bpf, selftests: Add tests for ctx access in sock_ops with single register

To verify fix ("bpf: sock_ops ctx access may stomp registers in corner case")
we want to force compiler to generate the following code when accessing a
field with BPF_TCP_SOCK_GET_COMMON,

     r1 = *(u32 *)(r1 + 96) // r1 is skops ptr

Rather than depend on clang to do this we add the test with inline asm to
the tcpbpf test. This saves us from having to create another runner and
ensures that if we break this again test_tcpbpf will crash.

With above code we get the xlated code,

  11: (7b) *(u64 *)(r1 +32) = r9
  12: (61) r9 = *(u32 *)(r1 +28)
  13: (15) if r9 == 0x0 goto pc+4
  14: (79) r9 = *(u64 *)(r1 +32)
  15: (79) r1 = *(u64 *)(r1 +0)
  16: (61) r1 = *(u32 *)(r1 +2348)
  17: (05) goto pc+1
  18: (79) r9 = *(u64 *)(r1 +32)

We also add the normal case where src_reg != dst_reg so we can compare
code generation easily from llvm-objdump and ensure that case continues
to work correctly. The normal code is xlated to,

  20: (b7) r1 = 0
  21: (61) r1 = *(u32 *)(r3 +28)
  22: (15) if r1 == 0x0 goto pc+2
  23: (79) r1 = *(u64 *)(r3 +0)
  24: (61) r1 = *(u32 *)(r1 +2348)

Where the temp variable is not used.

Signed-off-by: John Fastabend <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Song Liu <[email protected]>
Acked-by: Martin KaFai Lau <[email protected]>
Link: https://lore.kernel.org/bpf/159718351457.4728.3295119261717842496.stgit@john-Precision-5820-Tower

Author: jrfastab

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

@@ -54,6 +54,7 @@ SEC("sockops")
 int bpf_testcb(struct bpf_sock_ops *skops)
 {
 	char header[sizeof(struct ipv6hdr) + sizeof(struct tcphdr)];
+	struct bpf_sock_ops *reuse = skops;
 	struct tcphdr *thdr;
 	int good_call_rv = 0;
 	int bad_call_rv = 0;
@@ -62,6 +63,18 @@ int bpf_testcb(struct bpf_sock_ops *skops)
 	int v = 0;
 	int op;
 
+	/* Test reading fields in bpf_sock_ops using single register */
+	asm volatile (
+		"%[reuse] = *(u32 *)(%[reuse] +96)"
+		: [reuse] "+r"(reuse)
+		:);
+
+	asm volatile (
+		"%[op] = *(u32 *)(%[skops] +96)"
+		: [op] "+r"(op)
+		: [skops] "r"(skops)
+		:);
+
 	op = (int) skops->op;
 
 	update_event_map(op);