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

[Dan Carpenter <dan.carpenter@oracle.com>]

On Mon, Jan 25, 2021 at 11:39:08AM -0800, Jakub Kicinski wrote:
> 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@wunner.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@redhat.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?
> 

This was a famous tun.c bug back in the day.  In those days we weren't
careful to disallow remapping NULL to a different pointer.  See
/proc/sys/vm/mmap_min_addr.  The exploit was to remap NULL to be a valid
user controlled pointer.  It should have been impossible to exploit
because the code had a check for NULL, but the compiler optimized it
away.

https://lwn.net/Articles/342330/

> 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

If I had to guess why GCC doesn't warn about this I would say that
probably it's because a lot of macros have NULL checks built in.  Most
static analysis tools have a warning about inconsistent NULL checks but
Smatch won't warn about it unless it can lead to a NULL dereference.
The fact that pointless NULL checks slow down the code has never
bothered anyone up to now.

regards,
dan carpenter