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Message-ID: <20251019170118.2955346-3-chen.dylane@linux.dev>
Date: Mon, 20 Oct 2025 01:01:18 +0800
From: Tao Chen <chen.dylane@...ux.dev>
To: peterz@...radead.org,
mingo@...hat.com,
acme@...nel.org,
namhyung@...nel.org,
mark.rutland@....com,
alexander.shishkin@...ux.intel.com,
jolsa@...nel.org,
irogers@...gle.com,
adrian.hunter@...el.com,
kan.liang@...ux.intel.com,
song@...nel.org,
ast@...nel.org,
daniel@...earbox.net,
andrii@...nel.org,
martin.lau@...ux.dev,
eddyz87@...il.com,
yonghong.song@...ux.dev,
john.fastabend@...il.com,
kpsingh@...nel.org,
sdf@...ichev.me,
haoluo@...gle.com
Cc: linux-perf-users@...r.kernel.org,
linux-kernel@...r.kernel.org,
bpf@...r.kernel.org,
Tao Chen <chen.dylane@...ux.dev>
Subject: [PATCH bpf-next v3 2/2] bpf: Use per-cpu BPF callchain entry to save callchain
As Alexei noted, get_perf_callchain() return values may be reused
if a task is preempted after the BPF program enters migrate disable
mode. Drawing on the per-cpu design of bpf_bprintf_buffers,
per-cpu BPF callchain entry is used here.
Signed-off-by: Tao Chen <chen.dylane@...ux.dev>
---
kernel/bpf/stackmap.c | 98 ++++++++++++++++++++++++++++++++-----------
1 file changed, 74 insertions(+), 24 deletions(-)
diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index 94e46b7f340..3513077c57d 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -31,6 +31,52 @@ struct bpf_stack_map {
struct stack_map_bucket *buckets[] __counted_by(n_buckets);
};
+struct bpf_perf_callchain_entry {
+ u64 nr;
+ u64 ip[PERF_MAX_STACK_DEPTH];
+};
+
+#define MAX_PERF_CALLCHAIN_PREEMPT 3
+static DEFINE_PER_CPU(struct bpf_perf_callchain_entry[MAX_PERF_CALLCHAIN_PREEMPT],
+ bpf_perf_callchain_entries);
+static DEFINE_PER_CPU(int, bpf_perf_callchain_preempt_cnt);
+
+static int bpf_get_perf_callchain_or_entry(struct perf_callchain_entry **entry,
+ struct pt_regs *regs, bool kernel,
+ bool user, u32 max_stack, bool crosstack,
+ bool add_mark, bool get_callchain)
+{
+ struct bpf_perf_callchain_entry *bpf_entry;
+ struct perf_callchain_entry *perf_entry;
+ int preempt_cnt;
+
+ preempt_cnt = this_cpu_inc_return(bpf_perf_callchain_preempt_cnt);
+ if (WARN_ON_ONCE(preempt_cnt > MAX_PERF_CALLCHAIN_PREEMPT)) {
+ this_cpu_dec(bpf_perf_callchain_preempt_cnt);
+ return -EBUSY;
+ }
+
+ bpf_entry = this_cpu_ptr(&bpf_perf_callchain_entries[preempt_cnt - 1]);
+ if (!get_callchain) {
+ *entry = (struct perf_callchain_entry *)bpf_entry;
+ return 0;
+ }
+
+ perf_entry = get_perf_callchain(regs, (struct perf_callchain_entry *)bpf_entry,
+ kernel, user, max_stack,
+ crosstack, add_mark);
+ *entry = perf_entry;
+
+ return 0;
+}
+
+static void bpf_put_perf_callchain(void)
+{
+ if (WARN_ON_ONCE(this_cpu_read(bpf_perf_callchain_preempt_cnt) == 0))
+ return;
+ this_cpu_dec(bpf_perf_callchain_preempt_cnt);
+}
+
static inline bool stack_map_use_build_id(struct bpf_map *map)
{
return (map->map_flags & BPF_F_STACK_BUILD_ID);
@@ -192,11 +238,11 @@ get_callchain_entry_for_task(struct task_struct *task, u32 max_depth)
{
#ifdef CONFIG_STACKTRACE
struct perf_callchain_entry *entry;
- int rctx;
-
- entry = get_callchain_entry(&rctx);
+ int ret;
- if (!entry)
+ ret = bpf_get_perf_callchain_or_entry(&entry, NULL, false, false, 0, false, false,
+ false);
+ if (ret)
return NULL;
entry->nr = stack_trace_save_tsk(task, (unsigned long *)entry->ip,
@@ -216,7 +262,7 @@ get_callchain_entry_for_task(struct task_struct *task, u32 max_depth)
to[i] = (u64)(from[i]);
}
- put_callchain_entry(rctx);
+ bpf_put_perf_callchain();
return entry;
#else /* CONFIG_STACKTRACE */
@@ -305,6 +351,7 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
bool user = flags & BPF_F_USER_STACK;
struct perf_callchain_entry *trace;
bool kernel = !user;
+ int err;
if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK |
BPF_F_FAST_STACK_CMP | BPF_F_REUSE_STACKID)))
@@ -314,14 +361,15 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
if (max_depth > sysctl_perf_event_max_stack)
max_depth = sysctl_perf_event_max_stack;
- trace = get_perf_callchain(regs, NULL, kernel, user, max_depth,
- false, false);
+ err = bpf_get_perf_callchain_or_entry(&trace, regs, kernel, user, max_depth,
+ false, false, true);
+ if (err)
+ return err;
- if (unlikely(!trace))
- /* couldn't fetch the stack trace */
- return -EFAULT;
+ err = __bpf_get_stackid(map, trace, flags);
+ bpf_put_perf_callchain();
- return __bpf_get_stackid(map, trace, flags);
+ return err;
}
const struct bpf_func_proto bpf_get_stackid_proto = {
@@ -443,20 +491,23 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
if (sysctl_perf_event_max_stack < max_depth)
max_depth = sysctl_perf_event_max_stack;
- if (may_fault)
- rcu_read_lock(); /* need RCU for perf's callchain below */
-
if (trace_in)
trace = trace_in;
- else if (kernel && task)
+ else if (kernel && task) {
trace = get_callchain_entry_for_task(task, max_depth);
- else
- trace = get_perf_callchain(regs, NULL, kernel, user, max_depth,
- crosstask, false);
+ } else {
+ err = bpf_get_perf_callchain_or_entry(&trace, regs, kernel, user, max_depth,
+ false, false, true);
+ if (err)
+ return err;
+ }
+
+ if (unlikely(!trace))
+ goto err_fault;
- if (unlikely(!trace) || trace->nr < skip) {
- if (may_fault)
- rcu_read_unlock();
+ if (trace->nr < skip) {
+ if (!trace_in)
+ bpf_put_perf_callchain();
goto err_fault;
}
@@ -475,9 +526,8 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
memcpy(buf, ips, copy_len);
}
- /* trace/ips should not be dereferenced after this point */
- if (may_fault)
- rcu_read_unlock();
+ if (!trace_in)
+ bpf_put_perf_callchain();
if (user_build_id)
stack_map_get_build_id_offset(buf, trace_nr, user, may_fault);
--
2.48.1
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