linux-kernel - RE: [PATCH 1/4] s390: convert debug_info.ref_count from atomic_t to refcount

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Date:   Mon, 20 Feb 2017 13:35:36 +0000
From:   "Reshetova, Elena" <elena.reshetova@...el.com>
To:     Heiko Carstens <heiko.carstens@...ibm.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "hpa@...or.com" <hpa@...or.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        Kees Cook <keescook@...omium.org>,
        David Windsor <dwindsor@...il.com>
Subject: RE: [PATCH 1/4] s390: convert debug_info.ref_count from atomic_t to
 refcount_t

> On Mon, Feb 20, 2017 at 01:06:18PM +0200, Elena Reshetova wrote:
> > refcount_t type and corresponding API should be
> > used instead of atomic_t when the variable is used as
> > a reference counter. This allows to avoid accidental
> > refcounter overflows that might lead to use-after-free
> > situations.
> >
> > Signed-off-by: Elena Reshetova <elena.reshetova@...el.com>
> > Signed-off-by: Hans Liljestrand <ishkamiel@...il.com>
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > Signed-off-by: David Windsor <dwindsor@...il.com>
> > ---
> >  arch/s390/include/asm/debug.h | 3 ++-
> >  arch/s390/kernel/debug.c      | 8 ++++----
> >  2 files changed, 6 insertions(+), 5 deletions(-)
> 
> I can only see a pull request from Ingo a couple of hours ago for Peter's
> refcount code. So the refcount code is not merged yet. It would have been
> good if you would have waited until it is really merged to avoid confusion.

Sorry, I guess I was a bit too rushy, but I also want to be able to fix all things that come up as I post these before next merge window closes. 
> 
> > @@ -361,7 +361,7 @@ debug_info_create(const char *name, int
> pages_per_area, int nr_areas,
> >          debug_area_last = rc;
> >          rc->next = NULL;
> >
> > -	debug_info_get(rc);
> > +	refcount_set(&rc->ref_count, 1);
> 
> This is not wrong, but I will remove this hunk before applying your patch,
> since this doesn't look like an obvious correct change at first glance.

It isn't obvious, but needed unfortunately. refcount_inc is done in the way that it won't increment on zero value.
And since for this variable you set the initial refcounter value to zero and then call debug_info_get (that does inc), this
would only WARN and not increment. So for this initial case, we changed it to call refcount_set to "1" to make sure things work
as before.

Best Regards,
Elena.