linux-kernel - Re: [RFC PATCH v2 25/27] x86/cet: Add PTRACE interface for CET

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Message-ID: <1531498079.8494.16.camel@intel.com>
Date:   Fri, 13 Jul 2018 09:07:59 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...nel.org>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-mm@...ck.org, linux-arch@...r.kernel.org,
        linux-api@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...capital.net>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omiun.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>
Subject: Re: [RFC PATCH v2 25/27] x86/cet: Add PTRACE interface for CET

On Fri, 2018-07-13 at 01:08 +0200, Thomas Gleixner wrote:
> On Thu, 12 Jul 2018, Yu-cheng Yu wrote:
> > 
> > On Thu, 2018-07-12 at 16:03 +0200, Ingo Molnar wrote:
> > > 
> > > * Yu-cheng Yu <yu-cheng.yu@...el.com> wrote:
> > > > 
> > > > > 
> > > > > > 
> > > > > > diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
> > > > > > index e2ee403865eb..ac2bc3a18427 100644
> > > > > > --- a/arch/x86/kernel/ptrace.c
> > > > > > +++ b/arch/x86/kernel/ptrace.c
> > > > > > @@ -49,7 +49,9 @@ enum x86_regset {
> > > > > >  	REGSET_IOPERM64 = REGSET_XFP,
> > > > > >  	REGSET_XSTATE,
> > > > > >  	REGSET_TLS,
> > > > > > +	REGSET_CET64 = REGSET_TLS,
> > > > > >  	REGSET_IOPERM32,
> > > > > > +	REGSET_CET32,
> > > > > >  };
> > > > > Why does REGSET_CET64 alias on REGSET_TLS?
> > > > In x86_64_regsets[], there is no [REGSET_TLS].  The core dump code
> > > > cannot handle holes in the array.
> > > Is there a fundamental (ABI) reason for that?
> > What I did was, ran Linux with 'slub_debug', and forced a core dump
> > (kill -abrt <pid>), then there was a red zone warning in the dmesg.
> > My feeling is there could be issues in the core dump code.  These
> Kernel development is not about feelings.

I got that :-)

> 
> Either you can track down the root cause or you cannot. There is no place
> for feelings and no place in between. And if you cannot track down the root
> cause and explain it proper then the resulting patch is just papering over
> the symptoms and will come back to hunt you (or others) sooner than later.
> 
> No if, no could, no feelings. Facts is what matters. Really.

In kernel/ptrace.c,

find_regset(const struct user_regset_view *view, unsigned int type)
{
	const struct user_regset *regset;
	int n;

	for (n = 0; n < view->n; ++n) {
		regset = view->regsets + n;
		if (regset->core_note_type == type)
			return regset;
	}

	return NULL;
}

If there is a hole in the regset array, the empty slot's
regset->core_note_type is not defined.

We can add some comments near those enum's.

Yu-cheng