linux-kernel - Re: [PATCH] jprobes: Ensure that the probepoint is at function entry

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Date:   Thu, 6 Jul 2017 19:03:23 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] jprobes: Ensure that the probepoint is at function
 entry

On Wed, 5 Jul 2017 12:42:16 +0200
Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com> wrote:
> 
> > Similar to commit 90ec5e89e393c ("kretprobes: Ensure probe location is
> > at function entry"), ensure that the jprobe probepoint is at function
> > entry.

Sorry I missed it.

> > 
> > Signed-off-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
> > ---
> >  kernel/kprobes.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> > index adfe3b4cfe05..950018609339 100644
> > --- a/kernel/kprobes.c
> > +++ b/kernel/kprobes.c
> > @@ -1776,9 +1776,14 @@ int register_jprobes(struct jprobe **jps, int num)
> >  		jp = jps[i];
> >  		addr = arch_deref_entry_point(jp->entry);
> >  
> > -		/* Verify probepoint is a function entry point */
> > +		/*
> > +		 * Verify probepoint as well as the jprobe handler are
> > +		 * function entry points.
> > +		 */
> >  		if (kallsyms_lookup_size_offset(addr, NULL, &offset) &&
> > -		    offset == 0) {
> > +				offset == 0 &&
> > +				function_offset_within_entry(jp->kp.addr,
> > +					jp->kp.symbol_name, jp->kp.offset)) {

Here, you are agressively use tab, please align the indent to 
same level of kallsyms_lookup_size_offset?

> >  			jp->kp.pre_handler = setjmp_pre_handler;
> >  			jp->kp.break_handler = longjmp_break_handler;
> >  			ret = register_kprobe(&jp->kp);
> 
> Yeah, so I agree with the fix, but the line breaks there are disgusting.
> 
> One solution would be to split out the iterator into a register_jprobe() function. 
> Also, introduce a 'kp' temporary variable to simplify and shorten usage.
> 

Agreed.

> Also, 'function_offset_within_entry' is way too long a name, and it's also a 
> minomer I think. The purpose of this function is to enforce that the relative 
> 'offset' of a new probe is at the standard function entry offset: i.e. 0 on most 
> architectures, and some ABI dependent constant on PowerPC, right?
> 
> That's not at all clear from that name, plus it's a global namespace symbol, yet 
> has no 'kprobes' prefix. So it should be named something like 
> 'kprobe_offset_valid()' or such, with an arch_kprobe_offset_valid() counterpart.

Hmm, I would rather like kprobe_within_entry(), since offset != 0 is
actually valid for normal kprobe, that is kretprobe and jprobe limitation.

Thank you,

> 
> All of these cleanups should be in separate patches - with the fix in the end.
> 
> Thanks,
> 
> 	Ingo


-- 
Masami Hiramatsu <mhiramat@...nel.org>