linux-kernel - Re: x86/entry vs kgdb

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Date:   Tue, 26 May 2020 17:16:21 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, sumit.garg@...aro.org, jason.wessel@...driver.com,
        dianders@...omium.org, kgdb-bugreport@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, pmladek@...e.com,
        sergey.senozhatsky@...il.com, will@...nel.org
Subject: Re: x86/entry vs kgdb

On Mon, May 25, 2020 at 11:18:32AM +0200, Peter Zijlstra wrote:
> On Mon, May 25, 2020 at 10:36:05AM +0200, Peter Zijlstra wrote:
> > Hi!
> > 
> > Since you seem to care about kgdb, I figured you might want to fix this
> > before I mark it broken on x86 (we've been considering doing that for a
> > while).
> > 
> > AFAICT the whole debugreg usage of kgdb-x86_64 is completely hosed; it
> > doesn't respsect the normal exclusion zones as per arch_build_bp_info().
> > 
> > That is, breakpoints must never be in:
> > 
> >   - in the cpu_entry_area
> >   - in .entry.text
> >   - in .noinstr.text
> >   - in anything else marked NOKPROBE
> > 
> > by not respecting these constraints it is trivial to completely and
> > utterly hose the machine. The entry rework that is current underway will
> > explicitly not deal with #DB triggering in any of those places.
> 
> This also very much includes single stepping those bits.  Which KGDB
> obviously also does not respects.

For breakpoints there's already a pre-poke validation hook that
architectures can override if they want to. I can modify the default
implementation to include checking the nokprobe list.

Stepping is a bit more complex. There are hooks for some of the
underlying work but not pre-step validation hook. I'll see if we can add
one.


Daniel.