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

[Mathieu Desnoyers]

On 2024-10-09 01:23, Steven Rostedt wrote:
> On Fri,  4 Oct 2024 10:58:15 -0400
> Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> > 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/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.

I plan to add this comment on top of __TO_TRACE:

+ *
+ * With @syscall=0, the tracepoint callback array dereference is
+ * protected by disabling preemption.
+ * With @syscall=1, the tracepoint callback array dereference is
+ * protected by Tasks Trace RCU, which allows probes to handle page
+ * faults.

Thanks,

Mathieu


> -- 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"
> >   

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