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

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Date:   Fri, 7 Jul 2017 16:56:09 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     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 2017/07/07 12:49PM, Ingo Molnar wrote:
> 
> * Masami Hiramatsu <mhiramat@...nel.org> wrote:
> 
> > On Thu, 6 Jul 2017 14:15:49 +0200
> > Ingo Molnar <mingo@...nel.org> wrote:
> > 
> > > * Masami Hiramatsu <mhiramat@...nel.org> wrote:
> > > 
> > > > > 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.
> > > 
> > > But what entry? That it's within a range or that offset is always 0 is really an 
> > > implementational detail: depending on what type of kprobe it is, it is either 
> > > validly within the confines of the specified function symbol or not.
> > 
> > Hmm, right. In most cases, it just checks the address (symbol+offset) is
> > on the function entry.
> > 
> > > What _really_ matters to callers is whether it's a valid kprobe to be inserted 
> > > into that function, right?
> > 
> > No, for that purpose, kprobes checks it in other places (kprobe_addr() and check_kprobe_address_safe()). This function is an additional safety check
> > only for kretprobe and jprobe which must be placed on the function entry.
> > (kprobe can probe function body but kretprobe and jprobes are not)
> >  
> > > I.e. the long name came from over-specifying what is done by the function - while 
> > > simplifying makes it actually more meaningful to read.
> > 
> > I see, but kprobe_offset_valid is too simple. How about kprobe_on_func_entry()?
> 
> Ok, kprobe_on_func_entry() works for me.

Ingo, Masami,
Thanks for the review/feedback. I will accommodate these changes and 
post a revised version.

- Naveen