linux-kernel - Re: [PATCH 3/3] kprobes/x86: Simplify indirect-jump check in retpoline

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Date:   Wed, 31 Oct 2018 15:00:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Zhenzhong Duan <zhenzhong.duan@...cle.com>
Cc:     Linux-Kernel <linux-kernel@...r.kernel.org>, mingo@...hat.com,
        konrad.wilk@...cle.com, dwmw@...zon.co.uk, tglx@...utronix.de,
        Srinivas REDDY Eeda <srinivas.eeda@...cle.com>, bp@...e.de,
        hpa@...or.com
Subject: Re: [PATCH 3/3] kprobes/x86: Simplify indirect-jump check in
 retpoline

On Wed, Oct 31, 2018 at 02:53:20PM +0100, Peter Zijlstra wrote:
> On Wed, Oct 31, 2018 at 02:01:20PM +0800, Zhenzhong Duan wrote:
> > On 2018/10/30 16:36, Peter Zijlstra wrote:
> > > On Mon, Oct 29, 2018 at 11:55:06PM -0700, Zhenzhong Duan wrote:
> > > > Since CONFIG_RETPOLINE hard depends on compiler support now, so
> > > > replacing indirect-jump check with the range check is safe in that case.
> > > 
> > > Can we put kprobes on module init text before we run alternatives on it?
> > 
> > Forgive me I doesn't understand your question. Do you mean this patch impact
> > kprobes on module init text?
> 
> In that case we would still see the indirect paravirt calls for example,
> and we'd still need that cascade you took out.
> 
> Now, I'm not at all sure we're able to use kprobes at those times, so it
> might be a non-issue.

Hmm, what about the case where we have RETPOLINE runtime disabled? Then
the CALL_NOSPEC alternative patches in an indirect call again, and the
retpolines are gone.

Does that not need the __insn_is_indirect_jump() thing?

> > > > @@ -240,20 +242,16 @@ static int insn_jump_into_range(struct insn *insn, unsigned long start, int len)
> > > >   static int insn_is_indirect_jump(struct insn *insn)
> > > >   {
> > > > -	int ret = __insn_is_indirect_jump(insn);
> > > > +	int ret;
> > > >   #ifdef CONFIG_RETPOLINE
> > > > -	/*
> > > > -	 * Jump to x86_indirect_thunk_* is treated as an indirect jump.
> > > > -	 * Note that even with CONFIG_RETPOLINE=y, the kernel compiled with
> > > > -	 * older gcc may use indirect jump. So we add this check instead of
> > > > -	 * replace indirect-jump check.
> > > > -	 */
> > > > -	if (!ret)
> > > > +	/* Jump to x86_indirect_thunk_* is treated as an indirect jump. */
> > > >   		ret = insn_jump_into_range(insn,
> > > >   				(unsigned long)__indirect_thunk_start,
> > > >   				(unsigned long)__indirect_thunk_end -
> > > >   				(unsigned long)__indirect_thunk_start);
> > > > +#else
> > > > +		ret = __insn_is_indirect_jump(insn);
> > > >   #endif
> > > >   	return ret;
> > > >   }
> > > 
> > > The resulting code is indented wrong.
> > > 
> > 
> > Oh, yes. Thanks for point out.
> > 
> > Zhenzhong