netdev - Re: [PATHv3 net-next] bpf: enable verifier to add 0 to packet ptr

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Date:   Sat, 4 Feb 2017 18:53:47 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     William Tu <u9012063@...il.com>
Cc:     netdev@...r.kernel.org, Mihai Budiu <mbudiu@...are.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATHv3 net-next] bpf: enable verifier to add 0 to packet ptr

On Sat, Feb 04, 2017 at 08:37:29AM -0800, William Tu wrote:
> The patch fixes the case when adding a zero value to the packet
> pointer.  The zero value could come from src_reg equals type
> BPF_K or CONST_IMM.  The patch fixes both, otherwise the verifer
> reports the following error:
>   [...]
>     R0=imm0,min_value=0,max_value=0
>     R1=pkt(id=0,off=0,r=4)
>     R2=pkt_end R3=fp-12
>     R4=imm4,min_value=4,max_value=4
>     R5=pkt(id=0,off=4,r=4)
>   269: (bf) r2 = r0     // r2 becomes imm0
>   270: (77) r2 >>= 3
>   271: (bf) r4 = r1     // r4 becomes pkt ptr
>   272: (0f) r4 += r2    // r4 += 0
>   addition of negative constant to packet pointer is not allowed
> 
> Signed-off-by: William Tu <u9012063@...il.com>
> Signed-off-by: Mihai Budiu <mbudiu@...are.com>
> Cc: Daniel Borkmann <daniel@...earbox.net>
> Cc: Alexei Starovoitov <ast@...nel.org>
> Acked-by: Daniel Borkmann <daniel@...earbox.net>
...
> +		"direct packet access: test14 (pkt_ptr += 0, CONST_IMM, good access)",
> +		.insns = {
> +			BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1,
> +				    offsetof(struct __sk_buff, data)),
> +			BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1,
> +				    offsetof(struct __sk_buff, data_end)),
> +			BPF_MOV64_REG(BPF_REG_0, BPF_REG_2),
> +			BPF_ALU64_IMM(BPF_ADD, BPF_REG_0, 22),
> +			BPF_JMP_REG(BPF_JGT, BPF_REG_0, BPF_REG_3, 7),
> +			BPF_MOV64_IMM(BPF_REG_5, 12),
> +			BPF_ALU64_IMM(BPF_RSH, BPF_REG_5, 4),
> +			BPF_MOV64_REG(BPF_REG_6, BPF_REG_2),
> +			BPF_ALU64_REG(BPF_ADD, BPF_REG_6, BPF_REG_5),
> +			BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_6, 0),

Now looks great. Thanks!
Acked-by: Alexei Starovoitov <ast@...nel.org>