[MPTCP] Re: [PATCH] Squash-to: "sock: Make sk_protocol a 16-bit value"

[Matthieu Baerts <matthieu.baerts at tessares.net>]

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Hi Paolo,

On 07/01/2020 15:05, Paolo Abeni wrote:
> We need to update the ebpf accessors for both sk_type and
> sk_protocol. The code previously in place has some special
> hack to take care such fields being part of a bitfield.
> 
> The fix actually clean-up a bit the existing code.

Thank you for looking at that!

I have 3 questions below.

> Note: I'm not 100% sure about dropping the 'target_size' handling.
> This changes the 'target_size' from 1 to 2 for sk_protocol.
> Should not matter, as the ebpf target field is 4 bytes wide.
> 
> To be appended to the orig commit message:
> """
> Take care of BPF field access for sk_type/sk_protocol.
> Both of them are now outside the bitfield, so we can use
> load instructions without fourther shifting/masking.

(detail: s/fourther/further)

> 
> v5 -> v6:
>   - update eBPF accessors, too.
> """
> 
> Additionally, Eric's Reviewed-by should be dropped.

Good point!

> ---
>   include/net/sock.h | 15 ------------
>   net/core/filter.c  | 58 ++++++++++++++++------------------------------
>   2 files changed, 20 insertions(+), 53 deletions(-)
> 
> diff --git a/include/net/sock.h b/include/net/sock.h
> index c63e9088b4e3..8766f9bc3e70 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -436,21 +436,6 @@ struct sock {
>   	 * Because of non atomicity rules, all
>   	 * changes are protected by socket lock.
>   	 */
> -	unsigned int		__sk_flags_offset[0];
> -#ifdef __BIG_ENDIAN_BITFIELD
> -#define SK_FL_PROTO_SHIFT  16
> -#define SK_FL_PROTO_MASK   0x00ff0000
> -
> -#define SK_FL_TYPE_SHIFT   0
> -#define SK_FL_TYPE_MASK    0x0000ffff
> -#else
> -#define SK_FL_PROTO_SHIFT  8
> -#define SK_FL_PROTO_MASK   0x0000ff00
> -
> -#define SK_FL_TYPE_SHIFT   16
> -#define SK_FL_TYPE_MASK    0xffff0000
> -#endif
> -
>   	u8			sk_padding : 1,
>   				sk_kern_sock : 1,
>   				sk_no_check_tx : 1,
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 42fd17c48c5f..37759d6ce3ea 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -7607,21 +7607,19 @@ u32 bpf_sock_convert_ctx_access(enum bpf_access_type type,
>   		break;
>   
>   	case offsetof(struct bpf_sock, type):
> -		BUILD_BUG_ON(HWEIGHT32(SK_FL_TYPE_MASK) != BITS_PER_BYTE * 2);

Should we not keep it?

I see that some others are still doing that:

     BUILD_BUG_ON(sizeof_field(struct sock, sk_mark) != 4);

But not all, e.g. for sk_state, sk_num, sk_dport.

> -		*insn++ = BPF_LDX_MEM(BPF_W, si->dst_reg, si->src_reg,
> -				      offsetof(struct sock, __sk_flags_offset));
> -		*insn++ = BPF_ALU32_IMM(BPF_AND, si->dst_reg, SK_FL_TYPE_MASK);
> -		*insn++ = BPF_ALU32_IMM(BPF_RSH, si->dst_reg, SK_FL_TYPE_SHIFT);
> -		*target_size = 2;
> +		*insn++ = BPF_LDX_MEM(BPF_H, si->dst_reg, si->src_reg,

Instead of BPF_H, should we use this macro?

     BPF_FIELD_SIZEOF(struct sock, sk_type)

