Re: [PATCH 1/2] net: store KCOV remote handle in sk_buff

[Jakub Kicinski]

On Sat, 10 Oct 2020 09:54:57 +0200 Dmitry Vyukov wrote:
> On Sat, Oct 10, 2020 at 1:16 AM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
> > On Wed,  7 Oct 2020 10:17:25 +0000 Aleksandr Nogikh wrote:  
> > > From: Aleksandr Nogikh <nogikh@xxxxxxxxxx>
> > >
> > > Remote KCOV coverage collection enables coverage-guided fuzzing of the
> > > code that is not reachable during normal system call execution. It is
> > > especially helpful for fuzzing networking subsystems, where it is
> > > common to perform packet handling in separate work queues even for the
> > > packets that originated directly from the user space.
> > >
> > > Enable coverage-guided frame injection by adding a kcov_handle
> > > parameter to sk_buff structure. Initialization in __alloc_skb ensures
> > > that no socket buffer that was generated during a system call will be
> > > missed.
> > >
> > > Code that is of interest and that performs packet processing should be
> > > annotated with kcov_remote_start()/kcov_remote_stop().
> > >
> > > An alternative approach is to determine kcov_handle solely on the
> > > basis of the device/interface that received the specific socket
> > > buffer. However, in this case it would be impossible to distinguish
> > > between packets that originated from normal background network
> > > processes and those that were intentionally injected from the user
> > > space.
> > >
> > > Signed-off-by: Aleksandr Nogikh <nogikh@xxxxxxxxxx>  
> >
> > Could you use skb_extensions for this?  
> 
> Why? If for space, this is already under a non-production ifdef.

I understand, but the skb_ext infra is there for uncommon use cases
like this one. Any particular reason you don't want to use it? 
The slight LoC increase?

Is there any precedent for adding the kcov field to other performance
critical structures?