From deb59400464468352b4d7827420e04f1add94726 Mon Sep 17 00:00:00 2001 From: Jakub Sitnicki Date: Thu, 17 Mar 2022 12:39:20 +0100 Subject: [PATCH] selftests/bpf: Fix test for 4-byte load from dst_port on big-endian The check for 4-byte load from dst_port offset into bpf_sock is failing on big-endian architecture - s390. The bpf access converter rewrites the 4-byte load to a 2-byte load from sock_common at skc_dport offset, as shown below. * s390 / llvm-objdump -S --no-show-raw-insn 00000000000002a0 : 84: r1 = *(u32 *)(r1 + 48) 85: w0 = 1 86: if w1 == 51966 goto +1 87: w0 = 0 00000000000002c0 : 88: exit * s390 / bpftool prog dump xlated _Bool sk_dst_port__load_word(struct bpf_sock * sk): 35: (69) r1 = *(u16 *)(r1 +12) 36: (bc) w1 = w1 37: (b4) w0 = 1 38: (16) if w1 == 0xcafe goto pc+1 39: (b4) w0 = 0 40: (95) exit * x86_64 / llvm-objdump -S --no-show-raw-insn 00000000000002a0 : 84: r1 = *(u32 *)(r1 + 48) 85: w0 = 1 86: if w1 == 65226 goto +1 87: w0 = 0 00000000000002c0 : 88: exit * x86_64 / bpftool prog dump xlated _Bool sk_dst_port__load_word(struct bpf_sock * sk): 33: (69) r1 = *(u16 *)(r1 +12) 34: (b4) w0 = 1 35: (16) if w1 == 0xfeca goto pc+1 36: (b4) w0 = 0 37: (95) exit This leads to surprises if we treat the destination register contents as a 32-bit value, ignoring the fact that in reality it contains a 16-bit value. On little-endian the register contents reflect the bpf_sock struct definition, where the lower 16-bits contain the port number: struct bpf_sock { ... __be16 dst_port; /* offset 48 */ __u16 :16; ... }; However, on big-endian the register contents suggest that field the layout of bpf_sock struct is as so: struct bpf_sock { ... __u16 :16; /* offset 48 */ __be16 dst_port; ... }; Account for this quirky access conversion in the test case exercising the 4-byte load by treating the result as 16-bit wide. Signed-off-by: Jakub Sitnicki Signed-off-by: Daniel Borkmann Acked-by: Martin KaFai Lau Link: https://lore.kernel.org/bpf/20220317113920.1068535-5-jakub@cloudflare.com --- tools/testing/selftests/bpf/progs/test_sock_fields.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/tools/testing/selftests/bpf/progs/test_sock_fields.c b/tools/testing/selftests/bpf/progs/test_sock_fields.c index 43a17fd..9f4b8f9 100644 --- a/tools/testing/selftests/bpf/progs/test_sock_fields.c +++ b/tools/testing/selftests/bpf/progs/test_sock_fields.c @@ -251,10 +251,16 @@ int ingress_read_sock_fields(struct __sk_buff *skb) return CG_OK; } +/* + * NOTE: 4-byte load from bpf_sock at dst_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. + */ static __noinline bool sk_dst_port__load_word(struct bpf_sock *sk) { __u32 *word = (__u32 *)&sk->dst_port; - return word[0] == bpf_htonl(0xcafe0000); + return word[0] == bpf_htons(0xcafe); } static __noinline bool sk_dst_port__load_half(struct bpf_sock *sk) -- 2.7.4