linux-kernel - Re: [RFC] convert ftrace syscall tracer to TRACE

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Date:	Sat, 9 May 2009 16:06:29 +0200
From:	Frédéric Weisbecker <fweisbec@...il.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Ingo Molnar <mingo@...e.hu>, Jason Baron <jbaron@...hat.com>,
	Tom Zanussi <tzanussi@...il.com>, linux-kernel@...r.kernel.org,
	laijs@...fujitsu.com, rostedt@...dmis.org, peterz@...radead.org,
	jiayingz@...gle.com, mbligh@...gle.com, roland@...hat.com,
	fche@...hat.com
Subject: Re: [RFC] convert ftrace syscall tracer to TRACE_EVENT()

2009/5/9 Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>:
> * Ingo Molnar (mingo@...e.hu) wrote:
>>
>> * Frédéric Weisbecker <fweisbec@...il.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.


Yeah, for structures we would just need the copy_from_user.


> This should probably be done at the TP_fast_assign level.
>
> Note that ftrace fields do not support variable length strings, AFAIK.


It does! Look at the __string() field :)



> Mathieu
>
>> Can you see any complication?
>>
>>       Ingo
>
> --
> Mathieu Desnoyers
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
>
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