Re: [PATCH bpf-next v7 2/2] bpf: Hold the perf callchain entry until used completely

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

在 2026/1/10 07:47, Andrii Nakryiko 写道:
> On Tue, Jan 6, 2026 at 8:00 AM Tao Chen <chen.dylane@linux.dev> wrote:
>>
>> 在 2025/12/23 14:29, Tao Chen 写道:
>>> 在 2025/12/17 17:33, 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. The perf_callchain_entres has a small stack of entries, and
>>>> we can reuse it as follows:
>>>>
>>>> 1. get the perf callchain entry
>>>> 2. BPF use...
>>>> 3. put the perf callchain entry
>>>>
>>>> And Peter suggested that get_recursion_context used with preemption
>>>> disabled, so we should disable preemption at BPF side.
>>>>
>>>> Acked-by: Yonghong Song <yonghong.song@linux.dev>
>>>> Signed-off-by: Tao Chen <chen.dylane@linux.dev>
>>>> ---
>>>>    kernel/bpf/stackmap.c | 68 +++++++++++++++++++++++++++++++++++--------
>>>>    1 file changed, 56 insertions(+), 12 deletions(-)
>>>>
>>>> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
>>>> index da3d328f5c1..3bdd99a630d 100644
>>>> --- a/kernel/bpf/stackmap.c
>>>> +++ b/kernel/bpf/stackmap.c
>>>> @@ -210,13 +210,14 @@ static void stack_map_get_build_id_offset(struct
>>>> bpf_stack_build_id *id_offs,
>>>>    }
>>>>    static struct perf_callchain_entry *
>>>> -get_callchain_entry_for_task(struct task_struct *task, u32 max_depth)
>>>> +get_callchain_entry_for_task(int *rctx, struct task_struct *task, u32
>>>> max_depth)
>>>>    {
>>>>    #ifdef CONFIG_STACKTRACE
>>>>        struct perf_callchain_entry *entry;
>>>> -    int rctx;
>>>> -    entry = get_callchain_entry(&rctx);
>>>> +    preempt_disable();
>>>> +    entry = get_callchain_entry(rctx);
>>>> +    preempt_enable();
>>>>        if (!entry)
>>>>            return NULL;
>>>> @@ -238,8 +239,6 @@ get_callchain_entry_for_task(struct task_struct
>>>> *task, u32 max_depth)
>>>>                to[i] = (u64)(from[i]);
>>>>        }
>>>> -    put_callchain_entry(rctx);
>>>> -
>>>>        return entry;
>>>>    #else /* CONFIG_STACKTRACE */
>>>>        return NULL;
>>>> @@ -320,6 +319,34 @@ static long __bpf_get_stackid(struct bpf_map *map,
>>>>        return id;
>>>>    }
>>>> +static struct perf_callchain_entry *
>>>> +bpf_get_perf_callchain(int *rctx, struct pt_regs *regs, bool kernel,
>>>> bool user,
>>>> +               int max_stack, bool crosstask)
>>>> +{
>>>> +    struct perf_callchain_entry_ctx ctx;
>>>> +    struct perf_callchain_entry *entry;
>>>> +
>>>> +    preempt_disable();
>>>> +    entry = get_callchain_entry(rctx);
>>>> +    preempt_enable();
>>>> +
>>>> +    if (unlikely(!entry))
>>>> +        return NULL;
>>>> +
>>>> +    __init_perf_callchain_ctx(&ctx, entry, max_stack, false);
>>>> +    if (kernel)
>>>> +        __get_perf_callchain_kernel(&ctx, regs);
>>>> +    if (user && !crosstask)
>>>> +        __get_perf_callchain_user(&ctx, regs, 0);
>>>> +
>>>> +    return entry;
>>>> +}
>>>> +
>>>> +static void bpf_put_perf_callchain(int rctx)
>>>> +{
>>>> +    put_callchain_entry(rctx);
>>>> +}
>>>> +
>>>>    BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map
>>>> *, map,
>>>>           u64, flags)
>>>>    {
>>>> @@ -328,20 +355,25 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *,
>>>> regs, struct bpf_map *, map,
>>>>        struct perf_callchain_entry *trace;
>>>>        bool kernel = !user;
>>>>        u32 max_depth;
>>>> +    int rctx, ret;
>>>>        if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK |
