Re: [PATCH bpf] bpf: fix nested bpf tracepoints with per-cpu data

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Jun 6, 2019 at 1:17 PM Matt Mullins <mmullins@fb.com> wrote:
>
> BPF_PROG_TYPE_RAW_TRACEPOINTs can be executed nested on the same CPU, as
> they do not increment bpf_prog_active while executing.
>
> This enables three levels of nesting, to support
>   - a kprobe or raw tp or perf event,
>   - another one of the above that irq context happens to call, and
>   - another one in nmi context

Can NMIs be nested?

> (at most one of which may be a kprobe or perf event).
>
> Fixes: 20b9d7ac4852 ("bpf: avoid excessive stack usage for perf_sample_data")
> ---
> This is more lines of code, but possibly less intrusive than the
> per-array-element approach.
>
> I don't necessarily like that I duplicated the nest_level logic in two
> places, but I don't see a way to unify them:
>   - kprobes' bpf_perf_event_output doesn't use bpf_raw_tp_regs, and does
>     use the perf_sample_data,
>   - raw tracepoints' bpf_get_stackid uses bpf_raw_tp_regs, but not
>     the perf_sample_data, and
>   - raw tracepoints' bpf_perf_event_output uses both...
>
>  kernel/trace/bpf_trace.c | 95 +++++++++++++++++++++++++++++++++-------
>  1 file changed, 80 insertions(+), 15 deletions(-)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index f92d6ad5e080..4f5419837ddd 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -410,8 +410,6 @@ static const struct bpf_func_proto bpf_perf_event_read_value_proto = {
>         .arg4_type      = ARG_CONST_SIZE,
>  };
>
> -static DEFINE_PER_CPU(struct perf_sample_data, bpf_trace_sd);
> -
>  static __always_inline u64
>  __bpf_perf_event_output(struct pt_regs *regs, struct bpf_map *map,
>                         u64 flags, struct perf_sample_data *sd)
> @@ -442,24 +440,47 @@ __bpf_perf_event_output(struct pt_regs *regs, struct bpf_map *map,
>         return perf_event_output(event, sd, regs);
>  }
>
> +/*
> + * Support executing tracepoints in normal, irq, and nmi context that each call
> + * bpf_perf_event_output
> + */
> +struct bpf_trace_sample_data {
> +       struct perf_sample_data sds[3];
> +};
> +
> +static DEFINE_PER_CPU(struct bpf_trace_sample_data, bpf_trace_sds);
> +static DEFINE_PER_CPU(int, bpf_trace_nest_level);
>  BPF_CALL_5(bpf_perf_event_output, struct pt_regs *, regs, struct bpf_map *, map,
>            u64, flags, void *, data, u64, size)
>  {
> -       struct perf_sample_data *sd = this_cpu_ptr(&bpf_trace_sd);
> +       struct bpf_trace_sample_data *sds = this_cpu_ptr(&bpf_trace_sds);
> +       struct perf_sample_data *sd;
> +       int nest_level = this_cpu_inc_return(bpf_trace_nest_level);

reverse Christmas tree?

>         struct perf_raw_record raw = {
>                 .frag = {
>                         .size = size,
>                         .data = data,
>                 },
>         };
> +       int err = -EBUSY;
>
> +       if (WARN_ON_ONCE(nest_level > ARRAY_SIZE(sds->sds)))
> +               goto out;

consider this a nit, but I find it much simpler to follow when err is
set just before goto, so that it's clear what's going to be returned:

int err;

if (something_bad) {
    err = -EBAD_ERR_CODE1;
    goto out;
}


> +
> +       sd = &sds->sds[nest_level - 1];
> +
> +       err = -EINVAL;
>         if (unlikely(flags & ~(BPF_F_INDEX_MASK)))
> -               return -EINVAL;
> +               goto out;

Same here.

