netdev - Re: [PATCH bpf 03/11] bpf: Add write access to tcp

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Date:   Tue, 19 Dec 2017 17:10:31 -0800
From:   Alexei Starovoitov <ast@...com>
To:     Lawrence Brakmo <brakmo@...com>, netdev <netdev@...r.kernel.org>
CC:     Kernel Team <kernel-team@...com>, Blake Matheny <bmatheny@...com>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH bpf 03/11] bpf: Add write access to tcp_sock and sock
 fields

On 12/18/17 10:21 PM, Lawrence Brakmo wrote:
> +#define SOCK_OPS_SET_FIELD(FIELD_NAME, OBJ)				      \
> +	do {								      \
> +		int reg = BPF_REG_9;					      \
> +		BUILD_BUG_ON(FIELD_SIZEOF(OBJ, FIELD_NAME) >		      \
> +			     FIELD_SIZEOF(struct bpf_sock_ops, FIELD_NAME));  \
> +		while (si->dst_reg == reg || si->src_reg == reg)	      \
> +			reg--;						      \
> +		*insn++ = BPF_STX_MEM(BPF_DW, si->dst_reg, reg,		      \
> +				      offsetof(struct bpf_sock_ops_kern,      \
> +					       temp));			      \
> +		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(			      \
> +						struct bpf_sock_ops_kern,     \
> +						is_fullsock),		      \
> +				      reg, si->dst_reg,			      \
> +				      offsetof(struct bpf_sock_ops_kern,      \
> +					       is_fullsock));		      \
> +		*insn++ = BPF_JMP_IMM(BPF_JEQ, reg, 0, 2);		      \
> +		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(			      \
> +						struct bpf_sock_ops_kern, sk),\
> +				      reg, si->dst_reg,			      \
> +				      offsetof(struct bpf_sock_ops_kern, sk));\
> +		*insn++ = BPF_STX_MEM(BPF_FIELD_SIZEOF(OBJ, FIELD_NAME),      \
> +				      reg, si->src_reg,			      \
> +				      offsetof(OBJ, FIELD_NAME));	      \
> +		*insn++ = BPF_LDX_MEM(BPF_DW, reg, si->dst_reg,		      \
> +				      offsetof(struct bpf_sock_ops_kern,      \
> +					       temp));			      \
> +	} while (0)

that's neat. I like it.
I guess the prog can check is_fullsock on its own to see whether writes
will fail or not, so JEQ above is ok.
Only while() loop looks a bit scary.
May be replace with two 'if' ?
if (si->dst_reg == reg || si->src_reg == reg)
   reg --;
if (si->dst_reg == reg || si->src_reg == reg)
   reg --;
so it's clear that tmp reg will be reg_7, 8 or 9.