selftests/bpf: Make bpf get_preempt_count() work for v6.14+ kernels

Recent x86 kernels export __preempt_count as a ksym, while some old kernels
between v6.1 and v6.14 expose the preemption counter via
pcpu_hot.preempt_count. The existing selftest helper unconditionally
dereferenced __preempt_count, which breaks BPF program loading on such old
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_ksym_exists() and bpf_core_field_exists().

This allows a single BPF binary to run correctly across kernel versions
(e.g., v6.18 vs. v6.13) without relying on compile-time version checks.

Signed-off-by: Changwoo Min <changwoo@igalia.com>
Link: https://lore.kernel.org/r/20260130021843.154885-1-changwoo@igalia.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: multics69

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 +630,19 @@ 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);
+	/* By default, read the per-CPU __preempt_count. */
+	if (bpf_ksym_exists(&__preempt_count))
+		return *(int *) bpf_this_cpu_ptr(&__preempt_count);
+
+	/*
+	 * If __preempt_count does not exist, try to read preempt_count under
+	 * struct pcpu_hot. Between v6.1 and v6.14 -- more specifically,
+	 * [64701838bf057, 46e8fff6d45fe), preempt_count had been managed
+	 * under struct pcpu_hot.
+	 */
+	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