Re: [PATCH 2/8] tracing/ftrace: guard syscall probe with preempt_notrace

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Mon,  9 Sep 2024 16:16:46 -0400
Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:

> In preparation for allowing system call enter/exit instrumentation to
> handle page faults, make sure that ftrace can handle this change by
> explicitly disabling preemption within the ftrace system call tracepoint
> probes to respect the current expectations within ftrace ring buffer
> code.
> 
> This change does not yet allow ftrace to take page faults per se within
> its probe, but allows its existing probes to adapt to the upcoming
> change.

OK, this looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thank you,

> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Michael Jeanson <mjeanson@efficios.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Yonghong Song <yhs@fb.com>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Cc: Namhyung Kim <namhyung@kernel.org>
> Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
> Cc: bpf@vger.kernel.org
> Cc: Joel Fernandes <joel@joelfernandes.org>
> ---
>  include/trace/trace_events.h  | 38 ++++++++++++++++++++++++++++-------
>  kernel/trace/trace_syscalls.c | 12 +++++++++++
>  2 files changed, 43 insertions(+), 7 deletions(-)
> 
> diff --git a/include/trace/trace_events.h b/include/trace/trace_events.h
> index 8bcbb9ee44de..0228d9ed94a3 100644
> --- a/include/trace/trace_events.h
> +++ b/include/trace/trace_events.h
> @@ -263,6 +263,9 @@ static struct trace_event_fields trace_event_fields_##call[] = {	\
>  	tstruct								\
>  	{} };
>  
> +#undef DECLARE_EVENT_SYSCALL_CLASS
> +#define DECLARE_EVENT_SYSCALL_CLASS DECLARE_EVENT_CLASS
> +
>  #undef DEFINE_EVENT_PRINT
>  #define DEFINE_EVENT_PRINT(template, name, proto, args, print)
>  
> @@ -396,11 +399,11 @@ static inline notrace int trace_event_get_offsets_##call(		\
>  
>  #include "stages/stage6_event_callback.h"
>  
> -#undef DECLARE_EVENT_CLASS
> -#define DECLARE_EVENT_CLASS(call, proto, args, tstruct, assign, print)	\
> -									\
> +
> +#undef __DECLARE_EVENT_CLASS
> +#define __DECLARE_EVENT_CLASS(call, proto, args, tstruct, assign, print) \
>  static notrace void							\
> -trace_event_raw_event_##call(void *__data, proto)			\
> +do_trace_event_raw_event_##call(void *__data, proto)			\
>  {									\
>  	struct trace_event_file *trace_file = __data;			\
>  	struct trace_event_data_offsets_##call __maybe_unused __data_offsets;\
> @@ -425,15 +428,34 @@ trace_event_raw_event_##call(void *__data, proto)			\
>  									\
>  	trace_event_buffer_commit(&fbuffer);				\
>  }
> +
> +#undef DECLARE_EVENT_CLASS
> +#define DECLARE_EVENT_CLASS(call, proto, args, tstruct, assign, print)	\
> +__DECLARE_EVENT_CLASS(call, PARAMS(proto), PARAMS(args), PARAMS(tstruct), \
> +		      PARAMS(assign), PARAMS(print))			\
> +static notrace void							\
> +trace_event_raw_event_##call(void *__data, proto)			\
> +{									\
> +	do_trace_event_raw_event_##call(__data, args);			\
> +}
> +
> +#undef DECLARE_EVENT_SYSCALL_CLASS
> +#define DECLARE_EVENT_SYSCALL_CLASS(call, proto, args, tstruct, assign, print) \
> +__DECLARE_EVENT_CLASS(call, PARAMS(proto), PARAMS(args), PARAMS(tstruct), \
> +		      PARAMS(assign), PARAMS(print))			\
> +static notrace void							\
> +trace_event_raw_event_##call(void *__data, proto)			\
> +{									\
> +	guard(preempt_notrace)();					\
> +	do_trace_event_raw_event_##call(__data, args);			\
> +}
> +
>  /*
>   * The ftrace_test_probe is compiled out, it is only here as a build time check
>   * to make sure that if the tracepoint handling changes, the ftrace probe will
>   * fail to compile unless it too is updated.
>   */
>  
> -#undef DECLARE_EVENT_SYSCALL_CLASS
> -#define DECLARE_EVENT_SYSCALL_CLASS DECLARE_EVENT_CLASS
> -
>  #undef DEFINE_EVENT
>  #define DEFINE_EVENT(template, call, proto, args)			\
>  static inline void ftrace_test_probe_##call(void)			\
> @@ -443,6 +465,8 @@ static inline void ftrace_test_probe_##call(void)			\
>  
>  #include TRACE_INCLUDE(TRACE_INCLUDE_FILE)
>  
> +#undef __DECLARE_EVENT_CLASS
> +
>  #include "stages/stage7_class_define.h"
>  
>  #undef DECLARE_EVENT_CLASS
> diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
> index 067f8e2b930f..abf0e0b7cd0b 100644
> --- a/kernel/trace/trace_syscalls.c
> +++ b/kernel/trace/trace_syscalls.c
> @@ -299,6 +299,12 @@ static void ftrace_syscall_enter(void *data, struct pt_regs *regs, long id)
>  	int syscall_nr;
>  	int size;
>  
> +	/*
> +	 * Syscall probe called with preemption enabled, but the ring
> +	 * buffer and per-cpu data require preemption to be disabled.
> +	 */
> +	guard(preempt_notrace)();
> +
>  	syscall_nr = trace_get_syscall_nr(current, regs);
>  	if (syscall_nr < 0 || syscall_nr >= NR_syscalls)
>  		return;
> @@ -338,6 +344,12 @@ static void ftrace_syscall_exit(void *data, struct pt_regs *regs, long ret)
>  	struct trace_event_buffer fbuffer;
>  	int syscall_nr;
>  
> +	/*
> +	 * Syscall probe called with preemption enabled, but the ring
> +	 * buffer and per-cpu data require preemption to be disabled.
> +	 */
> +	guard(preempt_notrace)();
> +
>  	syscall_nr = trace_get_syscall_nr(current, regs);
>  	if (syscall_nr < 0 || syscall_nr >= NR_syscalls)
>  		return;
> -- 
> 2.39.2
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>