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

From: Daniel Borkmann <[email-masked]>
Date: Tue, 04 Mar 2014 10:59:42 +0100

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 <[email-masked]>

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>".

> ---
>   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.

...
> +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.

>   /* 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 <[email-masked]>
> @@ -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 ?

> +			insn->a_reg = 0;
> +			insn->x_reg = 6;
> +			insn->imm = fp->k;
> +
> +			insn++;
> +			insn->code = BPF_RET | BPF_K;
> +			break;
...
> +	/* RET */
> +BPF_RET_BPF_K_0:
> +	return regs[0/* R0 */];