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

[Tao Chen]

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@xxxxxxxxx>
> > Signed-off-by: Tao Chen <chen.dylane@xxxxxxxxx>
> > ---
> >   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...

--
Best Regards
Tao Chen