>> Raw performance data from BPF micro-benchmark:
>>>>>> SK_RUN_FILTER on same SKB (cache-hit) or 10k SKBs (cache-miss)
>>>>>> time in nsec per call, smaller is better
>>>>>> --x86_64--
>>>>>>            fprog #1  fprog #1   fprog #2  fprog #2
>>>>>>            cache-hit cache-miss cache-hit cache-miss
>>>>>> old BPF     90       101       192       202
>>>>>> ext BPF     31        71       47         97
>>>>>> old BPF jit 12        34       17         44
>>>>>> ext BPF jit TBD
>>>>>>
>>>>>> --i386--
>>>>>>            fprog #1  fprog #1   fprog #2  fprog #2
>>>>>>            cache-hit cache-miss cache-hit cache-miss
>>>>>> old BPF    107        136      227       252
>>>>>> ext BPF     40        119       69       172
>>>>>>
>>>>>> --arm32--
>>>>>>            fprog #1  fprog #1   fprog #2  fprog #2
>>>>>>            cache-hit cache-miss cache-hit cache-miss
>>>>>> old BPF    202        300      475       540
>>>>>> ext BPF    180        270      330       470
>>>>>> old BPF jit 26        182       37       202
>>>>>> new BPF jit TBD
>>>>>>
>>>>>> Tested with trinify BPF fuzzer
>>>>>>
>>>>>> Future work:
>>>>>>
>>>>>> 0. add bpf/ebpf testsuite to tools/testing/selftests/net/bpf
>>>>>>
>>>>>> 1. add extended BPF JIT for x86_64
>>>>>>
>>>>>> 2. add inband old/new demux and extended BPF verifier, so that new
>>>>>> programs
>>>>>>      can be loaded through old sk_attach_filter() and
>>>>>> sk_unattached_filter_create()
>>>>>>      interfaces
>>>>>>
>>>>>> 3. tracing filters systemtap-like with extended BPF
>>>>>>
>>>>>> 4. OVS with extended BPF
>>>>>>
>>>>>> 5. nftables with extended BPF
>>>>>>
>>>>>> Signed-off-by: Alexei Starovoitov <[email-masked]>
>>>>>> Acked-by: Hagen Paul Pfeifer <[email-masked]>
>>>>>> Reviewed-by: Daniel Borkmann <[email-masked]>
>>>>>
>>>>>
>>>>> One more question or possible issue that came through my mind: When
>>>>> someone attaches a socket filter from user space, and bpf_ext_enable=1
>>>>> then the old filter will transparently be converted to the new
>>>>> representation. If then user space (e.g. through checkpoint restore)
>>>>> will issue a sk_get_filter() and thus we're calling sk_decode_filter()
>>>>> on sk->sk_filter and, therefore, try to decode what we stored in
>>>>> insns_ext[] with the assumption we still have the old code. Would that
>>>>> actually crash (or leak memory, or just return garbage), as we access
>>>>> decodes[] array with filt->code? Would be great if you could double-check.
>>>>
>>>> ohh. yes. missed that.
>>>> when bpf_ext_enable=1 I think it's cleaner to return ebpf filter.
>>>> This way the user space can see how old bpf filter was converted.
>>>>
>>>> Of course we can allocate extra memory and keep original bpf code there
>>>> just to return it via sk_get_filter(), but that seems overkill.
>>>
>>> Cc'ing Pavel for a8fc92778080 ("sk-filter: Add ability to get socket
>>> filter program (v2)").
>>>
>>> I think the issue can be that when applications could get migrated
>>> from one machine to another and their kernel won't support ebpf yet,
>>> then filter could not get loaded this way as it's expected to return
>>> what the user loaded. The trade-off, however, is that the original
>>> BPF code needs to be stored as well. :(
>>
>> Sorry if I miss the point, but isn't the original filter kept on socket?
>> The sk_attach_filter() does so, then calls __sk_prepare_filter, which
>> in turn calls bpf_jit_compile(), and the latter two keep the insns in place.
> 
> Yes. in V8/V9 series original filter is kept on socket.

Ah, I see :)

> and your crtools/test/zdtm/live/static/socket_filter.c test passes.
> Let me know if there are any other tests I can try.

No, that's the only test we need wrt sk-filter.
Thanks for keeping an eye on it :)