selftests/bpf: Workaround a verifier issue for test exhandler

The llvm patch [1] enabled opaque pointer which caused selftest
'exhandler' failure.
  ...
  ; work = task->task_works;
  7: (79) r1 = *(u64 *)(r6 +2120)       ; R1_w=ptr_callback_head(off=0,imm=0) R6_w=ptr_task_struct(off=0,imm=0)
  ; func = work->func;
  8: (79) r2 = *(u64 *)(r1 +8)          ; R1_w=ptr_callback_head(off=0,imm=0) R2_w=scalar()
  ; if (!work && !func)
  9: (4f) r1 |= r2
  math between ptr_ pointer and register with unbounded min value is not allowed

  below is insn 10 and 11
  10: (55) if r1 != 0 goto +5
  11: (18) r1 = 0 ll
  ...

In llvm, the code generation of 'r1 |= r2' happened in codegen
selectiondag phase due to difference of opaque pointer vs. non-opaque pointer.
Without [1], the related code looks like:
  r2 = *(u64 *)(r6 + 2120)
  r1 = *(u64 *)(r2 + 8)
  if r2 != 0 goto +6 <LBB0_4>
  if r1 != 0 goto +5 <LBB0_4>
  r1 = 0 ll
  ...

I haven't found a good way in llvm to fix this issue. So let us workaround the
problem first so bpf CI won't be blocked.

  [1] https://reviews.llvm.org/D123300Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220419050900.3136024-1-yhs@fb.com

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

@@ -7,6 +7,8 @@
 #include <bpf/bpf_tracing.h>
 #include <bpf/bpf_core_read.h>
 
+#define barrier_var(var) asm volatile("" : "=r"(var) : "0"(var))
+
 char _license[] SEC("license") = "GPL";
 
 unsigned int exception_triggered;
@@ -37,7 +39,16 @@ int BPF_PROG(trace_task_newtask, struct task_struct *task, u64 clone_flags)
 	 */
 	work = task->task_works;
 	func = work->func;
-	if (!work && !func)
-		exception_triggered++;
+	/* Currently verifier will fail for `btf_ptr |= btf_ptr` * instruction.
+	 * To workaround the issue, use barrier_var() and rewrite as below to
+	 * prevent compiler from generating verifier-unfriendly code.
+	 */
+	barrier_var(work);
+	if (work)
+		return 0;
+	barrier_var(func);
+	if (func)
+		return 0;
+	exception_triggered++;
 	return 0;
 }