linux-kernel - Re: [PATCH] net/ipv4: fib_trie: Avoid cryptic ternary expressions

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Message-ID: <20190618225721.GV137143@google.com>
Date:   Tue, 18 Jun 2019 15:57:21 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Alexander Duyck <alexander.h.duyck@...hat.com>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH] net/ipv4: fib_trie: Avoid cryptic ternary expressions

On Tue, Jun 18, 2019 at 02:45:34PM -0700, Doug Anderson wrote:
> Hi,
> 
> On Tue, Jun 18, 2019 at 2:14 PM Matthias Kaehlcke <mka@...omium.org> wrote:
> >
> > empty_child_inc/dec() use the ternary operator for conditional
> > operations. The conditions involve the post/pre in/decrement
> > operator and the operation is only performed when the condition
> > is *not* true. This is hard to parse for humans, use a regular
> > 'if' construct instead and perform the in/decrement separately.
> >
> > This also fixes two warnings that are emitted about the value
> > of the ternary expression being unused, when building the kernel
> > with clang + "kbuild: Remove unnecessary -Wno-unused-value"
> > (https://lore.kernel.org/patchwork/patch/1089869/):
> >
> > CC      net/ipv4/fib_trie.o
> > net/ipv4/fib_trie.c:351:2: error: expression result unused [-Werror,-Wunused-value]
> >         ++tn_info(n)->empty_children ? : ++tn_info(n)->full_children;
> >
> > Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> > ---
> > I have no good understanding of the fib_trie code, but the
> > disentangled code looks wrong, and it should be equivalent to the
> > cryptic version, unless I messed it up. In empty_child_inc()
> > 'full_children' is only incremented when 'empty_children' is -1. I
> > suspect a bug in the cryptic code, but am surprised why it hasn't
> > blown up yet. Or is it intended behavior that is just
> > super-counterintuitive?
> >
> > For now I'm leaving it at disentangling the cryptic expressions,
> > if there is a bug we can discuss what action to take.
> > ---
> >  net/ipv4/fib_trie.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> I have no knowledge of this code either but Matthias's patch looks
> sane to me and I agree with the disentangling before making functional
> changes.

I in terms of the -stable process it might make sense to either
disentangle & fix in a single step, or first fix the cryptic code
(shudder!) and then disentangle it. I guess if we make it a series
disentangle & fix could be separate steps.

I'm open to whatever maintainers & stable folks prefer.

> My own personal belief is that this is pointing out a bug somewhere.
> Since "empty_children" ends up being an unsigned type it doesn't feel
> like it was by-design that -1 is ever a value that should be in there.

good point that 'empty_children' is unsigned, that indeed reinforces
the bug theory.

> In any case:
> 
> Reviewed-by: Douglas Anderson <dianders@...omium.org>

Thanks!