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Date:   Mon, 12 Oct 2020 08:04:33 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Aleksandr Nogikh <a.nogikh@...il.com>,
        David Miller <davem@...emloft.net>,
        Johannes Berg <johannes@...solutions.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Marco Elver <elver@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        linux-wireless <linux-wireless@...r.kernel.org>,
        Aleksandr Nogikh <nogikh@...gle.com>
Subject: Re: [PATCH 1/2] net: store KCOV remote handle in sk_buff

On Sat, Oct 10, 2020 at 5:14 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Sat, 10 Oct 2020 09:54:57 +0200 Dmitry Vyukov wrote:
> > On Sat, Oct 10, 2020 at 1:16 AM Jakub Kicinski <kuba@...nel.org> wrote:
> > > On Wed,  7 Oct 2020 10:17:25 +0000 Aleksandr Nogikh wrote:
> > > > From: Aleksandr Nogikh <nogikh@...gle.com>
> > > >
> > > > 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@...gle.com>
> > >
> > > 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?

I see. Yes, increase in complexity for no gain.
No, KCOV context wasn't added to anything as critical as sk_buff.
It seems there is no established practice both ways -- I don't see
anything debug-related in sk_buff nor in skb_ext_id...

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