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

From: Peter Zijlstra
Date: Wed Aug 26 2009 - 02:49:28 EST

On Tue, 2009-08-25 at 21:42 +0200, Frederic Weisbecker wrote:
> On Tue, Aug 25, 2009 at 02:31:19PM -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 ?
> I really can't see the point in doing this. I don't expect the kernel
> behaviour to change soon and have explicit syscalls interrupts done
> from it. It's not about a current kernel implementation fashion,
> it's about kernel design sanity that is not likely to go backward.
> Is it worth it to trace kernel threads, maintain their tracing
> specificities (such as workarounds with ret_from_fork that implies)
> just because we want to support tracing on some silly proprietary drivers?
> >
> > 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.
> If developers of out of tree drivers want to implement buggy things
> that would never be accepted after a minimal review here, and then instrument
> their bugs, then I would suggest them to implement their own ad hoc instrumentation,
> really :-/
> What's the point in supporting out of tree bugs?
> Well, the only advantage of doing this would be to support reverse engineering
> in tiny and rare corner cases. Not that worth the effort.
> > 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.
> >
> > Mathieu
> It means we have to support and check this corner case in every archs
> that support syscall tracing, deal with crashes because we omitted it, etc...
> For all the things I've explained above I don't think it's worth the effort.
> But it's just my opinion...

I fully agree, let out of tree people deal with their own crap.
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