linux-kernel - Re: [RFC PATCH -tip 0/5] kprobes: Abolish jprobe APIs

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Date:   Tue, 10 Oct 2017 10:02:48 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Jonathan Corbet <corbet@....net>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S . Miller" <davem@...emloft.net>,
        Ian McDonald <ian.mcdonald@...di.co.nz>,
        Vlad Yasevich <vyasevich@...il.com>,
        Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [RFC PATCH -tip 0/5] kprobes: Abolish jprobe APIs

On Mon, 9 Oct 2017 09:33:50 -0600
Jonathan Corbet <corbet@....net> wrote:

> > 		Using ftrace to hook to functions
> > 		=================================
> > 
> > Copyright 2017 VMware Inc.
> >    Author:   Steven Rostedt <srostedt@...dmis.org>
> >   License:   The GNU Free Documentation License, Version 1.2
> >                (dual licensed under the GPL v2)
> > 
> > Written for: 4.14  
> 
> Experience says that strings like this go obsolete in a hurry, so they
> don't really reflect the state of the document.  The git history tends to
> be rather more reliable.

And this document is about to become stale by 4.15 ;-)


> > The ftrace FLAGS
> > ================
> > 
> > The ftrace_ops flags are all defined and documented in include/linux/ftrace.h.
> > Some of the flags are used for internal infrastructure of ftrace, but the
> > ones that users should be aware of are the following:
> > 
> > (All of these are prefixed with FTRACE_OPS_FL_)  
> 
> I hate to say it, but you'll reduce the cognitive load on the reader if you
> just spell the flags out.
> 
> > PER_CPU - When set, the callback can be enabled or disabled per cpu with the
> >       following functions:
> > 
> >       void ftrace_function_local_enable(struct ftrace_ops *ops);
> >       void ftrace_function_local_disable(struct ftrace_ops *ops);
> > 
> >       These two functions must be called with preemption disabled.  
> 
> s/per cpu/per-CPU/
> 
> More importantly, though, what does this actually mean?  Presumably that
> the callback is only called if the function is hit on an enabled CPU?  It
> seems you can only enable/disable the local CPU?  Is the default state
> enabled or disabled?  Inquiring minds want to know.

This may be removed by 4.15.

Link: http://lkml.kernel.org/r/20171010130448.4goubbka7e4bguar@hirez.programming.kicks-ass.net

-- Steve