linux-kernel - Re: [PATCH v2 3/3] kgdb: Add NOKPROBE labels on the trap handler functions

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Date:   Mon, 20 Jul 2020 09:13:23 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Jason Wessel <jason.wessel@...driver.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sumit Garg <sumit.garg@...aro.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Will Deacon <will@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        kgdb-bugreport@...ts.sourceforge.net,
        LKML <linux-kernel@...r.kernel.org>,
        Patch Tracking <patches@...aro.org>
Subject: Re: [PATCH v2 3/3] kgdb: Add NOKPROBE labels on the trap handler
 functions

On Fri, Jul 17, 2020 at 03:39:58PM -0700, Doug Anderson wrote:
> Hi,
> 
> On Thu, Jul 16, 2020 at 8:20 AM Daniel Thompson
> <daniel.thompson@...aro.org> wrote:
> >
> > Currently kgdb honours the kprobe blocklist but doesn't place its own
> > trap handling code on the list. Add labels to discourage attempting to
> > use kgdb to debug itself.
> >
> > These changes do not make it impossible to provoke recursive trapping
> > since they do not cover all the calls that can be made on kgdb's entry
> > logic. However going much further whilst we are sharing the kprobe
> > blocklist risks reducing the capabilities of kprobe and this would be a
> > bad trade off (especially so given kgdb's users are currently conditioned
> > to avoid recursive traps).
> >
> > Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
> > ---
> >  kernel/debug/debug_core.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> 
> I could just be missing something, but...
> 
> I understand not adding "NOKPROBE_SYMBOL" to generic kernel functions
> that kgdb happens to call, but I'm not quite sure I understand why all
> of the kdb / kgdb code itself shouldn't be in the blocklist.  I
> certainly don't object to the functions you added to the blocklist, I
> guess I'm must trying to understand why it's a bad idea to add more or
> how you came up with the list of functions that you did.

Relatively early in the trap handler execution (just after we bring the
other CPUs to a halt) all breakpoints are replaced with the original
opcodes. Therefore I only marked up functions that run between the trap
firing and the breakpoints being removed (and also between the
breakpoints being reinstated and trap exit).


Daniel.