Re: [PATCH v4 net-next 1/3] Extended BPF interpreter and converter

[Alexei Starovoitov]

On Tue, Mar 4, 2014 at 1:59 AM, Daniel Borkmann <dborkman@xxxxxxxxxx> wrote:
> On 03/04/2014 06:18 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    139        270      296       470
>> old BPF jit 26        182       37       202
>> new BPF jit TBD
>>
>> Tested with trinify BPF fuzzer
>>
>> Future work:
>>
>> 0. seccomp
>>
>> 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 <ast@xxxxxxxxxxxx>
>
>
> Looks great, imho, some comments/questions inline:
>
> Nit: subject line of your patches should be, e.g.
>
>  "filter: add Extended BPF interpreter and converter"
>  "doc: filter: add Extended BPF documentation"
>  ...
>
> so first "<subsystem>: <summary phrase>".

sure. Will send v5 :)

>> ---
>>   include/linux/filter.h      |    8 +-
>>   include/linux/netdevice.h   |    1 +
>>   include/uapi/linux/filter.h |   34 +-
>>   net/core/filter.c           |  802
>> ++++++++++++++++++++++++++++++++++++++++++-
>>   net/core/sysctl_net_core.c  |    7 +
>>   5 files changed, 830 insertions(+), 22 deletions(-)
>>
>> diff --git a/include/linux/filter.h b/include/linux/filter.h
>> index e568c8ef896b..0e84ff6e991b 100644
>> --- a/include/linux/filter.h
>> +++ b/include/linux/filter.h
>> @@ -52,7 +52,13 @@ extern int sk_detach_filter(struct sock *sk);
>>   extern int sk_chk_filter(struct sock_filter *filter, unsigned int flen);
>>   extern int sk_get_filter(struct sock *sk, struct sock_filter __user
>> *filter, unsigned len);
>>   extern void sk_decode_filter(struct sock_filter *filt, struct
>> sock_filter *to);
>> +/* function remaps 'sock_filter' insns to 'sock_filter_ext' insns */
>> +int sk_convert_filter(struct sock_filter *old_prog, int len,
>> +                     struct sock_filter_ext *new_prog, int *p_new_len);
>> +/* execute extended bpf program */
>
>
> I think this and the above comment can be omitted, as both have a kernel doc
> in its implementation in net/core/filter.c that is more precise.

ok.
I like comments in .h, because this is how 'make tags' orders them
and 'vim -t sk_convert_filter' jumps there instead of .c
but ok, will remove them, since they indeed look out of place there.

> ...
>
>> +struct sock_filter_ext {
>> +       __u8    code;    /* opcode */
>> +       __u8    a_reg:4; /* dest register */
>> +       __u8    x_reg:4; /* source register */
>> +       __s16   off;     /* signed offset */
>> +       __s32   imm;     /* signed immediate constant */
>> +};
>> +
>>   struct sock_fprog {   /* Required for SO_ATTACH_FILTER. */
>>         unsigned short          len;    /* Number of filter blocks */
>>         struct sock_filter __user *filter;
>> @@ -45,12 +54,15 @@ struct sock_fprog { /* Required for SO_ATTACH_FILTER.
>> */
>>   #define         BPF_JMP         0x05
>>   #define         BPF_RET         0x06
>>   #define         BPF_MISC        0x07
>> +#define         BPF_ALU64       0x07
>> +
>>
>
> Please do not add empty newline above.

ohh. yes. good catch.
Right below this line in my temp_filter.h I have few macros
(similar to BPF_STMT and BPF_JUMP) that makes manual coding of
ebpf easier, but I sed them out of this diff just to include in the
future diffs.
will double check for empty lines.

>
>>   /* ld/ldx fields */
>>   #define BPF_SIZE(code)  ((code) & 0x18)
>>   #define         BPF_W           0x00
>>   #define         BPF_H           0x08
>>   #define         BPF_B           0x10
>
> ...
>
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index ad30d626a5bd..1494421486b7 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -1,5 +1,6 @@
>>   /*
>>    * Linux Socket Filter - Kernel level socket filtering
>> + * Extended BPF is Copyright (c) 2011-2014 PLUMgrid, http://plumgrid.com
>>    *
>>    * Author:
>>    *     Jay Schulist <jschlst@xxxxxxxxx>
>> @@ -40,6 +41,8 @@
>>   #include <linux/seccomp.h>
>>   #include <linux/if_vlan.h>
>>
>> +int bpf_ext_enable __read_mostly;
>> +
>>   /* No hurry in this branch
>>    *
>>    * Exported for the bpf jit load helper.
>> @@ -399,6 +402,7 @@ load_b:
>>         }
>>
>>         return 0;
>> +#undef K
>>   }
>>   EXPORT_SYMBOL(sk_run_filter);
>
> ...
>
>> +               /* RET_K, RET_A are remaped into 2 insns */
>> +               case BPF_RET | BPF_A:
>> +               case BPF_RET | BPF_K:
>> +                       insn->code = BPF_ALU | BPF_MOV |
>> +                               (BPF_SRC(fp->code) == BPF_K ? BPF_K :
>> BPF_X);
>
>
> Hmm, so the case statement is about BPF_RET | BPF_A and BPF_RET | BPF_K
> but BPF_RET | BPF_X is not mentioned. However, in BPF_SRC(fp->code)
> selection you fall back to BPF_X if it doesn't equal BPF_K? Is that
> correct? And, you probably also need to handle BPF_RET | BPF_X ?

:) that design choice of original BPF always puzzled me.
BPF_A macro only used in one insn: BPF_RET + BPF_A
and all other insns use BPF_K and BPF_X
and though comment in uapi/filter.h says "ret - BPF_K and BPF_X also apply"
this is not true, since sk_chk_filter() only allows ret+a and ret+k
libpcap is equally confused. It never generates ret+x, but has few
places in the code
that can recognize it. I guess that's an artifact of distant past.

epbf has only one RET insn that takes register R0 and returns it.
That is similar to real CPU 'ret' insn and done to make epbf easier
to generate from gcc/llvm point of view.
ebpf jit converts 'ret' into 'leave; ret' on x86_64.

so original bpf+k and bpf+a are converted into 'mov r0, [a or k]; ret r0'

btw, if there is interest I can put ebpf testsuite into tools/net/

Thanks!
Alexei
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