From: Menglong Dong <imagedong@xxxxxxxxxxx>
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 <imagedong@xxxxxxxxxxx>
---
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;
--
2.34.1
