linux-kernel - Re: [BUGFIX PATCH] kprobes/x86: Remove IRQ disabling from jprobe handlers

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Message-Id: <20171005172836.d084ef709be2e34037ccc0d6@kernel.org>
Date:   Thu, 5 Oct 2017 17:28:36 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     mingo@...hat.com, x86@...nel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Alexei Starovoitov <ast@...com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [BUGFIX PATCH] kprobes/x86: Remove IRQ disabling from jprobe
 handlers

On Thu, 5 Oct 2017 09:57:04 +0200
Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Masami Hiramatsu <mhiramat@...nel.org> wrote:
> 
> > On Wed, 4 Oct 2017 12:41:01 +0200
> > Ingo Molnar <mingo@...nel.org> wrote:
> > 
> > > 
> > > * Masami Hiramatsu <mhiramat@...nel.org> wrote:
> > > 
> > > > Hmm, actually we can not disable jprobe, that has no separate Kconfig.
> > > > So we need to introduce new kconfig for that.
> > > > 
> > > > And, there are several network protocols using jprobe to trace events.
> > > > (e.g. NET_DCCPPROBE and NET_TCPPROBE)
> > > > I think they need to migrate to trace-event at first.
> > > > 
> > > > So, how about below idea?
> > > > 
> > > > 1. Introduce CONFIG_JPROBE_API which only separate jprobe general parts
> > > >      (no arch dependent code involves) and make it default n.
> > > > 2. Mark break_handler and jprobe APIs deprecated so that no new user comes up.
> > > > 3. migrate in-kernel jprobe user to trace-event or ftrace.
> > > >    (may take some time)
> > > 
> > > So my suggestion would be to just return from register_jprobe() and don't register 
> > > anything.
> > 
> > with CONFIG_JPROBE_API=n, is that right?
> 
> No, unconditionally off with a WARN_ON_ONCE() warning in the registry function and 
> the deactivation of all in-kernel uses (such as self-tests).
> 
> The point is to make people that _truly_ rely on it complain - not just make them 
> silently turn on a Kconfig option ...

Hmm, but in that case, anyway we have to remove or rename the function
like register_jprobe. Or would that "unconditionally" mean "#if 0"?

> > > Yes, there are usecases of jprobes in the kernel, but they all look 
> > > pretty ancient and unused.
> > 
> > Hmm, in that case, should we also remove those users? If we disable such way
> > those features are just useless.
> 
> My hypothesis is that those features are not used (hence useless), but we should 
> first test whether there's any reliance before we remove code.

Agreed.

Thank you,

> 
> Thanks,
> 
> 	Ingo


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