netdev - [PATCH bpf-next v2 1/2] bpf: Make remote_port field in struct bpf_sk

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Date:   Wed,  9 Feb 2022 19:43:32 +0100
From:   Jakub Sitnicki <jakub@...udflare.com>
To:     bpf@...r.kernel.org
Cc:     netdev@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        kernel-team@...udflare.com, Yonghong Song <yhs@...com>
Subject: [PATCH bpf-next v2 1/2] bpf: Make remote_port field in struct bpf_sk_lookup 16-bit wide

remote_port is another case of a BPF context field documented as a 32-bit
value in network byte order for which the BPF context access converter
generates a load of a zero-padded 16-bit integer in network byte order.

First such case was dst_port in bpf_sock which got addressed in commit
4421a582718a ("bpf: Make dst_port field in struct bpf_sock 16-bit wide").

Loading 4-bytes from the remote_port offset and converting the value with
bpf_ntohl() leads to surprising results, as the expected value is shifted
by 16 bits.

Reduce the confusion by splitting the field in two - a 16-bit field holding
a big-endian integer, and a 16-bit zero-padding anonymous field that
follows it.

Suggested-by: Alexei Starovoitov <ast@...nel.org>
Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
---
 include/uapi/linux/bpf.h | 3 ++-
 net/bpf/test_run.c       | 4 ++--
 net/core/filter.c        | 3 ++-
 3 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index a7f0ddedac1f..afe3d0d7f5f2 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -6453,7 +6453,8 @@ struct bpf_sk_lookup {
 	__u32 protocol;		/* IP protocol (IPPROTO_TCP, IPPROTO_UDP) */
 	__u32 remote_ip4;	/* Network byte order */
 	__u32 remote_ip6[4];	/* Network byte order */
-	__u32 remote_port;	/* Network byte order */
+	__be16 remote_port;	/* Network byte order */
+	__u16 :16;		/* Zero padding */
 	__u32 local_ip4;	/* Network byte order */
 	__u32 local_ip6[4];	/* Network byte order */
 	__u32 local_port;	/* Host byte order */
diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
index 0220b0822d77..8c2608567555 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -1146,7 +1146,7 @@ int bpf_prog_test_run_sk_lookup(struct bpf_prog *prog, const union bpf_attr *kat
 	if (!range_is_zero(user_ctx, offsetofend(typeof(*user_ctx), local_port), sizeof(*user_ctx)))
 		goto out;
 
-	if (user_ctx->local_port > U16_MAX || user_ctx->remote_port > U16_MAX) {
+	if (user_ctx->local_port > U16_MAX) {
 		ret = -ERANGE;
 		goto out;
 	}
@@ -1154,7 +1154,7 @@ int bpf_prog_test_run_sk_lookup(struct bpf_prog *prog, const union bpf_attr *kat
 	ctx.family = (u16)user_ctx->family;
 	ctx.protocol = (u16)user_ctx->protocol;
 	ctx.dport = (u16)user_ctx->local_port;
-	ctx.sport = (__force __be16)user_ctx->remote_port;
+	ctx.sport = user_ctx->remote_port;
 
 	switch (ctx.family) {
 	case AF_INET:
diff --git a/net/core/filter.c b/net/core/filter.c
index 99a05199a806..83f06d3b2c52 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -10843,7 +10843,8 @@ static bool sk_lookup_is_valid_access(int off, int size,
 	case bpf_ctx_range(struct bpf_sk_lookup, local_ip4):
 	case bpf_ctx_range_till(struct bpf_sk_lookup, remote_ip6[0], remote_ip6[3]):
 	case bpf_ctx_range_till(struct bpf_sk_lookup, local_ip6[0], local_ip6[3]):
-	case bpf_ctx_range(struct bpf_sk_lookup, remote_port):
+	case offsetof(struct bpf_sk_lookup, remote_port) ...
+	     offsetof(struct bpf_sk_lookup, local_ip4) - 1:
 	case bpf_ctx_range(struct bpf_sk_lookup, local_port):
 	case bpf_ctx_range(struct bpf_sk_lookup, ingress_ifindex):
 		bpf_ctx_record_field_size(info, sizeof(__u32));
-- 
2.31.1