Re: thoughts about exception-handling requirements for kprobes

[Keshavamurthy Anil S <anil dot s dot keshavamurthy at intel dot com>]

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.

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);

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.

Let me know if you more questions.

-Anil