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Date: Tue, 8 Dec 2020 23:17:46 +0100
From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To: Lorenzo Bianconi <lorenzo@...nel.org>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org, davem@...emloft.net,
kuba@...nel.org, ast@...nel.org, daniel@...earbox.net,
shayagr@...zon.com, sameehj@...zon.com, john.fastabend@...il.com,
dsahern@...nel.org, brouer@...hat.com, echaudro@...hat.com,
lorenzo.bianconi@...hat.com, jasowang@...hat.com
Subject: Re: [PATCH v5 bpf-next 13/14] bpf: add new frame_length field to the
XDP ctx
On Mon, Dec 07, 2020 at 05:32:42PM +0100, Lorenzo Bianconi wrote:
> From: Eelco Chaudron <echaudro@...hat.com>
>
> This patch adds a new field to the XDP context called frame_length,
> which will hold the full length of the packet, including fragments
> if existing.
The approach you took for ctx access conversion is barely described :/
>
> eBPF programs can determine if fragments are present using something
> like:
>
> if (ctx->data_end - ctx->data < ctx->frame_length) {
> /* Fragements exists. /*
> }
>
> Signed-off-by: Eelco Chaudron <echaudro@...hat.com>
> Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> ---
> include/net/xdp.h | 22 +++++++++
> include/uapi/linux/bpf.h | 1 +
> kernel/bpf/verifier.c | 2 +-
> net/core/filter.c | 83 ++++++++++++++++++++++++++++++++++
> tools/include/uapi/linux/bpf.h | 1 +
> 5 files changed, 108 insertions(+), 1 deletion(-)
>
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index 09078ab6644c..e54d733c90ed 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -73,8 +73,30 @@ struct xdp_buff {
> void *data_hard_start;
> struct xdp_rxq_info *rxq;
> struct xdp_txq_info *txq;
> + /* If any of the bitfield lengths for frame_sz or mb below change,
> + * make sure the defines here are also updated!
> + */
> +#ifdef __BIG_ENDIAN_BITFIELD
> +#define MB_SHIFT 0
> +#define MB_MASK 0x00000001
> +#define FRAME_SZ_SHIFT 1
> +#define FRAME_SZ_MASK 0xfffffffe
> +#else
> +#define MB_SHIFT 31
> +#define MB_MASK 0x80000000
> +#define FRAME_SZ_SHIFT 0
> +#define FRAME_SZ_MASK 0x7fffffff
> +#endif
> +#define FRAME_SZ_OFFSET() offsetof(struct xdp_buff, __u32_bit_fields_offset)
> +#define MB_OFFSET() offsetof(struct xdp_buff, __u32_bit_fields_offset)
> + /* private: */
> + u32 __u32_bit_fields_offset[0];
Why? I don't get that. Please explain.
Also, looking at all the need for masking/shifting, I wonder if it would
be better to have u32 frame_sz and u8 mb...
> + /* public: */
> u32 frame_sz:31; /* frame size to deduce data_hard_end/reserved tailroom*/
> u32 mb:1; /* xdp non-linear buffer */
> +
> + /* Temporary registers to make conditional access/stores possible. */
> + u64 tmp_reg[2];
IMHO this kills the bitfield approach we have for vars above.
> };
>
> /* Reserve memory area at end-of data area.
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 30b477a26482..62c50ab28ea9 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -4380,6 +4380,7 @@ struct xdp_md {
> __u32 rx_queue_index; /* rxq->queue_index */
>
> __u32 egress_ifindex; /* txq->dev->ifindex */
> + __u32 frame_length;
> };
>
> /* DEVMAP map-value layout
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 93def76cf32b..c50caea29fa2 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -10526,7 +10526,7 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
> const struct bpf_verifier_ops *ops = env->ops;
> int i, cnt, size, ctx_field_size, delta = 0;
> const int insn_cnt = env->prog->len;
> - struct bpf_insn insn_buf[16], *insn;
> + struct bpf_insn insn_buf[32], *insn;
> u32 target_size, size_default, off;
> struct bpf_prog *new_prog;
> enum bpf_access_type type;
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 4c4882d4d92c..278640db9e0a 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -8908,6 +8908,7 @@ static u32 xdp_convert_ctx_access(enum bpf_access_type type,
> struct bpf_insn *insn_buf,
> struct bpf_prog *prog, u32 *target_size)
> {
> + int ctx_reg, dst_reg, scratch_reg;
> struct bpf_insn *insn = insn_buf;
>
> switch (si->off) {
> @@ -8954,6 +8955,88 @@ static u32 xdp_convert_ctx_access(enum bpf_access_type type,
> *insn++ = BPF_LDX_MEM(BPF_W, si->dst_reg, si->dst_reg,
> offsetof(struct net_device, ifindex));
> break;
> + case offsetof(struct xdp_md, frame_length):
> + /* Need tmp storage for src_reg in case src_reg == dst_reg,
> + * and a scratch reg */
> + scratch_reg = BPF_REG_9;
> + dst_reg = si->dst_reg;
> +
> + if (dst_reg == scratch_reg)
> + scratch_reg--;
> +
> + ctx_reg = (si->src_reg == si->dst_reg) ? scratch_reg - 1 : si->src_reg;
> + while (dst_reg == ctx_reg || scratch_reg == ctx_reg)
> + ctx_reg--;
> +
> + /* Save scratch registers */
> + if (ctx_reg != si->src_reg) {
> + *insn++ = BPF_STX_MEM(BPF_DW, si->src_reg, ctx_reg,
> + offsetof(struct xdp_buff,
> + tmp_reg[1]));
> +
> + *insn++ = BPF_MOV64_REG(ctx_reg, si->src_reg);
> + }
> +
> + *insn++ = BPF_STX_MEM(BPF_DW, ctx_reg, scratch_reg,
> + offsetof(struct xdp_buff, tmp_reg[0]));
Why don't you push regs to stack, use it and then pop it back? That way I
suppose you could avoid polluting xdp_buff with tmp_reg[2].
