Re: [PATCH] filter: added BPF random opcode

[Alexei Starovoitov <ast@plumgrid.com>]

On Tue, Apr 15, 2014 at 11:16 AM, Chema Gonzalez <chema@google.com> wrote:
> This should allow random packet sampling.

I think commit log should have more than one line,
especially for the new feature that affects uapi

In particular I don't find the reason of moving random
packet sampling into kernel to be that great.
pcap cannot receive them quickly enough anyway,
so there are drops already.
Just randomly pick packets that reached the user space.
Why add this to the kernel? I don't see how it can improve accuracy.

At the same time I think the extension technic itself is nice and clean :)
The call approach is definitely how we envisioned it to be used.
Right now we have only x86-64 jit that is waiting for net-next to be opened,
but this extension will be automatically jit-ed due to 'call approach'. Nice.

When you post patch revision please add vN tag.

Minor nits below:

> Signed-off-by: Chema Gonzalez <chema@google.com>
> ---
>  Documentation/networking/filter.txt | 13 +++++++++++++
>  include/linux/filter.h              |  1 +
>  include/uapi/linux/filter.h         |  3 ++-
>  net/core/filter.c                   | 12 ++++++++++++
>  tools/net/bpf_exp.l                 |  1 +
>  tools/net/bpf_exp.y                 | 11 ++++++++++-
>  6 files changed, 39 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/networking/filter.txt b/Documentation/networking/filter.txt
> index 81f940f..82e1cb0 100644
> --- a/Documentation/networking/filter.txt
> +++ b/Documentation/networking/filter.txt
> @@ -281,6 +281,7 @@ Possible BPF extensions are shown in the following table:
>    cpu                                   raw_smp_processor_id()
>    vlan_tci                              vlan_tx_tag_get(skb)
>    vlan_pr                               vlan_tx_tag_present(skb)
> +  rand                                  prandom_u32()
>
>  These extensions can also be prefixed with '#'.
>  Examples for low-level BPF:
> @@ -308,6 +309,18 @@ Examples for low-level BPF:
>    ret #-1
>    drop: ret #0
>
> +** icmp random packet sampling, 1 in 4
> +  ldh [12]
> +  jne #0x800, drop
> +  ldb [23]
> +  jneq #1, drop
> +  # get a random uint32 number
> +  ld rand
> +  mod #4
> +  jneq #1, drop
> +  ret #-1
> +  drop: ret #0
> +
>  ** SECCOMP filter example:
>
>    ld [4]                  /* offsetof(struct seccomp_data, arch) */
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 262dcbb..49c28aa 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -224,6 +224,7 @@ enum {
>         BPF_S_ANC_VLAN_TAG,
>         BPF_S_ANC_VLAN_TAG_PRESENT,
>         BPF_S_ANC_PAY_OFFSET,
> +       BPF_S_ANC_RANDOM,
>  };
>
>  #endif /* __LINUX_FILTER_H__ */
> diff --git a/include/uapi/linux/filter.h b/include/uapi/linux/filter.h
> index 8eb9cca..253b4d4 100644
> --- a/include/uapi/linux/filter.h
> +++ b/include/uapi/linux/filter.h
> @@ -130,7 +130,8 @@ struct sock_fprog { /* Required for SO_ATTACH_FILTER. */
>  #define SKF_AD_VLAN_TAG        44
>  #define SKF_AD_VLAN_TAG_PRESENT 48
>  #define SKF_AD_PAY_OFFSET      52
> -#define SKF_AD_MAX     56
> +#define SKF_AD_RANDOM  56
> +#define SKF_AD_MAX     60
>  #define SKF_NET_OFF   (-0x100000)
>  #define SKF_LL_OFF    (-0x200000)
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 765556b..b2a80a1 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -637,6 +637,12 @@ static u64 __get_raw_cpu_id(u64 ctx, u64 A, u64 X, u64 r4, u64 r5)
>         return raw_smp_processor_id();
>  }
>
> +/* note that this only generates 32-bit random numbers */
> +static u64 __skb_get_random(u64 ctx, u64 A, u64 X, u64 r4, u64 r5)

function name is misleading. It has nothing to do with 'skb'. Please
drop that prefix.

> +{
> +       return (u64)prandom_u32();

we have 64-bit registers now, so would be nice to generalize it to 64-bit random
since 8/16/32 can be made with bpf_and operation.
but I don't see how to cleanly do it yet, so I guess the current form is fine.

I like the approach, but would be nice to hear better justification
for extending uapi.

Thanks!

