Re: [PATCH v3 5/8] tracing: Allow system call tracepoints to handle page faults

[Steven Rostedt <rostedt@goodmis.org>]

On Fri,  4 Oct 2024 10:58:15 -0400
Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:

> Use Tasks Trace RCU to protect iteration of system call enter/exit
> tracepoint probes to allow those probes to handle page faults.
> 
> In preparation for this change, all tracers registering to system call
> enter/exit tracepoints should expect those to be called with preemption
> enabled.
> 
> This allows tracers to fault-in userspace system call arguments such as
> path strings within their probe callbacks.
> 
> 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/linux/tracepoint.h | 12 ++++++++++--
>  init/Kconfig               |  1 +
>  2 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> index 014790495ad8..cefd44b7c91f 100644
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -17,6 +17,7 @@
>  #include <linux/errno.h>
>  #include <linux/types.h>
>  #include <linux/rcupdate.h>
> +#include <linux/rcupdate_trace.h>
>  #include <linux/tracepoint-defs.h>
>  #include <linux/static_call.h>
>  
> @@ -107,6 +108,7 @@ void for_each_tracepoint_in_module(struct module *mod,
>  #ifdef CONFIG_TRACEPOINTS
>  static inline void tracepoint_synchronize_unregister(void)
>  {
> +	synchronize_rcu_tasks_trace();
>  	synchronize_rcu();
>  }
>  #else
> @@ -204,11 +206,17 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>  		if (!(cond))						\
>  			return;						\
>  									\
> -		preempt_disable_notrace();				\

Should add a comment somewhere stating that the syscall version is to allow faults.

-- Steve

> +		if (syscall)						\
> +			rcu_read_lock_trace();				\
> +		else							\
> +			preempt_disable_notrace();			\
>  									\
>  		__DO_TRACE_CALL(name, TP_ARGS(args));			\
>  									\
> -		preempt_enable_notrace();				\
> +		if (syscall)						\
> +			rcu_read_unlock_trace();			\
> +		else							\
> +			preempt_enable_notrace();			\
>  	} while (0)
>  
>  /*
> diff --git a/init/Kconfig b/init/Kconfig
> index fbd0cb06a50a..eedd0064fb36 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1984,6 +1984,7 @@ config BINDGEN_VERSION_TEXT
>  #
>  config TRACEPOINTS
>  	bool
> +	select TASKS_TRACE_RCU
>  
>  source "kernel/Kconfig.kexec"
>