[PATCH] selftests/bpf: Make x86 preempt_count access compatible across v6.14+

[Changwoo Min]

Recent x86 kernels (v6.15+) export __preempt_count as a ksym, while older
kernels expose the preemption counter via pcpu_hot.preempt_count. The
existing selftest helper unconditionally dereferenced __preempt_count,
which breaks BPF program loading on older kernels.

Make the x86 preemption count lookup version-agnostic by:
- Marking __preempt_count and pcpu_hot as weak ksyms.
- Introducing a BTF-described pcpu_hot___local layout with
  preserve_access_index.
- Selecting the appropriate access path at runtime using ksym availability
  and bpf_core_field_exists().

This allows a single BPF binary to run correctly on both v6.14-and-older
and v6.15-and-newer kernels without relying on compile-time version checks.

Fixes: 4b69e31329b6 ("selftests/bpf: Introduce experimental bpf_in_interrupt()")
Signed-off-by: Changwoo Min <changwoo@xxxxxxxxxx>
---
 tools/testing/selftests/bpf/bpf_experimental.h | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/bpf_experimental.h b/tools/testing/selftests/bpf/bpf_experimental.h
index a39576c8ba04..0194c0090e50 100644
--- a/tools/testing/selftests/bpf/bpf_experimental.h
+++ b/tools/testing/selftests/bpf/bpf_experimental.h
@@ -614,7 +614,13 @@ extern int bpf_cgroup_read_xattr(struct cgroup *cgroup, const char *name__str,
 
 extern bool CONFIG_PREEMPT_RT __kconfig __weak;
 #ifdef bpf_target_x86
-extern const int __preempt_count __ksym;
+extern const int __preempt_count __ksym __weak;
+
+struct pcpu_hot___local {
+	int preempt_count;
+} __attribute__((preserve_access_index));
+
+extern struct pcpu_hot___local pcpu_hot __ksym __weak;
 #endif
 
 struct task_struct___preempt_rt {
@@ -624,7 +630,13 @@ struct task_struct___preempt_rt {
 static inline int get_preempt_count(void)
 {
 #if defined(bpf_target_x86)
-	return *(int *) bpf_this_cpu_ptr(&__preempt_count);
+	/* v6.15 or later */
+	if (&__preempt_count)
+		return *(int *) bpf_this_cpu_ptr(&__preempt_count);
+	/* v6.14 or older */
+	if (bpf_core_field_exists(pcpu_hot.preempt_count))
+		return ((struct pcpu_hot___local *)
+			bpf_this_cpu_ptr(&pcpu_hot))->preempt_count;
 #elif defined(bpf_target_arm64)
 	return bpf_get_current_task_btf()->thread_info.preempt.count;
 #endif
-- 
2.52.0