netdev - Re: [PATCH v2 bpf-next] bpf: allow BPF programs access skb_shared_info->gso

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Date:   Fri, 18 Jan 2019 18:42:20 +0000
From:   Martin Lau <kafai@...com>
To:     Eric Dumazet <edumazet@...gle.com>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Eddie Hao <eddieh@...gle.com>,
        Vlad Dumitrescu <vladum@...gle.com>,
        Xiaotian Pei <xiaotian@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH v2 bpf-next] bpf: allow BPF programs access
 skb_shared_info->gso_segs field

On Thu, Jan 17, 2019 at 03:31:57PM -0800, Eric Dumazet wrote:
> This adds the ability to read gso_segs from a BPF program.
> 
> v2: refined Eddie Hao patch to address Alexei feedback.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Eddie Hao <eddieh@...gle.com>
> Cc: Vlad Dumitrescu <vladum@...gle.com>
> Cc: Xiaotian Pei <xiaotian@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>
> ---
>  include/uapi/linux/bpf.h                    |  1 +
>  net/core/filter.c                           | 21 ++++++++++++
>  tools/include/uapi/linux/bpf.h              |  1 +
>  tools/testing/selftests/bpf/test_verifier.c | 36 +++++++++++++++++++++
>  4 files changed, 59 insertions(+)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 91c43884f295f60a85268ddf0020bf8aa47f8329..2940a9854f6d8e493518ca894e0c9c630ae4ab7a 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -2540,6 +2540,7 @@ struct __sk_buff {
>  	__bpf_md_ptr(struct bpf_flow_keys *, flow_keys);
>  	__u64 tstamp;
>  	__u32 wire_len;
> +	__u32 gso_segs;
>  };
>  
>  struct bpf_tunnel_key {
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 2b3b436ef5457bf44c99780d6dec0b5f403f005c..a6ff5d9a04cf06926ee75cbc523456d12baf25ae 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -6700,6 +6700,27 @@ static u32 bpf_convert_ctx_access(enum bpf_access_type type,
>  							     target_size));
>  		break;
>  
> +	case offsetof(struct __sk_buff, gso_segs):
> +		/* si->dst_reg = skb_shinfo(SKB); */
> +#ifdef NET_SKBUFF_DATA_USES_OFFSET
> +		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, head),
> +				      si->dst_reg, si->src_reg,
> +				      offsetof(struct sk_buff, head));
> +		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, end),
> +				      BPF_REG_TMP, si->src_reg,
> +				      offsetof(struct sk_buff, end));
I am not sure BPF_REG_TMP can be used for non-classic BPF.
The earlier insn could be using BPF_REG_TMP (which is BPF_REG_2) and
R2 would become loss after this BPF_LDX_MEM.

Daniel, can BPF_REG_AX be used here as a tmp?

> +		*insn++ = BPF_ALU64_REG(BPF_ADD, si->dst_reg, BPF_REG_TMP);
> +#else
> +		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, end),
> +				      si->dst_reg, si->src_reg,
> +				      offsetof(struct sk_buff, end));
> +#endif
> +		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct skb_shared_info, gso_segs),
> +				      si->dst_reg, si->dst_reg,
> +				      bpf_target_off(struct skb_shared_info,
> +						     gso_segs, 2,
> +						     target_size));
> +		break;
>  	case offsetof(struct __sk_buff, wire_len):
>  		BUILD_BUG_ON(FIELD_SIZEOF(struct qdisc_skb_cb, pkt_len) != 4);
>  
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index 91c43884f295f60a85268ddf0020bf8aa47f8329..2940a9854f6d8e493518ca894e0c9c630ae4ab7a 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -2540,6 +2540,7 @@ struct __sk_buff {
>  	__bpf_md_ptr(struct bpf_flow_keys *, flow_keys);
>  	__u64 tstamp;
>  	__u32 wire_len;
> +	__u32 gso_segs;
>  };
>  
>  struct bpf_tunnel_key {
> diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
> index 2fd90d4568926d13542783c870507d43a6d6bb64..2c46531044bdf9ec1e4fa47e2c94c9edb0ac3d08 100644
> --- a/tools/testing/selftests/bpf/test_verifier.c
> +++ b/tools/testing/selftests/bpf/test_verifier.c
> @@ -5663,6 +5663,42 @@ static struct bpf_test tests[] = {
>  		.result = ACCEPT,
>  		.prog_type = BPF_PROG_TYPE_CGROUP_SKB,
>  	},
> +	{
> +		"read gso_segs from CGROUP_SKB",
> +		.insns = {
> +			BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1,
> +				    offsetof(struct __sk_buff, gso_segs)),
> +			BPF_MOV64_IMM(BPF_REG_0, 0),
> +			BPF_EXIT_INSN(),
> +		},
> +		.result = ACCEPT,
> +		.prog_type = BPF_PROG_TYPE_CGROUP_SKB,
> +	},
> +	{
> +		"write gso_segs from CGROUP_SKB",
> +		.insns = {
> +			BPF_MOV64_IMM(BPF_REG_0, 0),
> +			BPF_STX_MEM(BPF_W, BPF_REG_1, BPF_REG_0,
> +				    offsetof(struct __sk_buff, gso_segs)),
> +			BPF_MOV64_IMM(BPF_REG_0, 0),
> +			BPF_EXIT_INSN(),
> +		},
> +		.result = REJECT,
> +		.result_unpriv = REJECT,
> +		.errstr = "invalid bpf_context access off=164 size=4",
> +		.prog_type = BPF_PROG_TYPE_CGROUP_SKB,
> +	},
> +	{
> +		"read gso_segs from CLS",
> +		.insns = {
> +			BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1,
> +				    offsetof(struct __sk_buff, gso_segs)),
> +			BPF_MOV64_IMM(BPF_REG_0, 0),
> +			BPF_EXIT_INSN(),
> +		},
> +		.result = ACCEPT,
> +		.prog_type = BPF_PROG_TYPE_SCHED_CLS,
> +	},
>  	{
>  		"multiple registers share map_lookup_elem result",
>  		.insns = {
> -- 
> 2.20.1.321.g9e740568ce-goog
>