Re: [PATCHv3 bpf-next 1/5] bpf: Add socket assign support

[Martin KaFai Lau <kafai@fb.com>]

On Thu, Mar 26, 2020 at 09:25:52PM -0700, Joe Stringer wrote:
> Add support for TPROXY via a new bpf helper, bpf_sk_assign().
> 
> This helper requires the BPF program to discover the socket via a call
> to bpf_sk*_lookup_*(), then pass this socket to the new helper. The
> helper takes its own reference to the socket in addition to any existing
> reference that may or may not currently be obtained for the duration of
> BPF processing. For the destination socket to receive the traffic, the
> traffic must be routed towards that socket via local route. The
> simplest example route is below, but in practice you may want to route
> traffic more narrowly (eg by CIDR):
> 
>   $ ip route add local default dev lo
> 
> This patch avoids trying to introduce an extra bit into the skb->sk, as
> that would require more invasive changes to all code interacting with
> the socket to ensure that the bit is handled correctly, such as all
> error-handling cases along the path from the helper in BPF through to
> the orphan path in the input. Instead, we opt to use the destructor
> variable to switch on the prefetch of the socket.
> 
> Signed-off-by: Joe Stringer <joe@wand.net.nz>
> ---
> v3: Check skb_sk_is_prefetched() in TC level redirect check
> v2: Use skb->destructor to determine socket prefetch usage instead of
>       introducing a new metadata_dst
>     Restrict socket assign to same netns as TC device
>     Restrict assigning reuseport sockets
>     Adjust commit wording
> v1: Initial version
> ---
>  include/net/sock.h             |  7 +++++++
>  include/uapi/linux/bpf.h       | 25 ++++++++++++++++++++++++-
>  net/core/filter.c              | 31 +++++++++++++++++++++++++++++++
>  net/core/sock.c                |  9 +++++++++
>  net/ipv4/ip_input.c            |  3 ++-
>  net/ipv6/ip6_input.c           |  3 ++-
>  net/sched/act_bpf.c            |  3 +++
>  tools/include/uapi/linux/bpf.h | 25 ++++++++++++++++++++++++-
>  8 files changed, 102 insertions(+), 4 deletions(-)
> 

[ ... ]

> diff --git a/net/core/sock.c b/net/core/sock.c
> index 0fc8937a7ff4..cfaf60267360 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -2071,6 +2071,15 @@ void sock_efree(struct sk_buff *skb)
>  }
>  EXPORT_SYMBOL(sock_efree);
>  
> +/* Buffer destructor for prefetch/receive path where reference count may
> + * not be held, e.g. for listen sockets.
> + */
> +void sock_pfree(struct sk_buff *skb)
> +{
> +	sock_edemux(skb);
Nit. may be directly call sock_gen_put().

> +}
> +EXPORT_SYMBOL(sock_pfree);
> +