linux-kernel - Re: [PATCH v3] kdb: Make memory allocations more robust

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Message-ID: <20210125081855.gfq3n6urcmght3ef@maple.lan>
Date:   Mon, 25 Jan 2021 08:18:55 +0000
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Sumit Garg <sumit.garg@...aro.org>,
        kgdb-bugreport@...ts.sourceforge.net,
        Jason Wessel <jason.wessel@...driver.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "# 4.0+" <stable@...r.kernel.org>
Subject: Re: [PATCH v3] kdb: Make memory allocations more robust

On Fri, Jan 22, 2021 at 09:25:44AM -0800, Doug Anderson wrote:
> Hi,
> 
> On Fri, Jan 22, 2021 at 3:06 AM Sumit Garg <sumit.garg@...aro.org> wrote:
> >
> > Currently kdb uses in_interrupt() to determine whether its library
> > code has been called from the kgdb trap handler or from a saner calling
> > context such as driver init.  This approach is broken because
> > in_interrupt() alone isn't able to determine kgdb trap handler entry from
> > normal task context. This can happen during normal use of basic features
> > such as breakpoints and can also be trivially reproduced using:
> > echo g > /proc/sysrq-trigger
> 
> I guess an alternative to your patch is to fully eliminate GFP_KDB.
> It always strikes me as a sub-optimal design to choose between
> GFP_ATOMIC and GFP_KERNEL like this.  Presumably others must agree
> because otherwise I'd expect that the overall kernel would have
> something like "GFP_AUTOMATIC"?
> 
> It doesn't feel like it'd be that hard to do something more explicit.
> From a quick glance:
> 
> * I think kdb_defcmd() and kdb_defcmd2() are always called in response
> to a user typing something on the kdb command line.  Those should
> always be GFP_ATOMIC, right?

No. I'm afraid not. The kdb parser is also used to execute
kernel/debug/kdb/kdb_cmds as part of the kdb initialization. This
initialization happens from the init calls rather than from the kgdb
trap handler code.

When I first looked at Sumit's patch I had a similar reaction to you
but, whilst it is clearly it's not impossible to pass flags into the
kdb parser and all its subcommands, I concluded that GFP_KDB is a
better approach.

BTW the reason I insisted on getting rid of the in_atomic() was to make
it clear that GFP_KDB discriminates between exactly two calling contexts
(normal and kgdb trap handler). I was didn't want any hints that imply
GFP_KDB is a (broken) implementation of something like GFP_AUTOMATIC!


Daniel.