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 <[email protected]>
---
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
