On Thu, Jan 29, 2026 at 6:19 PM Changwoo Min <[email protected]> wrote: > > 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
Please use proper kernel comment style in the future. We don't use old networking style in bpf anymore. I fixed it up while applying. Also use [PATCH bpf-next] in the subject to help CI identify the tree.
