[PATCH] bpf: Add BPF CO-RE compatibility for older kernels

[Wander Lairson Costa <wander@redhat.com>]

Introduce a `thread_info___legacy` struct to provide BPF CO-RE
compatibility for older kernels (e.g., RHEL 8.x with 4.18) where the
`thread_info` struct lacks the `cpu` field. This allows `bpf_core_field_exists()`
checks to correctly determine the availability of this field at runtime,
preventing build failures on such systems.

The `task_cpu` helper function is updated to utilize this new legacy
struct when accessing the `cpu` field from `thread_info`, ensuring
that the BPF program can adapt to different kernel versions.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Cc: Derek Barbosa <debarbos@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Chunsheng Luo <luochunsheng@ustc.edu>
---
 bpf/stalld.bpf.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/bpf/stalld.bpf.c b/bpf/stalld.bpf.c
index b0a62ac..bf55f5b 100644
--- a/bpf/stalld.bpf.c
+++ b/bpf/stalld.bpf.c
@@ -49,6 +49,16 @@ struct {
 #define log_task(p) log_task_prefix("", p)
 #define log_task_error(p) log_task_prefix("error: ", p)
 
+/*
+ * BPF CO-RE compatibility: In older kernels (e.g., RHEL 8.x with 4.18),
+ * thread_info lacks the cpu field. We define it here to enable
+ * bpf_core_field_exists() checks, allowing runtime detection of whether
+ * this field is available on the target kernel.
+ */
+struct thread_info___legacy {
+	int cpu;
+};
+
 /*
  * BPF CO-RE "weak" or "candidate" definition.
  *
@@ -105,10 +115,11 @@ static inline bool task_is_rt(const struct task_struct *p)
 static inline int task_cpu(const struct task_struct *p)
 {
 	const struct task_struct___legacy *lp = (const void *) p;
+	const struct thread_info___legacy *lt = (const void *) &p->thread_info;
 
 	return bpf_core_field_exists(lp->cpu)
 		? BPF_CORE_READ(lp, cpu)
-		: BPF_CORE_READ(p, thread_info.cpu);
+		: BPF_CORE_READ(lt, cpu);
 }
 
 /**
-- 
2.51.1