Re: [PATCH v6 3/5] tracing/bpf-trace: Add support for faultable tracepoints

Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> · Mon, 9 Sep 2024 11:11:39 -0400

On 2024-09-04 21:21, Andrii Nakryiko wrote:
On Wed, Aug 28, 2024 at 7:42 AM Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> wrote:

In preparation for converting system call enter/exit instrumentation
into faultable tracepoints, make sure that bpf can handle registering to
such tracepoints by explicitly disabling preemption within the bpf
tracepoint probes to respect the current expectations within bpf tracing
code.

This change does not yet allow bpf to take page faults per se within its
probe, but allows its existing probes to connect to faultable
tracepoints.

Link: https://lore.kernel.org/lkml/20231002202531.3160-1-mathieu.desnoyers@xxxxxxxxxxxx/
Co-developed-by: Michael Jeanson <mjeanson@xxxxxxxxxxxx>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Signed-off-by: Michael Jeanson <mjeanson@xxxxxxxxxxxx>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Yonghong Song <yhs@xxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
Cc: bpf@xxxxxxxxxxxxxxx
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
---
Changes since v4:
- Use DEFINE_INACTIVE_GUARD.
- Add brackets to multiline 'if' statements.
Changes since v5:
- Rebased on v6.11-rc5.
- Pass the TRACEPOINT_MAY_FAULT flag directly to tracepoint_probe_register_prio_flags.
---
  include/trace/bpf_probe.h | 21 ++++++++++++++++-----
  kernel/trace/bpf_trace.c  |  2 +-
  2 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/include/trace/bpf_probe.h b/include/trace/bpf_probe.h
index a2ea11cc912e..cc96dd1e7c3d 100644
--- a/include/trace/bpf_probe.h
+++ b/include/trace/bpf_probe.h
@@ -42,16 +42,27 @@
  /* tracepoints with more than 12 arguments will hit build error */
  #define CAST_TO_U64(...) CONCATENATE(__CAST, COUNT_ARGS(__VA_ARGS__))(__VA_ARGS__)

-#define __BPF_DECLARE_TRACE(call, proto, args)                         \
+#define __BPF_DECLARE_TRACE(call, proto, args, tp_flags)               \
  static notrace void                                                    \
  __bpf_trace_##call(void *__data, proto)                                        \
  {                                                                      \
-       CONCATENATE(bpf_trace_run, COUNT_ARGS(args))(__data, CAST_TO_U64(args));        \
+       DEFINE_INACTIVE_GUARD(preempt_notrace, bpf_trace_guard);        \
+                                                                       \
+       if ((tp_flags) & TRACEPOINT_MAY_FAULT) {                        \
+               might_fault();                                          \
+               activate_guard(preempt_notrace, bpf_trace_guard)();     \
+       }                                                               \
+                                                                       \
+       CONCATENATE(bpf_trace_run, COUNT_ARGS(args))(__data, CAST_TO_U64(args)); \
  }

  #undef DECLARE_EVENT_CLASS
  #define DECLARE_EVENT_CLASS(call, proto, args, tstruct, assign, print) \
-       __BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args))
+       __BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args), 0)
+
+#undef DECLARE_EVENT_CLASS_MAY_FAULT
+#define DECLARE_EVENT_CLASS_MAY_FAULT(call, proto, args, tstruct, assign, print) \
+       __BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args), TRACEPOINT_MAY_FAULT)

  /*
   * This part is compiled out, it is only here as a build time check
@@ -105,13 +116,13 @@ static inline void bpf_test_buffer_##call(void)                           \

  #undef DECLARE_TRACE
  #define DECLARE_TRACE(call, proto, args)                               \
-       __BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args))          \
+       __BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args), 0)       \
         __DEFINE_EVENT(call, call, PARAMS(proto), PARAMS(args), 0)

  #undef DECLARE_TRACE_WRITABLE
  #define DECLARE_TRACE_WRITABLE(call, proto, args, size) \
         __CHECK_WRITABLE_BUF_SIZE(call, PARAMS(proto), PARAMS(args), size) \
-       __BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args)) \
+       __BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args), 0) \
         __DEFINE_EVENT(call, call, PARAMS(proto), PARAMS(args), size)

  #include TRACE_INCLUDE(TRACE_INCLUDE_FILE)
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index c77eb80cbd7f..ed07283d505b 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -2473,7 +2473,7 @@ int bpf_probe_register(struct bpf_raw_event_map *btp, struct bpf_raw_tp_link *li

         return tracepoint_probe_register_prio_flags(tp, (void *)btp->bpf_func,
                                                     link, TRACEPOINT_DEFAULT_PRIO,
-                                                   TRACEPOINT_MAY_EXIST);
+                                                   TRACEPOINT_MAY_EXIST | (tp->flags & TRACEPOINT_MAY_FAULT));
  }

  int bpf_probe_unregister(struct bpf_raw_event_map *btp, struct bpf_raw_tp_link *link)
--
2.39.2



I wonder if it would be better to just do this, instead of that
preempt guard. I think we don't strictly need preemption to be
disabled, we just need to stay on the same CPU, just like we do that
for many other program types.

I'm worried about introducing any kind of subtle synchronization
change in this series, and moving from preempt-off to migrate-disable
definitely falls under that umbrella.

I would recommend auditing all uses of this_cpu_*() APIs to make sure
accesses to per-cpu data structures are using atomics and not just using
operations that expect use of preempt-off to prevent concurrent threads
from updating to the per-cpu data concurrently.

So what you are suggesting may be a good idea, but I prefer to leave
this kind of change to a separate bpf-specific series, and I would
leave this work to someone who knows more about ebpf than me.

Thanks,

Mathieu


We'll need some more BPF-specific plumbing to fully support faultable
(sleepable) tracepoints, but this should unblock your work, unless I'm
missing something. And we can take it from there, once your patches
land, to take advantage of faultable tracepoints in the BPF ecosystem.

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index b69a39316c0c..415639b7c7a4 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -2302,7 +2302,8 @@ void __bpf_trace_run(struct bpf_raw_tp_link
*link, u64 *args)
         struct bpf_run_ctx *old_run_ctx;
         struct bpf_trace_run_ctx run_ctx;

-       cant_sleep();
+       migrate_disable();
+
         if (unlikely(this_cpu_inc_return(*(prog->active)) != 1)) {
                 bpf_prog_inc_misses_counter(prog);
                 goto out;
@@ -2318,6 +2319,8 @@ void __bpf_trace_run(struct bpf_raw_tp_link
*link, u64 *args)
         bpf_reset_run_ctx(old_run_ctx);
  out:
         this_cpu_dec(*(prog->active));
+
+       migrate_enable();
  }

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com








