Re: [PATCH][RFC 2/23]: SCST core

Ingo Molnar
2008-12-16T22:22:49Z

* Frédéric Weisbecker <fweisbec@gmail.com> wrote:

> 2008/12/16 Ingo Molnar <mingo@elte.hu>:
> >
> > * Frédéric Weisbecker <fweisbec@gmail.com> wrote:
> >
> >> > All the above functionality is almost what we need. The only thing
> >> > left, which I forgot to mention, is possibility to log also functions
> >> > return value on exit. This is what TRACE_EXIT_RES() in SCST does. Is
> >> > it possible to add those?
> >>
> >> I want to add that on the function graph tracer. That can be done pretty
> >> easily. The only problem comes with the type of the return value. Would
> >> this tracer be supposed to always return a 64 bits value regardless of
> >> the real typ of the value? There would be some pointless bytes on most
> >> return values. I don't know how to proceed for this problem.
> >
> > Things like mov ...,%eax are zero-extend so they'll zap the high 32 bits.
>
>
> That's right, but the problem occurs under 32 bits. The return values
> for 64 bits are in eax and edx. And most of the time, the high part
> (edx) will be junk.

for wider types i'd suggest to just print the low bits. Most of the
interesting return types fit into machine word.

> > The real problem are byte return values generated via things like:
> >
> > movb $1, %al
> >
> > those wont zero-extend, so you could get garbage in the output. One
> > approach would be to try a quick hack just to see how common a problem
> > this is.
>
>
> Yes, I will try something.
>
> > We could extract the return type from the debuginfo, hash it in a
> > read-mostly table and then look it up, but that seems complex both in
> > terms of build overhead and in terms of runtime overhead.
>
> I thought about it too and as you say it's rather complex. And thinking
> about non primitive types (like pid_t....) that would require a second
> pass of analysis to retrieve the corresponding primitive...

structure returns would be rather evil to handle, agreed.

Unrelated:

it would be really nice if we could extend ftrace to trace system calls
and their parameters and return values. We used to have that in the
latency-tracer in -rt, and it was rather useful. Controlled by a
trace_option i guess - and blendable into any of the tracer outputs
(function tracer most notably).

Explicit calls in the entry.S files would be OK for this - but maybe we
can get this via the tricky use of a TIF_ flags as well, to force the code
into the ptrace callbacks and then divert it for ftrace's pleasure?

Ingo