Re: [PATCH 18/18] tracing/kprobes: Dump the culprit kprobe in caseof kprobe recursion

[Masami Hiramatsu]

Frederic Weisbecker wrote:
> On Thu, Aug 27, 2009 at 11:30:24AM -0400, Masami Hiramatsu wrote:
> > Hi Frederic,
> >
> > Frederic Weisbecker wrote:
> > > Kprobes can enter into a probing recursion, ie: a kprobe that does an
> > > endless loop because one of its core mechanism function used during
> > > probing is also probed itself.
> > >
> > > This patch helps pinpointing the kprobe that raised such recursion
> > > by dumping it and raising a BUG instead of a warning (we also disarm
> > > the kprobe to try avoiding recursion in BUG itself). Having a BUG
> > > instead of a warning stops the stacktrace in the right place and
> > > doesn't pollute the logs with hundreds of traces that eventually end
> > > up in a stack overflow.
> >
> > Thanks, but I also found similar bug cases.
> >
> > > Signed-off-by: Frederic Weisbecker<fweisbec@xxxxxxxxx>
> > > Cc: Masami Hiramatsu<mhiramat@xxxxxxxxxx>
> > > Cc: Ananth N Mavinakayanahalli<ananth@xxxxxxxxxx>
> > > ---
> > >    arch/x86/kernel/kprobes.c |    8 ++++++--
> > >    include/linux/kprobes.h   |    2 ++
> > >    kernel/kprobes.c          |    7 +++++++
> > >    3 files changed, 15 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/x86/kernel/kprobes.c b/arch/x86/kernel/kprobes.c
> > > index 16ae961..ecee3d2 100644
> > > --- a/arch/x86/kernel/kprobes.c
> > > +++ b/arch/x86/kernel/kprobes.c
> > > @@ -490,9 +490,13 @@ static int __kprobes reenter_kprobe(struct kprobe *p, struct pt_regs *regs,
> >
> > Before this, kprobes checks p != kprobe_running(), but it's a
> > meaningless branch. Hitting a kprobe while KPROBES_HIT_SS always
> > treated as unrecoverable.
>
>
>
> Yeah, but that's the place where a probe ends up when bad reentrancy happens
> right?

No, a place which is shared by kprobes and other subsystems, will cause a
problem.

for example, I found an irq_return case which will be p == kprobe_running()
on x86-64.

-> <some irq occurs>
 -> irq_return
   -> <hit int3>
     -> do_int3
     -> <handling kprobe (set kprobe_running)>
   -> irq_return (from do_int3)
     -> <hit int3>
       -> do_int3
       <handling kprobe (kprobe_running == p)> <- here!


Perhaps, the original code assumes that it will be caused by an int3
which another subsystem inserted on out-of-line singlestep buffer
if the hitting probe is same as current probe.

However, in that case, int3 hitting address is on the out-of-line
buffer and should be different from first (current) int3 address.

So, I think this part should also be removed.

              if (p == kprobe_running()) {
                      regs->flags &= ~X86_EFLAGS_TF;
                      regs->flags |= kcb->kprobe_saved_flags;
                      return 0;
              } else {

Thank you,

> > >    			/* A probe has been hit in the codepath leading up
> > >    			 * to, or just after, single-stepping of a probed
> > >    			 * instruction. This entire codepath should strictly
> > > -			 * reside in .kprobes.text section. Raise a warning
> > > -			 * to highlight this peculiar case.
> > > +			 * reside in .kprobes.text section.
> > > +			 * Raise a BUG or we'll continue in an endless
> > > +			 * reentering loop and eventually a stack overflow.
> > >    			 */
> > > +			arch_disarm_kprobe(p);
> > > +			dump_kprobe(p);
> > > +			BUG();
> > >    		}
> > >    	default:
> > >    		/* impossible cases */
> > > diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
> > > index bcd9c07..87eb79c 100644
> > > --- a/include/linux/kprobes.h
> > > +++ b/include/linux/kprobes.h
> > > @@ -296,6 +296,8 @@ void recycle_rp_inst(struct kretprobe_instance *ri, struct hlist_head *head);
> > >    int disable_kprobe(struct kprobe *kp);
> > >    int enable_kprobe(struct kprobe *kp);
> > >
> > > +void dump_kprobe(struct kprobe *kp);
> > > +
> > >    #else /* !CONFIG_KPROBES: */
> > >
> > >    static inline int kprobes_built_in(void)
> > > diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> > > index ef177d6..f72e96c 100644
> > > --- a/kernel/kprobes.c
> > > +++ b/kernel/kprobes.c
> > > @@ -1141,6 +1141,13 @@ static void __kprobes kill_kprobe(struct kprobe *p)
> > >    	arch_remove_kprobe(p);
> > >    }
> > >
> > > +void __kprobes dump_kprobe(struct kprobe *kp)
> > > +{
> > > +	printk(KERN_WARNING "Dumping kprobe:\n");
> > > +	printk(KERN_WARNING "Name: %s\nAddress: %p\nOffset: %x\n",
> > > +	       kp->symbol_name, kp->addr, kp->offset);
> > > +}
> >
> > Since kp->symbol_name + kp->offset = kp->addr, I recommend to show it
> > as "Kprobe at %s+%x:<%p>\n", kp->symbol_name, kp->offset, kp->addr.
>
>
> Ok I'll fix this, thanks.
>
>
> > > +
> > >    /* Module notifier call back, checking kprobes on the module */
> > >    static int __kprobes kprobes_module_callback(struct notifier_block *nb,
> > >    					     unsigned long val, void *data)
> >
> > Thank you,
--
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@xxxxxxxxxx

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