Subject: Re: [PATCH bpf-next] bpf: Add document for 'dst_port' of 'struct bpf_sock'

From: Jakub Sitnicki <[email-masked]>
Date: Tue, 25 Jan 2022 20:24:27 +0100

On Thu, Jan 13, 2022 at 08:02 AM CET, [email-masked] wrote:
> From: Menglong Dong <[email-masked]>
>
> The description of 'dst_port' in 'struct bpf_sock' is not accurated.
> In fact, 'dst_port' is not in network byte order, it is 'partly' in
> network byte order.
>
> We can see it in bpf_sock_convert_ctx_access():
>
>> case offsetof(struct bpf_sock, dst_port):
>> 	*insn++ = BPF_LDX_MEM(
>> 		BPF_FIELD_SIZEOF(struct sock_common, skc_dport),
>> 		si->dst_reg, si->src_reg,
>> 		bpf_target_off(struct sock_common, skc_dport,
>> 			       sizeof_field(struct sock_common,
>> 					    skc_dport),
>> 			       target_size));
>
> It simply passes 'sock_common->skc_dport' to 'bpf_sock->dst_port',
> which makes that the low 16-bits of 'dst_port' is equal to 'skc_port'
> and is in network byte order, but the high 16-bites of 'dst_port' is
> 0. And the actual port is 'bpf_ntohs((__u16)dst_port)', and
> 'bpf_ntohl(dst_port)' is totally not the right port.
>
> This is different form 'remote_port' in 'struct bpf_sock_ops' or
> 'struct __sk_buff':
>
>> case offsetof(struct __sk_buff, remote_port):
>> 	BUILD_BUG_ON(sizeof_field(struct sock_common, skc_dport) != 2);
>>
>> 	*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, sk),
>> 			      si->dst_reg, si->src_reg,
>> 				      offsetof(struct sk_buff, sk));
>> 	*insn++ = BPF_LDX_MEM(BPF_H, si->dst_reg, si->dst_reg,
>> 			      bpf_target_off(struct sock_common,
>> 					     skc_dport,
>> 					     2, target_size));
>> #ifndef __BIG_ENDIAN_BITFIELD
>> 	*insn++ = BPF_ALU32_IMM(BPF_LSH, si->dst_reg, 16);
>> #endif
>
> We can see that it will left move 16-bits in little endian, which makes
> the whole 'remote_port' is in network byte order, and the actual port
> is bpf_ntohl(remote_port).
>
> Note this in the document of 'dst_port'. ( Maybe this should be unified
> in the code? )
>
> Signed-off-by: Menglong Dong <[email-masked]>
> ---
>  include/uapi/linux/bpf.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index b0383d371b9a..891a182a749a 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -5500,7 +5500,11 @@ struct bpf_sock {
>  	__u32 src_ip4;
>  	__u32 src_ip6[4];
>  	__u32 src_port;		/* host byte order */
> -	__u32 dst_port;		/* network byte order */
> +	__u32 dst_port;		/* low 16-bits are in network byte order,
> +				 * and high 16-bits are filled by 0.
> +				 * So the real port in host byte order is
> +				 * bpf_ntohs((__u16)dst_port).
> +				 */
>  	__u32 dst_ip4;
>  	__u32 dst_ip6[4];
>  	__u32 state;

I'm probably missing something obvious, but is there anything stopping
us from splitting the field, so that dst_ports is 16-bit wide?

I gave a quick check to the change below and it seems to pass verifier
checks and sock_field tests.

IDK, just an idea. Didn't give it a deeper thought.

--8<--

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 4a2f7041ebae..344d62ccafba 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -5574,7 +5574,8 @@ struct bpf_sock {
 	__u32 src_ip4;
 	__u32 src_ip6[4];
 	__u32 src_port;		/* host byte order */
-	__u32 dst_port;		/* network byte order */
+	__u16 unused;
+	__u16 dst_port;		/* network byte order */
 	__u32 dst_ip4;
 	__u32 dst_ip6[4];
 	__u32 state;
diff --git a/net/core/filter.c b/net/core/filter.c
index a06931c27eeb..c56b8ba82de5 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -8276,7 +8276,6 @@ bool bpf_sock_is_valid_access(int off, int size, enum bpf_access_type type,
 	case offsetof(struct bpf_sock, family):
 	case offsetof(struct bpf_sock, type):
 	case offsetof(struct bpf_sock, protocol):
-	case offsetof(struct bpf_sock, dst_port):
 	case offsetof(struct bpf_sock, src_port):
 	case offsetof(struct bpf_sock, rx_queue_mapping):
 	case bpf_ctx_range(struct bpf_sock, src_ip4):
@@ -8285,6 +8284,9 @@ bool bpf_sock_is_valid_access(int off, int size, enum bpf_access_type type,
 	case bpf_ctx_range_till(struct bpf_sock, dst_ip6[0], dst_ip6[3]):
 		bpf_ctx_record_field_size(info, size_default);
 		return bpf_ctx_narrow_access_ok(off, size, size_default);
+	case offsetof(struct bpf_sock, dst_port):
+		bpf_ctx_record_field_size(info, sizeof(__u16));
+		return bpf_ctx_narrow_access_ok(off, size, sizeof(__u16));
 	}

 	return size == size_default;
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 4a2f7041ebae..344d62ccafba 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -5574,7 +5574,8 @@ struct bpf_sock {
 	__u32 src_ip4;
 	__u32 src_ip6[4];
 	__u32 src_port;		/* host byte order */
-	__u32 dst_port;		/* network byte order */
+	__u16 unused;
+	__u16 dst_port;		/* network byte order */
 	__u32 dst_ip4;
 	__u32 dst_ip6[4];
 	__u32 state;