> +
> + /* What does this code do?
> + * dst_reg = 0
> + *
> + * if (!ctx_reg->mb)
> + * goto no_mb:
> + *
> + * dst_reg = (struct xdp_shared_info *)xdp_data_hard_end(xdp)
> + * dst_reg = dst_reg->data_length
> + *
> + * NOTE: xdp_data_hard_end() is xdp->hard_start +
> + * xdp->frame_sz - sizeof(shared_info)
> + *
> + * no_mb:
> + * dst_reg += ctx_reg->data_end - ctx_reg->data
> + */
> + *insn++ = BPF_MOV64_IMM(dst_reg, 0);
> +
> + *insn++ = BPF_LDX_MEM(BPF_W, scratch_reg, ctx_reg, MB_OFFSET());
> + *insn++ = BPF_ALU32_IMM(BPF_AND, scratch_reg, MB_MASK);
> + *insn++ = BPF_JMP_IMM(BPF_JEQ, scratch_reg, 0, 7); /*goto no_mb; */
> +
> + *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct xdp_buff,
> + data_hard_start),
> + dst_reg, ctx_reg,
> + offsetof(struct xdp_buff, data_hard_start));
> + *insn++ = BPF_LDX_MEM(BPF_W, scratch_reg, ctx_reg,
> + FRAME_SZ_OFFSET());
> + *insn++ = BPF_ALU32_IMM(BPF_AND, scratch_reg, FRAME_SZ_MASK);
> + *insn++ = BPF_ALU32_IMM(BPF_RSH, scratch_reg, FRAME_SZ_SHIFT);
> + *insn++ = BPF_ALU64_REG(BPF_ADD, dst_reg, scratch_reg);
> + *insn++ = BPF_ALU64_IMM(BPF_SUB, dst_reg,
> + SKB_DATA_ALIGN(sizeof(struct skb_shared_info)));
> + *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct xdp_shared_info,
> + data_length),
> + dst_reg, dst_reg,
> + offsetof(struct xdp_shared_info,
> + data_length));
> +
> + /* no_mb: */
> + *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct xdp_buff, data_end),
> + scratch_reg, ctx_reg,
> + offsetof(struct xdp_buff, data_end));
> + *insn++ = BPF_ALU64_REG(BPF_ADD, dst_reg, scratch_reg);
> + *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct xdp_buff, data),
> + scratch_reg, ctx_reg,
> + offsetof(struct xdp_buff, data));
> + *insn++ = BPF_ALU64_REG(BPF_SUB, dst_reg, scratch_reg);
> +
> + /* Restore scratch registers */
> + *insn++ = BPF_LDX_MEM(BPF_DW, scratch_reg, ctx_reg,
> + offsetof(struct xdp_buff, tmp_reg[0]));
> +
> + if (ctx_reg != si->src_reg)
> + *insn++ = BPF_LDX_MEM(BPF_DW, ctx_reg, ctx_reg,
> + offsetof(struct xdp_buff,
> + tmp_reg[1]));
> + break;
> }
>
> return insn - insn_buf;
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index 30b477a26482..62c50ab28ea9 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -4380,6 +4380,7 @@ struct xdp_md {
> __u32 rx_queue_index; /* rxq->queue_index */
>
> __u32 egress_ifindex; /* txq->dev->ifindex */
> + __u32 frame_length;
> };
>
> /* DEVMAP map-value layout
> --
> 2.28.0
>
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