[PATCH 1/6] perf offcpu: Fix a build failure on old kernels

[Namhyung Kim <namhyung@kernel.org>]

Old kernels have task_struct which contains "state" field and newer
kernels have "__state".  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
the task struct directly and access them using new and old definitions
in a row.

Reported-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
 tools/perf/util/bpf_skel/off_cpu.bpf.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/tools/perf/util/bpf_skel/off_cpu.bpf.c b/tools/perf/util/bpf_skel/off_cpu.bpf.c
index 792ae2847080..cc6d7fd55118 100644
--- a/tools/perf/util/bpf_skel/off_cpu.bpf.c
+++ b/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)
-- 
2.37.0.rc0.161.g10f37bed90-goog