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SubjectRe: [PATCH bpf v2] bpf: fix nested bpf tracepoints with per-cpu data
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On Fri, 2019-06-14 at 00:47 +0200, Daniel Borkmann wrote:
> On 06/12/2019 07:00 AM, Andrii Nakryiko wrote:
> > On Tue, Jun 11, 2019 at 8:48 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
> > > (at most one of which may be a kprobe or perf event).
> > >
> > > Fixes: 20b9d7ac4852 ("bpf: avoid excessive stack usage for perf_sample_data")
>
> Generally, looks good to me. Two things below:
>
> Nit, for stable, shouldn't fixes tag be c4f6699dfcb8 ("bpf: introduce BPF_RAW_TRACEPOINT")
> instead of the one you currently have?

Ah, yeah, that's probably more reasonable; I haven't managed to come up
with a scenario where one could hit this without raw tracepoints. I'll
fix up the nits that've accumulated since v2.

> One more question / clarification: we have __bpf_trace_run() vs trace_call_bpf().
>
> Only raw tracepoints can be nested since the rest has the bpf_prog_active per-CPU
> counter via trace_call_bpf() and would bail out otherwise, iiuc. And raw ones use
> the __bpf_trace_run() added in c4f6699dfcb8 ("bpf: introduce BPF_RAW_TRACEPOINT").
>
> 1) I tried to recall and find a rationale for mentioned trace_call_bpf() split in
> the c4f6699dfcb8 log, but couldn't find any. Is the raison d'être purely because of
> performance overhead (and desire to not miss events as a result of nesting)? (This
> also means we're not protected by bpf_prog_active in all the map ops, of course.)
> 2) Wouldn't this also mean that we only need to fix the raw tp programs via
> get_bpf_raw_tp_regs() / put_bpf_raw_tp_regs() and won't need this duplication for
> the rest which relies upon trace_call_bpf()? I'm probably missing something, but
> given they have separate pt_regs there, how could they be affected then?

For the pt_regs, you're correct: I only used get/put_raw_tp_regs for
the _raw_tp variants. However, consider the following nesting:

trace_nest_level raw_tp_nest_level
(kprobe) bpf_perf_event_output 1 0
(raw_tp) bpf_perf_event_output_raw_tp 2 1
(raw_tp) bpf_get_stackid_raw_tp 2 2

I need to increment a nest level (and ideally increment it only once)
between the kprobe and the first raw_tp, because they would otherwise
share the struct perf_sample_data. But I also need to increment a nest
level between the two raw_tps, since they share the pt_regs -- I can't
use trace_nest_level for everything because it's not used by
get_stackid, and I can't use raw_tp_nest_level for everything because
it's not incremented by kprobes.

If raw tracepoints were to bump bpf_prog_active, then I could get away
with just using that count in these callsites -- I'm reluctant to do
that, though, since it would prevent kprobes from ever running inside a
raw_tp. I'd like to retain the ability to (e.g.)
trace.py -K htab_map_update_elem
and get some stack traces from at least within raw tracepoints.

That said, as I wrote up this example, bpf_trace_nest_level seems to be
wildly misnamed; I should name those after the structure they're
protecting...

> Thanks,
> Daniel
>
> > > Signed-off-by: Matt Mullins <mmullins@fb.com>
> > > ---
> >
> > LGTM, minor nit below.
> >
> > Acked-by: Andrii Nakryiko <andriin@fb.com>
> >
> > > v1->v2:
> > > * reverse-Christmas-tree-ize the declarations in bpf_perf_event_output
> > > * instantiate err more readably
> > >
> > > I've done additional testing with the original workload that hit the
> > > irq+raw-tp reentrancy problem, and as far as I can tell, it's still
> > > solved with this solution (as opposed to my earlier per-map-element
> > > version).
> > >
> > > kernel/trace/bpf_trace.c | 100 ++++++++++++++++++++++++++++++++-------
> > > 1 file changed, 84 insertions(+), 16 deletions(-)
> > >
> > > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > > index f92d6ad5e080..1c9a4745e596 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 +440,50 @@ __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);
> > > + int nest_level = this_cpu_inc_return(bpf_trace_nest_level);
> > > struct perf_raw_record raw = {
> > > .frag = {
> > > .size = size,
> > > .data = data,
> > > },
> > > };
> > > + struct perf_sample_data *sd;
> > > + int err;
> > >
> > > - if (unlikely(flags & ~(BPF_F_INDEX_MASK)))
> > > - return -EINVAL;
> > > + if (WARN_ON_ONCE(nest_level > ARRAY_SIZE(sds->sds))) {
> > > + err = -EBUSY;
> > > + goto out;
> > > + }
> > > +
> > > + sd = &sds->sds[nest_level - 1];
> > > +
> > > + if (unlikely(flags & ~(BPF_F_INDEX_MASK))) {
> > > + err = -EINVAL;
> > > + goto out;
> > > + }
> >
> > Feel free to ignore, but just stylistically, given this check doesn't
> > depend on sd, I'd move it either to the very top or right after
> > `nest_level > ARRAY_SIZE(sds->sds)` check, so that all the error
> > checking is grouped without interspersed assignment.
>
> Makes sense.
>
> > > 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 +846,48 @@ 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 +904,18 @@ 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 +930,17 @@ 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
> > >
>
>
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