On Thu, Jan 13, 2022 at 08:02 AM CET, menglong8.dong@xxxxxxxxx wrote:
> 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;
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;
