linux-kernel - Re: [PATCH RFC 3/5] kdbus: do explicit overflow check in kdbus_conn_quota

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Date:	Thu, 2 Jul 2015 17:06:29 +0300
From:	Sergei Zviagintsev <sergei@...v.net>
To:	David Herrmann <dh.herrmann@...il.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Daniel Mack <daniel@...que.org>,
	David Herrmann <dh.herrmann@...glemail.com>,
	Djalal Harouni <tixxdz@...ndz.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 3/5] kdbus: do explicit overflow check in
 kdbus_conn_quota_inc()

Hi,

On Thu, Jul 02, 2015 at 10:35:30AM +0200, David Herrmann wrote:
> Hi
> 
> On Sun, Jun 28, 2015 at 3:17 PM, Sergei Zviagintsev <sergei@...v.net> wrote:
> > Replace the use of max() with explicit and obvious overflow check.
> >
> > Signed-off-by: Sergei Zviagintsev <sergei@...v.net>
> > ---
> >  ipc/kdbus/connection.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/ipc/kdbus/connection.c b/ipc/kdbus/connection.c
> > index af044f93c14f..1d44e280eff0 100644
> > --- a/ipc/kdbus/connection.c
> > +++ b/ipc/kdbus/connection.c
> > @@ -668,9 +668,11 @@ int kdbus_conn_quota_inc(struct kdbus_conn *c, struct kdbus_user *u,
> >
> >         id = u ? u->id : KDBUS_USER_KERNEL_ID;
> >         if (id >= c->n_quota) {
> > -               unsigned int users;
> > +               unsigned int users = KDBUS_ALIGN8(id) + 8;
> > +
> > +               if (users < id) /* overflow */
> > +                       users = id;
> >
> > -               users = max(KDBUS_ALIGN8(id) + 8, id);
> 
> To me, the max() looks fine. I mean, it should be obvious that it
> checks for an overflow, right? Otherwise, I'd prefer adding a comment
> instead of the explicit conditional.

Well, at first sight I assumed that there was some clever algorithm
which later had been quick-fixed and so on...

I am not sure about commenting current variant, as CodingStyle
tells us to write obvious code instead of explanations. And if it is
already obvious, then it doesn't need a comment, right? :)

In another letter of this series I mentioned commits af8e2f750985
and ac5c385cc67a from out-of-tree kdbus repo, which show that not all of
these overflow things are obvious for maintainer, not talking about
someone who isn't familiar with the code.

> 
> Thanks
> David
> 
> >                 quota = krealloc(c->quota, users * sizeof(*quota),
> >                                  GFP_KERNEL | __GFP_ZERO);
> >                 if (!quota)
> > --
> > 1.8.3.1
> >
--
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