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 <[email protected]> --- .../testing/selftests/bpf/bpf_experimental.h | 21 +++++++++++++++++-- 1 file changed, 19 insertions(+), 2 deletions(-) diff --git a/tools/testing/selftests/bpf/bpf_experimental.h b/tools/testing/selftests/bpf/bpf_experimental.h index a39576c8ba04..acabf869d2c4 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 +630,18 @@ 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 -- 2.52.0
