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

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* 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
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