Re: [PATCH v1 4/8] tracing/bpf: guard syscall probe with preempt_notrace

[Steven Rostedt]

On Thu,  3 Oct 2024 11:16:34 -0400
Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:

> In preparation for allowing system call enter/exit instrumentation to
> handle page faults, make sure that bpf can handle this change by
> explicitly disabling preemption within the bpf system call 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 adapt to the upcoming change.
> 

I guess the BPF folks should state if this is needed or not?

Does the BPF hooks into the tracepoints expect preemption to be disabled
when called?

-- Steve


> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> Tested-by: Andrii Nakryiko <andrii@xxxxxxxxxx> # BPF parts
> Cc: Michael Jeanson <mjeanson@xxxxxxxxxxxx>
> 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: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
> Cc: bpf@xxxxxxxxxxxxxxx
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> ---
>  include/trace/bpf_probe.h | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/include/trace/bpf_probe.h b/include/trace/bpf_probe.h
> index c85bbce5aaa5..211b98d45fc6 100644
> --- a/include/trace/bpf_probe.h
> +++ b/include/trace/bpf_probe.h
> @@ -53,8 +53,17 @@ __bpf_trace_##call(void *__data, proto)					\
>  #define DECLARE_EVENT_CLASS(call, proto, args, tstruct, assign, print)	\
>  	__BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args))
>  
> +#define __BPF_DECLARE_TRACE_SYSCALL(call, proto, args)			\
> +static notrace void							\
> +__bpf_trace_##call(void *__data, proto)					\
> +{									\
> +	guard(preempt_notrace)();					\
> +	CONCATENATE(bpf_trace_run, COUNT_ARGS(args))(__data, CAST_TO_U64(args));	\
> +}
> +
>  #undef DECLARE_EVENT_SYSCALL_CLASS
> -#define DECLARE_EVENT_SYSCALL_CLASS DECLARE_EVENT_CLASS
> +#define DECLARE_EVENT_SYSCALL_CLASS(call, proto, args, tstruct, assign, print)	\
> +	__BPF_DECLARE_TRACE_SYSCALL(call, PARAMS(proto), PARAMS(args))
>  
>  /*
>   * This part is compiled out, it is only here as a build time check