linux-kernel - Re: x86/entry vs kgdb

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Message-ID: <20200525091832.GE325303@hirez.programming.kicks-ass.net>
Date:   Mon, 25 May 2020 11:18:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     daniel.thompson@...aro.org, x86@...nel.org
Cc:     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 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.