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