From: Changwoo Min <changwoo@igalia.com>
To: Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Andrii Nakryiko <andrii@kernel.org>
Cc: Martin KaFai Lau <martin.lau@linux.dev>,
Eduard Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>,
Yonghong Song <yonghong.song@linux.dev>,
John Fastabend <john.fastabend@gmail.com>,
KP Singh <kpsingh@kernel.org>,
Stanislav Fomichev <sdf@fomichev.me>, Hao Luo <haoluo@google.com>,
Jiri Olsa <jolsa@kernel.org>, Shuah Khan <shuah@kernel.org>,
kernel-dev@igalia.com, bpf@vger.kernel.org,
sched-ext@lists.linux.dev, linux-kernel@vger.kernel.org,
linux-kselftest@vger.kernel.org,
Changwoo Min <changwoo@igalia.com>
Subject: [PATCH v2] selftests/bpf: Make x86 preempt_count access compatible across v6.14+
Date: Fri, 30 Jan 2026 11:18:43 +0900 [thread overview]
Message-ID: <20260130021843.154885-1-changwoo@igalia.com> (raw)
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
next reply other threads:[~2026-01-30 2:19 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-01-30 2:18 Changwoo Min [this message]
2026-01-30 20:23 ` Alexei Starovoitov
2026-01-31 1:40 ` Changwoo Min
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