netdev - Re: [PATCH bpf-next 1/4] xdp: Support specifying expected existing program when attaching XDP

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Date:   Thu, 19 Mar 2020 19:13:17 -0700
From:   Yonghong Song <yhs@...com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>
CC:     Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        Andrii Nakryiko <andriin@...com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        Lorenz Bauer <lmb@...udflare.com>,
        Andrey Ignatov <rdna@...com>, <netdev@...r.kernel.org>,
        <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/4] xdp: Support specifying expected existing
 program when attaching XDP



On 3/19/20 6:13 AM, Toke Høiland-Jørgensen wrote:
> From: Toke Høiland-Jørgensen <toke@...hat.com>
> 
> While it is currently possible for userspace to specify that an existing
> XDP program should not be replaced when attaching to an interface, there is
> no mechanism to safely replace a specific XDP program with another.
> 
> This patch adds a new netlink attribute, IFLA_XDP_EXPECTED_FD, which can be
> set along with IFLA_XDP_FD. If set, the kernel will check that the program
> currently loaded on the interface matches the expected one, and fail the
> operation if it does not. This corresponds to a 'cmpxchg' memory operation.

The patch set itself looks good to me. But previously there is a
discussion regarding a potential similar functionality through bpf_link.
I guess maintainers (Alexei and Daniel) need to weigh in as some
future vision is involved.

> 
> A new companion flag, XDP_FLAGS_EXPECT_FD, is also added to explicitly
> request checking of the EXPECTED_FD attribute. This is needed for userspace
> to discover whether the kernel supports the new attribute.
> 
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---
>   include/linux/netdevice.h    |    2 +-
>   include/uapi/linux/if_link.h |    4 +++-
>   net/core/dev.c               |   25 ++++++++++++++++++++-----
>   net/core/rtnetlink.c         |   11 +++++++++++
>   4 files changed, 35 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index b6fedd54cd8e..40b12bd93913 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -3767,7 +3767,7 @@ struct sk_buff *dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
>   
>   typedef int (*bpf_op_t)(struct net_device *dev, struct netdev_bpf *bpf);
>   int dev_change_xdp_fd(struct net_device *dev, struct netlink_ext_ack *extack,
> -		      int fd, u32 flags);
> +		      int fd, int expected_fd, u32 flags);
>   u32 __dev_xdp_query(struct net_device *dev, bpf_op_t xdp_op,
>   		    enum bpf_netdev_command cmd);
>   int xdp_umem_query(struct net_device *dev, u16 queue_id);
[...]