Re: [patch] kprobes: optimize branch placement

Ananth N Mavinakayanahalli
2006-09-18T13:52:30Z

On Sun, Sep 17, 2006 at 01:30:38AM +0200, Ingo Molnar wrote:
>
> * Andrew Morton <akpm@osdl.org> wrote:
>
> > On Sat, 16 Sep 2006 22:43:42 +0200
> > Ingo Molnar <mingo@elte.hu> wrote:
> >
> > > --- linux.orig/arch/i386/kernel/kprobes.c
> > > +++ linux/arch/i386/kernel/kprobes.c
> > > @@ -354,9 +354,8 @@ no_kprobe:
> > > */
> > > fastcall void *__kprobes trampoline_handler(struct pt_regs *regs)
> > > {
> > > - struct kretprobe_instance *ri = NULL;
> > > - struct hlist_head *head;
> > > - struct hlist_node *node, *tmp;
> > > + struct kretprobe_instance *ri = NULL, *tmp;
> > > + struct list_head *head;
> > > unsigned long flags, orig_ret_address = 0;
> > > unsigned long trampoline_address =(unsigned long)&kretprobe_trampoline;
> >
> > Wanna fix the whitespace wreckage while you're there??
>
> will do. If you consider this for -mm then there's some djprobes noise
> in the patch [djprobes isnt upstream yet] - it's not completely
> sanitized yet. (but it should actually work if applied to upstream -
> kprobes and djprobes are disjunct.) Also, i havent tested with
> CONFIG_KPROBES turned off, etc. I'll do a clean queue.

Also, the hlist->list changes need to be taken care of for the other
archs too.

> > i386's kprobe_handler() appears to forget to reenable preemption in
> > the if (p->pre_handler && p->pre_handler(p, regs)) case?
>
> that portion seems a bit tricky - i think what happens is that the
> pre_handler() sets stuff up for single-stepping, and then we do this
> recursive single-stepping (during which preemption remains disabled),
> and _then_ do we re-enable preemption.

Well, that is the jprobes and return probes case. In the case of normal
kprobes, p->pre_handler() should always return 0.

In the case of a jprobe, the setjmp_pre_handler() resets the instruction
pointer to the instrumented routine (same signature as the routine being
jprobed), which later does a jprobe_return(), a placeholder for the
arch-specific trap instruction. We re-enter the kprobe_handler here and
then re-enable preemption via the longjmp_break_handler. As for the
return probe case, since the underlying instruction originally was a nop
(kretprobe_trampoline), we don't need to single-step.

Yes, its a bit convoluted, but we are currently covered for all cases.

Ananth