linux-kernel - Re: [PATCH bpf-next v2] bpf: Simplify expression for identify bpf mem type

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Date:   Thu, 18 Mar 2021 16:58:02 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jianlin Lv <Jianlin.Lv@....com>, bpf@...r.kernel.org
Cc:     kuba@...nel.org, simon.horman@...ronome.com, davem@...emloft.net,
        ast@...nel.org, alexei.starovoitov@...il.com, andrii@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        oss-drivers@...ronome.com, linux-api@...r.kernel.org,
        iecedge@...il.com
Subject: Re: [PATCH bpf-next v2] bpf: Simplify expression for identify bpf mem
 type

On 3/18/21 7:36 AM, Jianlin Lv wrote:
> Added BPF_LD_ST_SIZE_MASK macro as mask of size modifier that help to
> reduce the evaluation of expressions in if statements,
> and remove BPF_SIZE_MASK in netronome driver.
> 
> Signed-off-by: Jianlin Lv <Jianlin.Lv@....com>
> ---
> v2: Move the bpf_LD_ST_SIZE_MASK macro definition to include/linux/bpf.h
> ---
>   drivers/net/ethernet/netronome/nfp/bpf/main.h |  8 +++-----
>   include/linux/bpf.h                           |  1 +
>   kernel/bpf/verifier.c                         | 12 ++++--------
>   3 files changed, 8 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/net/ethernet/netronome/nfp/bpf/main.h b/drivers/net/ethernet/netronome/nfp/bpf/main.h
> index d0e17eebddd9..e90981e69763 100644
> --- a/drivers/net/ethernet/netronome/nfp/bpf/main.h
> +++ b/drivers/net/ethernet/netronome/nfp/bpf/main.h
> @@ -346,8 +346,6 @@ struct nfp_insn_meta {
>   	struct list_head l;
>   };
>   
> -#define BPF_SIZE_MASK	0x18
> -
>   static inline u8 mbpf_class(const struct nfp_insn_meta *meta)
>   {
>   	return BPF_CLASS(meta->insn.code);
> @@ -375,7 +373,7 @@ static inline bool is_mbpf_alu(const struct nfp_insn_meta *meta)
>   
>   static inline bool is_mbpf_load(const struct nfp_insn_meta *meta)
>   {
> -	return (meta->insn.code & ~BPF_SIZE_MASK) == (BPF_LDX | BPF_MEM);
> +	return (meta->insn.code & ~BPF_LD_ST_SIZE_MASK) == (BPF_LDX | BPF_MEM);
>   }
>   
>   static inline bool is_mbpf_jmp32(const struct nfp_insn_meta *meta)
> @@ -395,7 +393,7 @@ static inline bool is_mbpf_jmp(const struct nfp_insn_meta *meta)
>   
>   static inline bool is_mbpf_store(const struct nfp_insn_meta *meta)
>   {
> -	return (meta->insn.code & ~BPF_SIZE_MASK) == (BPF_STX | BPF_MEM);
> +	return (meta->insn.code & ~BPF_LD_ST_SIZE_MASK) == (BPF_STX | BPF_MEM);
>   }
>   
>   static inline bool is_mbpf_load_pkt(const struct nfp_insn_meta *meta)
> @@ -430,7 +428,7 @@ static inline bool is_mbpf_classic_store_pkt(const struct nfp_insn_meta *meta)
>   
>   static inline bool is_mbpf_atomic(const struct nfp_insn_meta *meta)
>   {
> -	return (meta->insn.code & ~BPF_SIZE_MASK) == (BPF_STX | BPF_ATOMIC);
> +	return (meta->insn.code & ~BPF_LD_ST_SIZE_MASK) == (BPF_STX | BPF_ATOMIC);
>   }
>   
>   static inline bool is_mbpf_mul(const struct nfp_insn_meta *meta)
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index a25730eaa148..e85924719c65 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -995,6 +995,7 @@ struct bpf_array {
>   				 BPF_F_RDONLY_PROG |	\
>   				 BPF_F_WRONLY |		\
>   				 BPF_F_WRONLY_PROG)
> +#define BPF_LD_ST_SIZE_MASK	0x18	/* mask of size modifier */
>   
>   #define BPF_MAP_CAN_READ	BIT(0)
>   #define BPF_MAP_CAN_WRITE	BIT(1)
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index f9096b049cd6..29fdfdb8abfa 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -11384,15 +11384,11 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
>   	for (i = 0; i < insn_cnt; i++, insn++) {
>   		bpf_convert_ctx_access_t convert_ctx_access;
>   
> -		if (insn->code == (BPF_LDX | BPF_MEM | BPF_B) ||
> -		    insn->code == (BPF_LDX | BPF_MEM | BPF_H) ||
> -		    insn->code == (BPF_LDX | BPF_MEM | BPF_W) ||
> -		    insn->code == (BPF_LDX | BPF_MEM | BPF_DW))
> +		/* opcode: BPF_MEM | <size> | BPF_LDX */
> +		if ((insn->code & ~BPF_LD_ST_SIZE_MASK) == (BPF_LDX | BPF_MEM))
>   			type = BPF_READ;
> -		else if (insn->code == (BPF_STX | BPF_MEM | BPF_B) ||
> -			 insn->code == (BPF_STX | BPF_MEM | BPF_H) ||
> -			 insn->code == (BPF_STX | BPF_MEM | BPF_W) ||
> -			 insn->code == (BPF_STX | BPF_MEM | BPF_DW))
> +		/* opcode: BPF_MEM | <size> | BPF_STX */
> +		else if ((insn->code & ~BPF_LD_ST_SIZE_MASK) == (BPF_STX | BPF_MEM))
>   			type = BPF_WRITE;
>   		else
>   			continue;
> 

To me this cleanup makes the code harder to read, in particular on verfier side,
I don't think it's worth it, especially given it's not in (highly) performance
critical code.

Thanks,
Daniel