>
>         perf_sample_data_init(sd, 0, 0);
>         sd->raw = &raw;
>
> -       return __bpf_perf_event_output(regs, map, flags, sd);
> +       err = __bpf_perf_event_output(regs, map, flags, sd);
> +
> +out:
> +       this_cpu_dec(bpf_trace_nest_level);
> +       return err;
>  }
>
>  static const struct bpf_func_proto bpf_perf_event_output_proto = {
> @@ -822,16 +843,48 @@ pe_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>  /*
>   * bpf_raw_tp_regs are separate from bpf_pt_regs used from skb/xdp
>   * to avoid potential recursive reuse issue when/if tracepoints are added
> - * inside bpf_*_event_output, bpf_get_stackid and/or bpf_get_stack
> + * inside bpf_*_event_output, bpf_get_stackid and/or bpf_get_stack.
> + *
> + * Since raw tracepoints run despite bpf_prog_active, support concurrent usage
> + * in normal, irq, and nmi context.
>   */
> -static DEFINE_PER_CPU(struct pt_regs, bpf_raw_tp_regs);
> +struct bpf_raw_tp_regs {
> +       struct pt_regs regs[3];
> +};
> +static DEFINE_PER_CPU(struct bpf_raw_tp_regs, bpf_raw_tp_regs);
> +static DEFINE_PER_CPU(int, bpf_raw_tp_nest_level);
> +static struct pt_regs *get_bpf_raw_tp_regs(void)
> +{
> +       struct bpf_raw_tp_regs *tp_regs = this_cpu_ptr(&bpf_raw_tp_regs);
> +       int nest_level = this_cpu_inc_return(bpf_raw_tp_nest_level);
> +
> +       if (WARN_ON_ONCE(nest_level > ARRAY_SIZE(tp_regs->regs))) {
> +               this_cpu_dec(bpf_raw_tp_nest_level);
> +               return ERR_PTR(-EBUSY);
> +       }
> +
> +       return &tp_regs->regs[nest_level - 1];
> +}
> +
> +static void put_bpf_raw_tp_regs(void)
> +{
> +       this_cpu_dec(bpf_raw_tp_nest_level);
> +}
> +
>  BPF_CALL_5(bpf_perf_event_output_raw_tp, struct bpf_raw_tracepoint_args *, args,
>            struct bpf_map *, map, u64, flags, void *, data, u64, size)
>  {
> -       struct pt_regs *regs = this_cpu_ptr(&bpf_raw_tp_regs);
> +       struct pt_regs *regs = get_bpf_raw_tp_regs();
> +       int ret;
> +
> +       if (IS_ERR(regs))
> +               return PTR_ERR(regs);
>
>         perf_fetch_caller_regs(regs);
> -       return ____bpf_perf_event_output(regs, map, flags, data, size);
> +       ret = ____bpf_perf_event_output(regs, map, flags, data, size);
> +
> +       put_bpf_raw_tp_regs();
> +       return ret;
>  }
>
>  static const struct bpf_func_proto bpf_perf_event_output_proto_raw_tp = {
> @@ -848,12 +901,18 @@ static const struct bpf_func_proto bpf_perf_event_output_proto_raw_tp = {
>  BPF_CALL_3(bpf_get_stackid_raw_tp, struct bpf_raw_tracepoint_args *, args,
>            struct bpf_map *, map, u64, flags)
>  {
> -       struct pt_regs *regs = this_cpu_ptr(&bpf_raw_tp_regs);
> +       struct pt_regs *regs = get_bpf_raw_tp_regs();
> +       int ret;
> +
> +       if (IS_ERR(regs))
> +               return PTR_ERR(regs);
>
>         perf_fetch_caller_regs(regs);
>         /* similar to bpf_perf_event_output_tp, but pt_regs fetched differently */
> -       return bpf_get_stackid((unsigned long) regs, (unsigned long) map,
> -                              flags, 0, 0);
> +       ret = bpf_get_stackid((unsigned long) regs, (unsigned long) map,
> +                             flags, 0, 0);
> +       put_bpf_raw_tp_regs();
> +       return ret;
>  }
>
>  static const struct bpf_func_proto bpf_get_stackid_proto_raw_tp = {
> @@ -868,11 +927,17 @@ static const struct bpf_func_proto bpf_get_stackid_proto_raw_tp = {
>  BPF_CALL_4(bpf_get_stack_raw_tp, struct bpf_raw_tracepoint_args *, args,
>            void *, buf, u32, size, u64, flags)
>  {
> -       struct pt_regs *regs = this_cpu_ptr(&bpf_raw_tp_regs);
> +       struct pt_regs *regs = get_bpf_raw_tp_regs();
> +       int ret;
> +
> +       if (IS_ERR(regs))
> +               return PTR_ERR(regs);
>
>         perf_fetch_caller_regs(regs);
> -       return bpf_get_stack((unsigned long) regs, (unsigned long) buf,
> -                            (unsigned long) size, flags, 0);
> +       ret = bpf_get_stack((unsigned long) regs, (unsigned long) buf,
> +                           (unsigned long) size, flags, 0);
> +       put_bpf_raw_tp_regs();
> +       return ret;
>  }
>
>  static const struct bpf_func_proto bpf_get_stack_proto_raw_tp = {
> --
> 2.17.1
>