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

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Date:   Mon, 25 Jan 2021 11:46:08 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     kgdb-bugreport@...ts.sourceforge.net,
        Jason Wessel <jason.wessel@...driver.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "# 4.0+" <stable@...r.kernel.org>
Subject: Re: [PATCH v3] kdb: Make memory allocations more robust

Thanks Doug for your comments.

On Fri, 22 Jan 2021 at 22:55, Doug Anderson <dianders@...omium.org> 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?
>
> * The one call that's not in kdb_defcmd() / kdb_defcmd2() is in
> kdb_register_flags().  That can be called either during init time or
> from kdb_defcmd2().  It doesn't seem like it'd be hard to rename
> kdb_register_flags() to _kdb_register_flags() and add a "gfp_t flags"
> to the end.  Then the exported kdb_register_flags() would pass
> GFP_KERNEL and the call from kdb_defcmd2() would pass GFP_ATOMIC:
>

Thanks for your suggestions. I agree with you that it's better to get
rid of GFP_KDB. But I think we need to backport this fix to stable
kernels as well, so IMO a minimal change like this would be better. I
will rather push a seperate code refactoring patch to incorporate your
suggestions.

>
> > We can improve this by adding check for in_dbg_master() instead which
>
> s/adding check/adding a check/
>

Ack. If we don't have any further comments, can this be incorporated
while applying this patch?

>
> > explicitly determines if we are running in debugger context.
> >
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
> > ---
> >
> > Changes in v3:
> > - Refined commit description and Cc: stable@...r.kernel.org.
> >
> > Changes in v2:
> > - Get rid of redundant in_atomic() check.
> >
> >  kernel/debug/kdb/kdb_private.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
>
> I would leave it up to Daniel to say whether he agrees that a full
> removal of "GFP_KDB" would be a better solution.  However, your patch
> clearly improves the state of things, so:
>
> Reviewed-by: Douglas Anderson <dianders@...omium.org>

Thanks,
Sumit