Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Sun, 24 Jan 2021 11:46:05 +0100
From:   Lukas Wunner <lukas@wunner.de>
To:     Jakub Kicinski <kuba@kernel.org>
Cc:     Pablo Neira Ayuso <pablo@netfilter.org>,
        Jozsef Kadlecsik <kadlec@netfilter.org>,
        Florian Westphal <fw@strlen.de>,
        netfilter-devel@vger.kernel.org, coreteam@netfilter.org,
        netdev@vger.kernel.org, Daniel Borkmann <daniel@iogearbox.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Eric Dumazet <edumazet@google.com>,
        Thomas Graf <tgraf@suug.ch>,
        Laura Garcia Liebana <nevola@gmail.com>,
        John Fastabend <john.fastabend@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH nf-next v4 1/5] net: sched: Micro-optimize egress handling
Message-ID: <20210124104605.GB1056@wunner.de>
References: <cover.1611304190.git.lukas@wunner.de>
 <a2a8af1622dff2bfd51d446aa8da2c1d2f6f543c.1611304190.git.lukas@wunner.de>
 <20210123192624.4cee3b7d@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20210123192624.4cee3b7d@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Precedence: bulk

On Sat, Jan 23, 2021 at 07:26:24PM -0800, Jakub Kicinski wrote:
> On Fri, 22 Jan 2021 09:47:01 +0100 Lukas Wunner wrote:
> > sch_handle_egress() returns either the skb or NULL to signal to its
> > caller __dev_queue_xmit() whether a packet should continue to be
> > processed.
> > 
> > The skb is always non-NULL, otherwise __dev_queue_xmit() would hit a
> > NULL pointer deref right at its top.
> > 
> > But the compiler doesn't know that.  So if sch_handle_egress() signals
> > success by returning the skb, the "if (!skb) goto out;" statement
> > results in a gratuitous NULL pointer check in the Assembler output.
> 
> Which exact compiler are we talking about it? Did you report this?
> As Eric pointed the compiler should be able to figure this out quite
> easily.

I tested with gcc 8, 9, 10.

No need to report as it's the expected behavior with
-fno-delete-null-pointer-checks, whose motivation appears
questionable though (per my preceding e-mail).

Thanks,

Lukas