Re: [PATCH 08/12] add trace events for each syscall entry/exit

From: Mathieu Desnoyers
Date: Wed Aug 26 2009 - 13:08:21 EST

* Peter Zijlstra (peterz@xxxxxxxxxxxxx) wrote:
> On Tue, 2009-08-25 at 14:31 -0400, Mathieu Desnoyers wrote:
> > (Well, I do not have time currently to look into the gory details
> > (sorry), but let's try to take a step back from the problem.)
> >
> > The design proposal for this kthread behavior wrt syscalls is based on a
> > very specific and current kernel behavior, that may happen to change and
> > that I have actually seen proven incorrect. For instance, some
> > proprietary Linux driver does very odd things with system calls within
> > kernel threads, like invoking them with int 0x80.
> >
> > Yes, this is odd, but do we really want to tie the tracer that much to
> > the actual OS implementation specificities ?
> >
> > That sounds like a recipe for endless breakages and missing bits of
> > instrumentation.
> >
> > So my advice would be: if we want to trace the syscall entry/exit paths,
> > let's trace them for the _whole_ system, and find ways to make it work
> > for corner-cases rather than finding clever ways to diminish
> > instrumentation coverage.
> >
> > Given the ret from fork example happens to be the first event fired
> > after the thread is created, we should be able to deal with this problem
> > by initializing the thread structure used by syscall exit tracing to an
> > initial "ret from fork" value.
> So you're saying we should let proprietary crap influence the design of
> the kernel in any way?

Nah. And I start to feel comfortable with syscall entry/exit being only
be traced for userspace threads. But as I pointed out in a follow-up
email, the lack of sys_*() tracing for invocation from within the kernel
might be problematic. This is actually my main point.


Mathieu Desnoyers
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