Thank you, Alexei, for quick review.
On 1/30/26 2:21 AM, Alexei Starovoitov wrote:
On Thu, Jan 29, 2026 at 5:54 AM Changwoo Min <[email protected]> wrote:
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.
See.. with bpf approach instead of kfunc this new helpers
can work on old kernels without backporting kfuncs :)
You are right. I love the flexibility of BPF in deployment! \o/
Fixes: 4b69e31329b6 ("selftests/bpf: Introduce experimental bpf_in_interrupt()")
fixes tag is not appropriate. It's not a bug fix.
Sure, I will drop the Fixes tag.
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);
please use bpf_ksym_exists().
It helps to catch missing __weak. This patch adds it,
but let's demonstrate best coding practices.
Sure, will change it as suggested.
+ /* 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;
iirc pcpu_hot approach was there for a short time.
Like 5.x kernel didn't have it. It was per-cpu var too.
Pls adjust the comment.
Sure, I found that pcpu_hot had been used only between 6.1 -- 6.14.
I will send out v2 shortly.
Regards,
Changwoo Min
pw-bot: cr
