netdev - Re: [PATCH bpf-next] net: Don't uninstall an XDP program when none is installed

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Message-ID: <CAJ+HfNg8C+teCywDDjKY6_gdPsg_dzm1cMNFhj7gLps6RYMAJQ@mail.gmail.com>
Date:   Thu, 13 Jun 2019 11:04:45 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     Maxim Mikityanskiy <maximmi@...lanox.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Björn Töpel <bjorn.topel@...el.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH bpf-next] net: Don't uninstall an XDP program when none is installed

On Wed, 12 Jun 2019 at 23:15, Jakub Kicinski
<jakub.kicinski@...ronome.com> wrote:
>
> On Wed, 12 Jun 2019 16:14:18 +0000, Maxim Mikityanskiy wrote:
> > dev_change_xdp_fd doesn't perform any checks in case it uninstalls an
> > XDP program. It means that the driver's ndo_bpf can be called with
> > XDP_SETUP_PROG asking to set it to NULL even if it's already NULL. This
> > case happens if the user runs `ip link set eth0 xdp off` when there is
> > no XDP program attached.
> >
> > The drivers typically perform some heavy operations on XDP_SETUP_PROG,
> > so they all have to handle this case internally to return early if it
> > happens. This patch puts this check into the kernel code, so that all
> > drivers will benefit from it.
> >
> > Signed-off-by: Maxim Mikityanskiy <maximmi@...lanox.com>
> > ---
> > Björn, please take a look at this, Saeed told me you were doing
> > something related, but I couldn't find it. If this fix is already
> > covered by your work, please tell about that.
> >
> >  net/core/dev.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index 66f7508825bd..68b3e3320ceb 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -8089,6 +8089,9 @@ int dev_change_xdp_fd(struct net_device *dev, struct netlink_ext_ack *extack,
> >                       bpf_prog_put(prog);
> >                       return -EINVAL;
> >               }
> > +     } else {
> > +             if (!__dev_xdp_query(dev, bpf_op, query))
> > +                     return 0;
>
> This will mask the error if program is installed with different flags.
>

Hmm, probably missing something, but I fail to see how the error is
being masked? This is to catch the NULL-to-NULL case early.

> You driver should do nothing is program installation state did not
> change.  I.e.:
>
>         if (!!prog == !!oldprog)
>
> You can't remove the active -> active check anyway, this change should
> make no difference.
>
> >       }
> >
> >       err = dev_xdp_install(dev, bpf_op, extack, flags, prog);