linux-kernel - Re: [patch 15/14] x86/dumpstack/64: Speedup in_exception

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Message-ID: <alpine.DEB.2.21.1904032038500.1833@nanos.tec.linutronix.de>
Date:   Wed, 3 Apr 2019 21:42:41 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...capital.net>
cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [patch 15/14] x86/dumpstack/64: Speedup in_exception_stack()

On Wed, 3 Apr 2019, Thomas Gleixner wrote:
> On Tue, 2 Apr 2019, Andy Lutomirski wrote:
> > > On Apr 2, 2019, at 1:29 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > >>> How about a much better fix: make the DB stack be the same size as all
> > >>> the others and just have 4 of them (DB0, DB1, DB2, and DB3.  After all,
> > >>> overflowing from one debug stack into another is just as much of a bug as
> > >>> overflowing into a different IST stack.
> > >> 
> > >> That makes sense.
> > > 
> > > Except that we just have two not four.
> > > 
> > > It needs some tweaking of the ist_shift stuff in entry_64.S but that's not
> > > rocket science. Famous last words....
> > > 
> > 
> > The ist_shift mess should probably be in C, but that’s a big can of
> > worms. That being said, why do we have it at all?  Once upon a time, we’d
> > do ICEBP from user mode (or a legit breakpoint), then send a signal and
> > hit a data breakpoint, and we’d recurse.  But we don’t run user debug
> > handlers on the IST stack at all anymore.
> >
> > Maybe we can convince ourselves it’s safe?
> 
> Maybe. Need to think about it for a while.

What about kprobes. It has nasty reentrancy stuff as well...

Thanks,

	tglx