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selftests/bpf: Make bpf get_preempt_count() work for v6.14+ kernels

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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>
Link: https://lore.kernel.org/r/20260130021843.154885-1-changwoo@igalia.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Changwoo Min 2026-01-30 11:18:43 +09:00 • committed by Alexei Starovoitov
parent b18a761ca0
commit cd77618c41
1 changed files with 20 additions and 2 deletions
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22
tools/testing/selftests/bpf/bpf_experimental.h
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@ -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 +630,19 @@ 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