[v2] selftests/bpf: Make x86 preempt_count access compatible across v6.14+

[PATCH v2] selftests/bpf: Make x86 preempt_count access compatible across v6.14+

Posted by Changwoo Min 1 week, 1 day ago

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>
---
 .../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

Re: [PATCH v2] selftests/bpf: Make x86 preempt_count access compatible across v6.14+

Posted by Alexei Starovoitov 1 week ago

On Thu, Jan 29, 2026 at 6:19 PM Changwoo Min <changwoo@igalia.com> 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 <changwoo@igalia.com>
> ---
>  .../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.

Re: [PATCH v2] selftests/bpf: Make x86 preempt_count access compatible across v6.14+

Posted by Changwoo Min 1 week ago

Hi Alexei,

On 1/31/26 5:23 AM, Alexei Starovoitov wrote:
> On Thu, Jan 29, 2026 at 6:19 PM Changwoo Min <changwoo@igalia.com> wrote:

> 
> 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.

Noted. Thanks a lot!

Regards,
Changwoo Min