>>>>                       BPF_F_FAST_STACK_CMP | BPF_F_REUSE_STACKID)))
>>>>            return -EINVAL;
>>>>        max_depth = stack_map_calculate_max_depth(map->value_size,
>>>> elem_size, flags);
>>>> -    trace = get_perf_callchain(regs, kernel, user, max_depth,
>>>> -                   false, false, 0);
>>>> +
>>>> +    trace = bpf_get_perf_callchain(&rctx, regs, kernel, user, max_depth,
>>>> +                       false);
>>>>        if (unlikely(!trace))
>>>>            /* couldn't fetch the stack trace */
>>>>            return -EFAULT;
>>>> -    return __bpf_get_stackid(map, trace, flags);
>>>> +    ret = __bpf_get_stackid(map, trace, flags);
>>>> +    bpf_put_perf_callchain(rctx);
>>>> +
>>>> +    return ret;
>>>>    }
>>>>    const struct bpf_func_proto bpf_get_stackid_proto = {
>>>> @@ -435,6 +467,7 @@ static long __bpf_get_stack(struct pt_regs *regs,
>>>> struct task_struct *task,
>>>>        bool kernel = !user;
>>>>        int err = -EINVAL;
>>>>        u64 *ips;
>>>> +    int rctx;
>>>>        if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK |
>>>>                       BPF_F_USER_BUILD_ID)))
>>>> @@ -467,18 +500,26 @@ static long __bpf_get_stack(struct pt_regs
>>>> *regs, struct task_struct *task,
>>>>            trace = trace_in;
>>>>            trace->nr = min_t(u32, trace->nr, max_depth);
>>>>        } else if (kernel && task) {
>>>> -        trace = get_callchain_entry_for_task(task, max_depth);
>>>> +        trace = get_callchain_entry_for_task(&rctx, task, max_depth);
>>>>        } else {
>>>> -        trace = get_perf_callchain(regs, kernel, user, max_depth,
>>>> -                       crosstask, false, 0);
>>>> +        trace = bpf_get_perf_callchain(&rctx, regs, kernel, user,
>>>> max_depth,
>>>> +                           crosstask);
>>>>        }
>>>> -    if (unlikely(!trace) || trace->nr < skip) {
>>>> +    if (unlikely(!trace)) {
>>>>            if (may_fault)
>>>>                rcu_read_unlock();
>>>>            goto err_fault;
>>>>        }
>>>> +    if (trace->nr < skip) {
>>>> +        if (may_fault)
>>>> +            rcu_read_unlock();
>>>> +        if (!trace_in)
>>>> +            bpf_put_perf_callchain(rctx);
>>>> +        goto err_fault;
>>>> +    }
>>>> +
>>>>        trace_nr = trace->nr - skip;
>>>>        copy_len = trace_nr * elem_size;
>>>> @@ -497,6 +538,9 @@ static long __bpf_get_stack(struct pt_regs *regs,
>>>> struct task_struct *task,
>>>>        if (may_fault)
>>>>            rcu_read_unlock();
>>>> +    if (!trace_in)
>>>> +        bpf_put_perf_callchain(rctx);
>>>> +
>>>>        if (user_build_id)
>>>>            stack_map_get_build_id_offset(buf, trace_nr, user, may_fault);
>>>
>>> Hi Peter,
>>>
>>> As Alexei said, the patch needs your ack, please review again, thanks.
>>>
>>
>> ping...
> 
> Peter, if I understand correctly, this will go through bpf-next tree,
> but it would be great if you could take a look and confirm this
> overall is not broken. Thanks!
> 
>>
>> --
>> Best Regards
>> Tao Chen

Hi Andrii, Peter

It appears that the code does not require a rebase, and the latest CI 
build is valid. Looking forward to your response. Thanks.

CI has tested the following submission:
Status:     SUCCESS
Name:       [RESEND,bpf-next,v7,0/2] Pass external callchain entry to 
get_perf_callchain
Patchwork: 
https://patchwork.kernel.org/project/netdevbpf/list/?series=1034091&state=*
Matrix:     https://github.com/kernel-patches/bpf/actions/runs/21051611369

-- 
Best Regards
Tao Chen