> +				      bpf_target_off(struct sock, sk_type,
> +						     sizeof_field(struct sock,
> +								  sk_type),
> +						     target_size));
>   		break;
>   
>   	case offsetof(struct bpf_sock, protocol):
> -		BUILD_BUG_ON(HWEIGHT32(SK_FL_PROTO_MASK) != BITS_PER_BYTE);
> -		*insn++ = BPF_LDX_MEM(BPF_W, si->dst_reg, si->src_reg,
> -				      offsetof(struct sock, __sk_flags_offset));
> -		*insn++ = BPF_ALU32_IMM(BPF_AND, si->dst_reg, SK_FL_PROTO_MASK);
> -		*insn++ = BPF_ALU32_IMM(BPF_RSH, si->dst_reg, SK_FL_PROTO_SHIFT);
> -		*target_size = 1;
> +		*insn++ = BPF_LDX_MEM(BPF_H, si->dst_reg, si->src_reg,
> +				      bpf_target_off(struct sock, sk_protocol,
> +						     sizeof_field(struct sock,
> +								  sk_protocol),
> +						     target_size));
>   		break;
>   
>   	case offsetof(struct bpf_sock, src_ip4):
> @@ -7903,20 +7901,13 @@ static u32 sock_addr_convert_ctx_access(enum bpf_access_type type,
>   		break;
>   
>   	case offsetof(struct bpf_sock_addr, type):
> -		SOCK_ADDR_LOAD_NESTED_FIELD_SIZE_OFF(
> -			struct bpf_sock_addr_kern, struct sock, sk,
> -			__sk_flags_offset, BPF_W, 0);
> -		*insn++ = BPF_ALU32_IMM(BPF_AND, si->dst_reg, SK_FL_TYPE_MASK);
> -		*insn++ = BPF_ALU32_IMM(BPF_RSH, si->dst_reg, SK_FL_TYPE_SHIFT);
> +		SOCK_ADDR_LOAD_NESTED_FIELD(struct bpf_sock_addr_kern,
> +					    struct sock, sk, sk_type);
>   		break;
>   
>   	case offsetof(struct bpf_sock_addr, protocol):
> -		SOCK_ADDR_LOAD_NESTED_FIELD_SIZE_OFF(
> -			struct bpf_sock_addr_kern, struct sock, sk,
> -			__sk_flags_offset, BPF_W, 0);
> -		*insn++ = BPF_ALU32_IMM(BPF_AND, si->dst_reg, SK_FL_PROTO_MASK);
> -		*insn++ = BPF_ALU32_IMM(BPF_RSH, si->dst_reg,
> -					SK_FL_PROTO_SHIFT);
> +		SOCK_ADDR_LOAD_NESTED_FIELD(struct bpf_sock_addr_kern,
> +					    struct sock, sk, sk_protocol);
>   		break;
>   
>   	case offsetof(struct bpf_sock_addr, msg_src_ip4):
> @@ -8835,11 +8826,11 @@ sk_reuseport_is_valid_access(int off, int size,
>   				    skb,				\
>   				    SKB_FIELD)
>   
> -#define SK_REUSEPORT_LOAD_SK_FIELD_SIZE_OFF(SK_FIELD, BPF_SIZE, EXTRA_OFF) \
> -	SOCK_ADDR_LOAD_NESTED_FIELD_SIZE_OFF(struct sk_reuseport_kern,	\
> -					     struct sock,		\
> -					     sk,			\
> -					     SK_FIELD, BPF_SIZE, EXTRA_OFF)
> +#define SK_REUSEPORT_LOAD_SK_FIELD(SK_FIELD)				\
> +	SOCK_ADDR_LOAD_NESTED_FIELD(struct sk_reuseport_kern,		\
> +				    struct sock,			\
> +				    sk,					\
> +				    SK_FIELD)

Why did you have to change that too?

>   
>   static u32 sk_reuseport_convert_ctx_access(enum bpf_access_type type,
>   					   const struct bpf_insn *si,
> @@ -8863,16 +8854,7 @@ static u32 sk_reuseport_convert_ctx_access(enum bpf_access_type type,
>   		break;
>   
>   	case offsetof(struct sk_reuseport_md, ip_protocol):
> -		BUILD_BUG_ON(HWEIGHT32(SK_FL_PROTO_MASK) != BITS_PER_BYTE);
> -		SK_REUSEPORT_LOAD_SK_FIELD_SIZE_OFF(__sk_flags_offset,
> -						    BPF_W, 0);
> -		*insn++ = BPF_ALU32_IMM(BPF_AND, si->dst_reg, SK_FL_PROTO_MASK);
> -		*insn++ = BPF_ALU32_IMM(BPF_RSH, si->dst_reg,
> -					SK_FL_PROTO_SHIFT);
> -		/* SK_FL_PROTO_MASK and SK_FL_PROTO_SHIFT are endian
> -		 * aware.  No further narrowing or masking is needed.
> -		 */
> -		*target_size = 1;
> +		SK_REUSEPORT_LOAD_SK_FIELD(sk_protocol);

Detail for me: why did they not modify the bitfield instead of adding 
__sk_flags_offset[0] in include/net/sock.h ?

Cheers,
Matt
-- 
Matthieu Baerts | R&D Engineer
matthieu.baerts(a)tessares.net
Tessares SA | Hybrid Access Solutions
www.tessares.net
1 Avenue Jean Monnet, 1348 Louvain-la-Neuve, Belgium