On Sun, 24 Jan 2021 11:33:01 +0100 Lukas Wunner wrote:
> 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@xxxxxxxxx> 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@xxxxxxxxxx>
> 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?
I wonder if modern compilers can't simply warn about this particular
case. Not to mention our static checkers..
Dan, do you think the concern from the above-quoted commit is still
valid? Is this something that smatch flags these days? We're apparently
paying a real performance price in networking for tying compiler's hands
with -fno-delete-null-pointer-checks
