Re: [3/3 PATCH] Kprobes: User space probes support- single stepping out-of-line

Andrew Morton
2006-03-20T20:44:35Z

Prasanna S Panchamukhi <prasanna@in.ibm.com> wrote:
>
> > > +
> > > + if (__copy_to_user_inatomic((unsigned long *)addr,
> > > + (unsigned long *)uprobe->kp.ainsn.insn, size))
> > > + return -EFAULT;
> > > +
> > > + regs->eip = addr;
> > > +
> > > + return 0;
> > > +}
> >
> > If we're going to use __copy_to_user_inatomic() then we'll need some nice
> > comments explaining why this is happening.
> >
> > And we'll need to actually *be* in-atomic. That means we need an
> > open-coded inc_preempt_count() and dec_preempt_count() in there and I don't
> > see them.
> >
>
> We come here, after probe is hit, through uporbe_handler() with
> interrupts disabled (since it is a interrupt gate). In uprobe_handler()
> preemption is disabled and remains disabled until original instruction
> is single stepped.
>
> I will add proper comments in next iteration.

preempt_disable() is insufficient - it is a no-op on !CONFIG_PREEMPT.

You _must_ run inc_preempt_count(). See how kmap_atomic() and
kunmap_atomic() work.

> > > + */
> > > +void __kprobes replace_original_insn(struct uprobe *uprobe,
> > > + struct pt_regs *regs, kprobe_opcode_t opcode)
> > > +{
> > > + kprobe_opcode_t *addr;
> > > + struct page *page;
> > > +
> > > + page = find_get_page(uprobe->inode->i_mapping,
> > > + uprobe->offset >> PAGE_CACHE_SHIFT);
> > > + BUG_ON(!page);
> > > +
> > > + __lock_page(page);
> >
> > Whoa. Why is __lock_page() being used here? It looks like a bug is being
> > covered up.
> >
>
> we come here with a spinlock held. I will add the comment.

Then the code is buggy. __lock_page() can schedule away, causing this CPU
to recur onto the same lock and deadlock.

> > > + addr = (kprobe_opcode_t *)kmap_atomic(page, KM_USER1);
> > > + addr = (kprobe_opcode_t *)((unsigned long)addr +
> > > + (unsigned long)(uprobe->offset & ~PAGE_MASK));
> > > + *addr = opcode;
> > > + /*TODO: flush vma ? */
> >
> > flush_dcache_page() would be needed.
> >
> > But then, what happens if the page is shared by other processes? Do they
> > all start taking debug traps?
>
> Yes, you are right. I think single stepping inline was a bad idea, disarming
> the probe looks to be a better option
>

You skipped my second question?