> +}
> +
>  /* Register mappings for user programs. */
>  #define A_REG          0
>  #define X_REG          7
> @@ -773,6 +779,7 @@ static bool convert_bpf_extensions(struct sock_filter *fp,
>         case SKF_AD_OFF + SKF_AD_NLATTR:
>         case SKF_AD_OFF + SKF_AD_NLATTR_NEST:
>         case SKF_AD_OFF + SKF_AD_CPU:
> +       case SKF_AD_OFF + SKF_AD_RANDOM:
>                 /* arg1 = ctx */
>                 insn->code = BPF_ALU64 | BPF_MOV | BPF_X;
>                 insn->a_reg = ARG1_REG;
> @@ -806,6 +813,9 @@ static bool convert_bpf_extensions(struct sock_filter *fp,
>                 case SKF_AD_OFF + SKF_AD_CPU:
>                         insn->imm = __get_raw_cpu_id - __bpf_call_base;
>                         break;
> +               case SKF_AD_OFF + SKF_AD_RANDOM:
> +                       insn->imm = __skb_get_random - __bpf_call_base;
> +                       break;
>                 }
>                 break;
>
> @@ -1356,6 +1366,7 @@ int sk_chk_filter(struct sock_filter *filter, unsigned int flen)
>                         ANCILLARY(VLAN_TAG);
>                         ANCILLARY(VLAN_TAG_PRESENT);
>                         ANCILLARY(PAY_OFFSET);
> +                       ANCILLARY(RANDOM);
>                         }
>
>                         /* ancillary operation unknown or unsupported */
> @@ -1741,6 +1752,7 @@ void sk_decode_filter(struct sock_filter *filt, struct sock_filter *to)
>                 [BPF_S_ANC_VLAN_TAG]    = BPF_LD|BPF_B|BPF_ABS,
>                 [BPF_S_ANC_VLAN_TAG_PRESENT] = BPF_LD|BPF_B|BPF_ABS,
>                 [BPF_S_ANC_PAY_OFFSET]  = BPF_LD|BPF_B|BPF_ABS,
> +               [BPF_S_ANC_RANDOM]      = BPF_LD|BPF_B|BPF_ABS,
>                 [BPF_S_LD_W_LEN]        = BPF_LD|BPF_W|BPF_LEN,
>                 [BPF_S_LD_W_IND]        = BPF_LD|BPF_W|BPF_IND,
>                 [BPF_S_LD_H_IND]        = BPF_LD|BPF_H|BPF_IND,
> diff --git a/tools/net/bpf_exp.l b/tools/net/bpf_exp.l
> index bf7be77..833a966 100644
> --- a/tools/net/bpf_exp.l
> +++ b/tools/net/bpf_exp.l
> @@ -92,6 +92,7 @@ extern void yyerror(const char *str);
>  "#"?("cpu")    { return K_CPU; }
>  "#"?("vlan_tci") { return K_VLANT; }
>  "#"?("vlan_pr")        { return K_VLANP; }
> +"#"?("rand")   { return K_RAND; }
>
>  ":"            { return ':'; }
>  ","            { return ','; }
> diff --git a/tools/net/bpf_exp.y b/tools/net/bpf_exp.y
> index d15efc9..e6306c5 100644
> --- a/tools/net/bpf_exp.y
> +++ b/tools/net/bpf_exp.y
> @@ -56,7 +56,7 @@ static void bpf_set_jmp_label(char *label, enum jmp_type type);
>  %token OP_LDXI
>
>  %token K_PKT_LEN K_PROTO K_TYPE K_NLATTR K_NLATTR_NEST K_MARK K_QUEUE K_HATYPE
> -%token K_RXHASH K_CPU K_IFIDX K_VLANT K_VLANP K_POFF
> +%token K_RXHASH K_CPU K_IFIDX K_VLANT K_VLANP K_POFF K_RAND
>
>  %token ':' ',' '[' ']' '(' ')' 'x' 'a' '+' 'M' '*' '&' '#' '%'
>
> @@ -164,6 +164,9 @@ ldb
>         | OP_LDB K_POFF {
>                 bpf_set_curr_instr(BPF_LD | BPF_B | BPF_ABS, 0, 0,
>                                    SKF_AD_OFF + SKF_AD_PAY_OFFSET); }
> +       | OP_LDB K_RAND {
> +               bpf_set_curr_instr(BPF_LD | BPF_B | BPF_ABS, 0, 0,
> +                                  SKF_AD_OFF + SKF_AD_RANDOM); }
>         ;
>
>  ldh
> @@ -212,6 +215,9 @@ ldh
>         | OP_LDH K_POFF {
>                 bpf_set_curr_instr(BPF_LD | BPF_H | BPF_ABS, 0, 0,
>                                    SKF_AD_OFF + SKF_AD_PAY_OFFSET); }
> +       | OP_LDH K_RAND {
> +               bpf_set_curr_instr(BPF_LD | BPF_H | BPF_ABS, 0, 0,
> +                                  SKF_AD_OFF + SKF_AD_RANDOM); }
>         ;
>
>  ldi
> @@ -265,6 +271,9 @@ ld
>         | OP_LD K_POFF {
>                 bpf_set_curr_instr(BPF_LD | BPF_W | BPF_ABS, 0, 0,
>                                    SKF_AD_OFF + SKF_AD_PAY_OFFSET); }
> +       | OP_LD K_RAND {
> +               bpf_set_curr_instr(BPF_LD | BPF_W | BPF_ABS, 0, 0,
> +                                  SKF_AD_OFF + SKF_AD_RANDOM); }
>         | OP_LD 'M' '[' number ']' {
>                 bpf_set_curr_instr(BPF_LD | BPF_MEM, 0, 0, $4); }
>         | OP_LD '[' 'x' '+' number ']' {
> --
> 1.9.1.423.g4596e3a
>