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Message-ID: <a0a4134e-66f0-4266-b4d3-0c080a684d96@linux.dev>
Date: Sun, 28 Sep 2025 12:11:28 +0800
From: Tao Chen <chen.dylane@...ux.dev>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: song@...nel.org, jolsa@...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, bpf@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v2] bpf: Add preempt_disable to protect
get_perf_callchain
在 2025/9/27 02:52, Andrii Nakryiko 写道:
> On Fri, Sep 26, 2025 at 8:40 AM Tao Chen <chen.dylane@...ux.dev> wrote:
>>
>> As Alexei noted, get_perf_callchain() return values may be reused
>> if a task is preempted after the BPF program enters migrate disable
>> mode. We therefore use bpf_perf_callchain_entries percpu entries
>> similarly to bpf_try_get_buffers to preserve the current task's
>> callchain and prevent overwriting by preempting tasks. And we also
>> add preempt_disable to protect get_perf_callchain.
>>
>> Reported-by: Alexei Starovoitov <ast@...nel.org>
>> Closes: https://lore.kernel.org/bpf/CAADnVQ+s8B7-fvR1TNO-bniSyKv57cH_ihRszmZV7pQDyV=VDQ@mail.gmail.com
>> Signed-off-by: Tao Chen <chen.dylane@...ux.dev>
>> ---
>> kernel/bpf/stackmap.c | 76 ++++++++++++++++++++++++++++++++++---------
>> 1 file changed, 61 insertions(+), 15 deletions(-)
>>
>> Change list:
>> v1 -> v2:
>> From Alexei
>> - create percpu entris to preserve current task's callchain
>> similarly to bpf_try_get_buffers.
>> v1: https://lore.kernel.org/bpf/20250922075333.1452803-1-chen.dylane@linux.dev
>>
>> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
>> index 2e182a3ac4c..8788c219926 100644
>> --- a/kernel/bpf/stackmap.c
>> +++ b/kernel/bpf/stackmap.c
>> @@ -31,6 +31,55 @@ 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(struct bpf_perf_callchain_entry **entry,
>> + struct pt_regs *regs, u32 init_nr, bool kernel,
>> + bool user, u32 max_stack, bool crosstack,
>> + bool add_mark)
>> +{
>> + 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]);
>> +
>> + preempt_disable();
>> + perf_entry = get_perf_callchain(regs, init_nr, kernel, user, max_stack,
>> + crosstack, add_mark);
>> + if (unlikely(!perf_entry)) {
>> + preempt_enable();
>> + this_cpu_dec(bpf_perf_callchain_preempt_cnt);
>> + return -EFAULT;
>> + }
>> + memcpy(bpf_entry, perf_entry, sizeof(u64) * (perf_entry->nr + 1));
>
> N copies of a stack trace is not good enough, let's have N + 1 now :)
>
> If we are going with our own buffers, we need to teach
> get_perf_callchain to let us pass that buffer directly to avoid that
> unnecessary copy.
>
> Also, I know it's about 1KB, but it would be so simple and efficient
> to just have this bpf_perf_callchain_entry on the stack. Kernel has a
> 16KB stack, right? It feels like for something like this using 1KB of
> the stack to simplify and speed up stack trace capture is a good
> enough reason.
>
It's a good idea, if that's the case, do we also not need
preempt_disable for get_perf_callchain?
>> + *entry = bpf_entry;
>> + preempt_enable();
>> +
>> + 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);
>> @@ -303,8 +352,9 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
>> u32 max_depth = map->value_size / stack_map_data_size(map);
>> u32 skip = flags & BPF_F_SKIP_FIELD_MASK;
>> bool user = flags & BPF_F_USER_STACK;
>> - struct perf_callchain_entry *trace;
>> + struct bpf_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 +364,15 @@ 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, 0, kernel, user, max_depth,
>> - false, false);
>> + err = bpf_get_perf_callchain(&trace, regs, 0, kernel, user, max_depth,
>> + false, false);
>> + if (err)
>> + return err;
>>
>> - if (unlikely(!trace))
>> - /* couldn't fetch the stack trace */
>> - return -EFAULT;
>> + err = __bpf_get_stackid(map, (struct perf_callchain_entry *)trace, flags);
>> + bpf_put_perf_callchain();
>>
>> - return __bpf_get_stackid(map, trace, flags);
>> + return err;
>> }
>>
>> const struct bpf_func_proto bpf_get_stackid_proto = {
>> @@ -443,8 +494,7 @@ 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 */
>> + preempt_disable();
>>
>> if (trace_in)
>> trace = trace_in;
>> @@ -455,8 +505,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
>> crosstask, false);
>>
>> if (unlikely(!trace) || trace->nr < skip) {
>> - if (may_fault)
>> - rcu_read_unlock();
>> + preempt_enable();
>> goto err_fault;
>> }
>>
>> @@ -474,10 +523,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
>> } else {
>> memcpy(buf, ips, copy_len);
>> }
>> -
>> - /* trace/ips should not be dereferenced after this point */
>> - if (may_fault)
>> - rcu_read_unlock();
>> + preempt_enable();
>>
>> if (user_build_id)
>> stack_map_get_build_id_offset(buf, trace_nr, user, may_fault);
>
> really it's just build_id resolution that can take a while, which is
> why we are trying to avoid preemption around it. But for non-build_id
> case, can we avoid extra copying?
>
>> --
>> 2.48.1
>>
--
Best Regards
Tao Chen
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