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?
Thanks,
Lukas
