selftests/bpf: Fix s390 sock_field test failure

llvm patch [1] enabled cross-function optimization for func arguments
(ArgumentPromotion) at -O2 level. And this caused s390 sock_fields
test failure ([2]). The failure is gone right now as patch [1] was
reverted in [3]. But it is possible that patch [3] will be reverted
again and then the test failure in [2] will show up again. So it is
desirable to fix the failure regardless.

The following is an analysis why sock_field test fails with
llvm patch [1].

The main problem is in
  static __noinline bool sk_dst_port__load_word(struct bpf_sock *sk)
  {
        __u32 *word = (__u32 *)&sk->dst_port;
        return word[0] == bpf_htons(0xcafe);
  }
  static __noinline bool sk_dst_port__load_half(struct bpf_sock *sk)
  {
        __u16 *half = (__u16 *)&sk->dst_port;
        return half[0] == bpf_htons(0xcafe);
  }
  ...
  int read_sk_dst_port(struct __sk_buff *skb)
  {
	...
        sk = skb->sk;
	...
        if (!sk_dst_port__load_word(sk))
                RET_LOG();
        if (!sk_dst_port__load_half(sk))
                RET_LOG();
	...
  }

Through some cross-function optimization by ArgumentPromotion
optimization, the compiler does:
  static __noinline bool sk_dst_port__load_word(__u32 word_val)
  {
        return word_val == bpf_htons(0xcafe);
  }
  static __noinline bool sk_dst_port__load_half(__u16 half_val)
  {
        return half_val == bpf_htons(0xcafe);
  }
  ...
  int read_sk_dst_port(struct __sk_buff *skb)
  {
        ...
        sk = skb->sk;
        ...
        __u32 *word = (__u32 *)&sk->dst_port;
        __u32 word_val = word[0];
        ...
        if (!sk_dst_port__load_word(word_val))
                RET_LOG();

        __u16 half_val = word_val >> 16;
        if (!sk_dst_port__load_half(half_val))
                RET_LOG();
        ...
  }

In current uapi bpf.h, we have
  struct bpf_sock {
	...
        __be16 dst_port;        /* network byte order */
        __u16 :16;              /* zero padding */
	...
  };
But the old kernel (e.g., 5.6) we have
  struct bpf_sock {
	...
	__u32 dst_port;         /* network byte order */
	...
  };

So for backward compatability reason, 4-byte load of
dst_port is converted to 2-byte load internally.
Specifically, 'word_val = word[0]' is replaced by 2-byte load
by the verifier and this caused the trouble for later
sk_dst_port__load_half() where half_val becomes 0.

Typical usr program won't have such a code pattern tiggering
the above bug, so let us fix the test failure with source
code change. Adding an empty asm volatile statement seems
enough to prevent undesired transformation.

  [1] https://reviews.llvm.org/D148269
  [2] https://lore.kernel.org/bpf/e7f2c5e8-a50c-198d-8f95-388165f1e4fd@meta.com/
  [3] https://reviews.llvm.org/rG141be5c062ecf22bd287afffd310e8ac4711444aTested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20230516214945.1013578-1-yhs@fb.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -265,7 +265,10 @@ static __noinline bool sk_dst_port__load_word(struct bpf_sock *sk)
 
 static __noinline bool sk_dst_port__load_half(struct bpf_sock *sk)
 {
-	__u16 *half = (__u16 *)&sk->dst_port;
+	__u16 *half;
+
+	asm volatile ("");
+	half = (__u16 *)&sk->dst_port;
 	return half[0] == bpf_htons(0xcafe);
 }