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Re: thoughts about exception-handling requirements for kprobes
On Mon, Mar 20, 2006 at 10:39:51AM -0800, Keshavamurthy Anil S wrote:
> On Sun, Mar 19, 2006 at 09:24:54AM -0800, Prasanna S Panchamukhi wrote:
> >
> > On Fri, Mar 17, 2006 at 01:50:57PM -0800, Keshavamurthy Anil S wrote:
> > > On Thu, Mar 09, 2006 at 07:57:18AM -0800, Richard J Moore wrote:
> > > >
> > > > I've been thinking about the need for exception-handling and
> > how the
> > > > current implementation has become a little muddled.
> > >
> > > Here is my thinking on this kprobe fault handling...
> > > Ideally we want the ability to recover from all
> > > the page faults happening from either pre-handler
> > > or happening from post-handler transparently in the
> > > same way as the normal kernel would recover from
> > > do_page_fault() function. In order for this to happen,
> > > I think we should not be calling pre-handler/post-handler
> > > by disabling preempt which is a major design change.
> > > Also in the current code if fixup_exception() fails to
> > > fixup the exception then falling back on the normal
> > > do_page_fault() is a bad thing with preempt disabled.
> > >
> > > I was thinking on this issue for the past several days
> > > and I believe that currently we are disabling preempt
> > > before calling pre/post handler, because we don;t
> > > want the process to get migrated to different CPU
> > > and we don't want another process to be scheduled
> > > while we are servicing kprobe as the newly scheduled
> > > process might trigger another probe and we don;t
> > > have space to save the kprobe control block(kprobe_ctlbk)
> > > info, because we save kprobe_ctlbk in the per cpu structure.
> > >
> > > If we move this saving kprobe_ctlbk to task struct then
> > > I think we will have the ability to call pre/post-handler
> > > without having to disable preempt and their by any faults
> > > happening from either pre/post handler can recover transparently
> > > in the same way as the normal kernel would recover.
> > >
> >
> > Kprobes user-specified pre/post handler are called within
> > the interrupt context and if we allow page faults while within
> > user-specified pre/post handler, then it might sleep.
> > Is is ok to sleep while within the interrupt handler?
> Prasanna,
> I am not getting what you are asking here, if you are
> asking is it okay to sleep while within the interrupt handler,
> then it is BIG NO.
Anil,
>
> What I am saying is that we should look into kprobes to see
> if we can support calling users pre/post handlers
> without having to disable preempt.
>
> Currenlty we are calling users pre_handler() and post_handler()
> with preempt disabled. If the user has put a probes on
> syscalls, then when his pre/post handlers are called he is
> bound to call copy_from_user(), which has a check might_sleep().
> The might_sleep() calls in_atomic() function which checks preempt_count()
> and if preempt_count() is greater than zero( in our case it indeed greater
> than zero, since we are calling pre/post handlers with preempt disabled)
> the kernel prints a error message
> printk(KERN_ERR "Debug: sleeping function called from invalid"
> " context at %s:%d\n", file, line);
Are you trying to tell here that by allowing preemption() in the
kprobes handler, the above debug message log can be avoided?
>
> Also if we want to fallback on do_page_fault() function in kprobe_fault_handler() to
> recover the page, then we should not be in preempt_disabled() state.
We actually do not want to fall back on system do_page_fault() because,
it might sleep. When pre/post handler page faults, we can just try
calling fixup_exception() (non-ia64 architectures) and try to avoid actual
do_page_fault() to be called because it might sleep().
Thanks
Prasanna
--
Prasanna S Panchamukhi
Linux Technology Center
India Software Labs, IBM Bangalore
Email: prasanna@in.ibm.com
Ph: 91-80-51776329