perf offcpu: Fix build failure on old kernels

Old kernels have a 'struct task_struct' which contains a "state" field
and newer kernels have "__state" instead.

While the get_task_state() in the BPF code handles that in some way, it
assumed the current kernel has the new definition and it caused a build
error on old kernels.

We should not assume anything and access them carefully.  Do not use
'task struct' directly access it instead using new and old definitions
in a row.

Fixes: edc41a1 ("perf record: Enable off-cpu analysis with BPF")
Reported-by: Ian Rogers <[email protected]>
Signed-off-by: Namhyung Kim <[email protected]>
Cc: Blake Jones <[email protected]>
Cc: Hao Luo <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Milian Wolff <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Song Liu <[email protected]>
Cc: [email protected]
Link: http://lore.kernel.org/lkml/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: namhyung

tools/perf/util/bpf_skel/off_cpu.bpf.c

@@ -71,6 +71,11 @@ struct {
 	__uint(max_entries, 1);
 } cgroup_filter SEC(".maps");
 
+/* new kernel task_struct definition */
+struct task_struct___new {
+	long __state;
+} __attribute__((preserve_access_index));
+
 /* old kernel task_struct definition */
 struct task_struct___old {
 	long state;
@@ -93,14 +98,17 @@ const volatile bool uses_cgroup_v1 = false;
  */
 static inline int get_task_state(struct task_struct *t)
 {
-	if (bpf_core_field_exists(t->__state))
-		return BPF_CORE_READ(t, __state);
+	/* recast pointer to capture new type for compiler */
+	struct task_struct___new *t_new = (void *)t;
 
-	/* recast pointer to capture task_struct___old type for compiler */
-	struct task_struct___old *t_old = (void *)t;
+	if (bpf_core_field_exists(t_new->__state)) {
+		return BPF_CORE_READ(t_new, __state);
+	} else {
+		/* recast pointer to capture old type for compiler */
+		struct task_struct___old *t_old = (void *)t;
 
-	/* now use old "state" name of the field */
-	return BPF_CORE_READ(t_old, state);
+		return BPF_CORE_READ(t_old, state);
+	}
 }
 
 static inline __u64 get_cgroup_id(struct task_struct *t)