Re: [PATCH] kretprobes: reject registration if a symbol offset is specified

[Naveen N. Rao]

Hi Masami,

On 2017/02/14 07:32PM, Masami Hiramatsu wrote:
> On Tue, 14 Feb 2017 14:01:18 +0530
> "Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > Users shouldn't be able to specify an offset with kretprobes, as we always
> > want to probe at function entry. Otherwise, we won't be able to capture
> > the proper return address resulting in the kretprobe never firing.
> > 
> 
> Nack, this should be checked by using kallsyms, since the
> many non-exported kernel functions have same name.
> Actually perf-probe is trying to put any probes(including return
> probe) by using relative address from text-start symbol (_stext
> or _text). In this case, kretprobe also can be set by _text+OFFSET.

Interesting. In my tests, 'perf probe' always chose the function name 
for return probes on both x86 and powerpc. So, looking into it further, 
I found commit 25dd9171f51c ("perf probe: Fix probing kretprobes") which 
changed perf probe behavior due to how kprobe_events behaved. And 
kprobe_events has always dis-allowed use of offset with kprobe_events.

But, I agree -- we should allow use of offset with kretprobes.  
Otherwise, we won't be able to probe functions that have the same name.

> 
> So please rewrite this by using kallsyms_lookup_size_offset()
> which tells you the address is actually on the beginning of
> function or not.

Sure. This makes use of offsets and/or absolute addresses with 
kretprobes safe. Patches on the way...

Thanks!
- Naveen

> 
> Thank you,
> 
> > With samples/kprobes/kretprobe_example.c including an offset:
> > 	my_kretprobe.kp.offset = 40;
> > 
> > Before this patch, the probe gets planted but never fires.
> > 
> > After this patch:
> > 	$ sudo insmod samples/kprobes/kretprobe_example.ko
> > 	[sudo] password for naveen: 
> > 	insmod: ERROR: could not insert module samples/kprobes/kretprobe_example.ko: Operation not permitted
> > 
> > And dmesg:
> > 	[48253.757629] register_kretprobe failed, returned -22
> > 
> > Signed-off-by: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
> > ---
> >  kernel/kprobes.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> > index 60a702a05684..83ad7e440417 100644
> > --- a/kernel/kprobes.c
> > +++ b/kernel/kprobes.c
> > @@ -1847,6 +1847,9 @@ int register_kretprobe(struct kretprobe *rp)
> >  	int i;
> >  	void *addr;
> >  
> > +	if (rp->kp.offset)
> > +		return -EINVAL;
> > +
> >  	if (kretprobe_blacklist_size) {
> >  		addr = kprobe_addr(&rp->kp);
> >  		if (IS_ERR(addr))
> > -- 
> > 2.11.0
> > 
> 
> 
> -- 
> Masami Hiramatsu <mhiramat@xxxxxxxxxx>
>