Merge branch 'Make 2-byte access to bpf_sk_lookup->remote_port endian-agnostic'

Jakub Sitnicki says:

====================

This patch set is a result of a discussion we had around the RFC patchset from
Ilya [1]. The fix for the narrow loads from the RFC series is still relevant,
but this series does not depend on it. Nor is it required to unbreak sk_lookup
tests on BE, if this series gets applied.

To summarize the takeaways from [1]:

 1) we want to make 2-byte load from ctx->remote_port portable across LE and BE,
 2) we keep the 4-byte load from ctx->remote_port as it is today - result varies
    on endianess of the platform.

[1] https://lore.kernel.org/bpf/20220222182559.2865596-2-iii@linux.ibm.com/

v1 -> v2:
- Remove needless check that 4-byte load is from &ctx->remote_port offset
  (Martin)

[v1]: https://lore.kernel.org/bpf/20220317165826.1099418-1-jakub@cloudflare.com/
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Alexei Starovoitov

net/core/filter.c

@@ -10989,13 +10989,24 @@ 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 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));
 		return bpf_ctx_narrow_access_ok(off, size, sizeof(__u32));
 
+	case bpf_ctx_range(struct bpf_sk_lookup, remote_port):
+		/* Allow 4-byte access to 2-byte field for backward compatibility */
+		if (size == sizeof(__u32))
+			return true;
+		bpf_ctx_record_field_size(info, sizeof(__be16));
+		return bpf_ctx_narrow_access_ok(off, size, sizeof(__be16));
+
+	case offsetofend(struct bpf_sk_lookup, remote_port) ...
+	     offsetof(struct bpf_sk_lookup, local_ip4) - 1:
+		/* Allow access to zero padding for backward compatibility */
+		bpf_ctx_record_field_size(info, sizeof(__u16));
+		return bpf_ctx_narrow_access_ok(off, size, sizeof(__u16));
+
 	default:
 		return false;
 	}
@@ -11077,6 +11088,11 @@ static u32 sk_lookup_convert_ctx_access(enum bpf_access_type type,
 						     sport, 2, target_size));
 		break;
 
+	case offsetofend(struct bpf_sk_lookup, remote_port):
+		*target_size = 2;
+		*insn++ = BPF_MOV32_IMM(si->dst_reg, 0);
+		break;
+
 	case offsetof(struct bpf_sk_lookup, local_port):
 		*insn++ = BPF_LDX_MEM(BPF_H, si->dst_reg, si->src_reg,
 				      bpf_target_off(struct bpf_sk_lookup_kern,

tools/testing/selftests/bpf/progs/test_sk_lookup.c

@@ -413,15 +413,20 @@ int ctx_narrow_access(struct bpf_sk_lookup *ctx)
 
 	/* Narrow loads from remote_port field. Expect SRC_PORT. */
 	if (LSB(ctx->remote_port, 0) != ((SRC_PORT >> 0) & 0xff) ||
-	    LSB(ctx->remote_port, 1) != ((SRC_PORT >> 8) & 0xff) ||
-	    LSB(ctx->remote_port, 2) != 0 || LSB(ctx->remote_port, 3) != 0)
+	    LSB(ctx->remote_port, 1) != ((SRC_PORT >> 8) & 0xff))
 		return SK_DROP;
 	if (LSW(ctx->remote_port, 0) != SRC_PORT)
 		return SK_DROP;
 
-	/* Load from remote_port field with zero padding (backward compatibility) */
+	/*
+	 * NOTE: 4-byte load from bpf_sk_lookup at remote_port offset
+	 * is quirky. It gets rewritten by the access converter to a
+	 * 2-byte load for backward compatibility. Treating the load
+	 * result as a be16 value makes the code portable across
+	 * little- and big-endian platforms.
+	 */
 	val_u32 = *(__u32 *)&ctx->remote_port;
-	if (val_u32 != bpf_htonl(bpf_ntohs(SRC_PORT) << 16))
+	if (val_u32 != SRC_PORT)
 		return SK_DROP;
 
 	/* Narrow loads from local_port field. Expect DST_PORT. */