netdev - Re: [PATCH nf-next v4 1/5] net: sched: Micro-optimize egress handling

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Date:   Sun, 24 Jan 2021 11:33:01 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Florian Westphal <fw@...len.de>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        netdev <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Thomas Graf <tgraf@...g.ch>,
        Laura Garcia Liebana <nevola@...il.com>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH nf-next v4 1/5] net: sched: Micro-optimize egress handling

On Fri, Jan 22, 2021 at 10:40:05AM +0100, Eric Dumazet wrote:
> On Fri, Jan 22, 2021 at 9:55 AM Lukas Wunner <lukas@...ner.de> 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.
> >
> > Avoid by telling the compiler that __dev_queue_xmit() is never passed a
> > NULL skb.
[...]
> > we're about to add a netfilter egress hook to __dev_queue_xmit()
> > and without the micro-optimization, it will result in a performance
> > degradation which is indeed measurable:
[...]
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > +__attribute__((nonnull(1)))
> >  static int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
> >  {
> >         struct net_device *dev = skb->dev;
> 
> It is a bit sad the compilers do not automatically get this knowledge
> from the very first instruction :
> 
>  struct net_device *dev = skb->dev;

The compiler (gcc) is capable of doing that, but the feature was disabled by:

    commit a3ca86aea507904148870946d599e07a340b39bf
    Author: Eugene Teo <eteo@...hat.com>
    Date:   Wed Jul 15 14:59:10 2009 +0800

    Add '-fno-delete-null-pointer-checks' to gcc CFLAGS

If -fno-delete-null-pointer-checks is dropped from the top-level Makefile
then the gratuitous NULL pointer checks disappear from the Assembler output,
obviating the need to litter hot paths with __attribute__((nonnull(1)))
annotations.

Taking a closer look at that commit, its rationale appears questionable:
It says that broken code such as ...

	struct agnx_priv *priv = dev->priv;

	if (!dev)
		return;

... would result in the NULL pointer check being optimized away.
The commit message claims that keeping the NULL pointer check in
"makes it harder to abuse" the broken code.

I don't see how that's the case:  If dev is NULL, the NULL pointer
dereference at the function's top causes termination of the task
in kernel/exit.c:do_exit().  So the NULL pointer check is never
reached by the task.  If on the other hand dev is non-NULL,
the task isn't terminated but then the NULL pointer check is
unnecessary as well.

So the point of the commit remains elusive to me.  I could submit
an RFC patch which drops -fno-delete-null-pointer-checks and see
if any security folks cry foul.  Thoughts?

Thanks,

Lukas