Re: [PATCH bpf-next v3 2/2] bpf: Use per-cpu BPF callchain entry to save callchain

[Tao Chen <chen.dylane@linux.dev>]

在 2025/10/20 01:01, Tao Chen 写道:
> 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@linux.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();

double-put issue here reported by AI, i will fix it.

>   
>   	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);


-- 
Best Regards
Tao Chen