Re: [RFC] convert ftrace syscall tracer to TRACE_EVENT()

Mathieu Desnoyers
2009-05-09T13:50:57Z

* Ingo Molnar (mingo@elte.hu) wrote:
>
> * Frédéric Weisbecker <fweisbec@gmail.com> wrote:
>
> > > Secondly, we should reuse the information we get in
> > > SYSCALL_DEFINE, to construct the TRACE_EVENT tracepoints
> > > directly - without having to list all syscalls again in a
> > > separate file.
> >
> > Indeed, that's not trivial though, but feasible. I'm not sure we
> > can reuse the TRACE_EVENT macro directly inside SYSCALL_DEFINE.
> > The resulting macro tempest effect that would occur confuses me
> > and I have troubles to imagine the result.
>
> Lets take an example. This syscall:
>
> SYSCALL_DEFINE3(sched_setscheduler, pid_t, pid, int, policy,
> struct sched_param __user *, param)
>
> Is equivalent to:
>
> SYSCALL_DEFINE3(name, t1, v1, t2, v2, t3, v3)
>
> ('t' for type, 'v' for variable/value).
>
> This would transform into the following TRACE_EVENT() construct:
>
> TRACE_EVENT_SYSCALL2():
>
> TRACE_EVENT(sys_##name,
> TP_PROTO(t1 v1, t2 v2),
> TP_ARGS(v1, v2),
> TP_STRUCT__entry(
> __field(t1, v1)
> __field(t2, v2)
> ),
> TP_fast_assign(
> __entry->v1 = v1;
> __entry->v2 = v2;
> ),
> TP_printk("%016Lx %016Lx", (u64)__entry->v1, (u64)__entry->v2)
> );
>
> We need TRACE_EVENT_SYSCALL[123456] definitions, and that's it.
>
> The only place where we lose type information is the printk format -
> but that's not a big issue, as i'd expect the event record to be the
> main user of this.
>
> [ In addition to this, we could extend DEFINE_SYSCALL[1..6] with a
> (optional) format string definition field, and fill that in for
> anything that matters. ]
>
> Note, this assumes that all syscall types can be described via
> __field() - i think that's correct. (we dont want to deref strings
> as they are untrusted, and there are no arrays in syscall
> parameters)
>

I would expect to use copy_string_from_user (for strings)
and copy_from_user for structures, because without any strings
(especially), the trace information become much less useful.

This should probably be done at the TP_fast_assign level.

Note that ftrace fields do not support variable length strings, AFAIK.

Mathieu

> Can you see any complication?
>
> Ingo

--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68