Re: [PATCH v2 bpf-next 1/5] bpf: Hooks for sys_sendmsg

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

On Tue, May 22, 2018 at 04:40:02PM -0700, Andrey Ignatov wrote:
> In addition to already existing BPF hooks for sys_bind and sys_connect,
> the patch provides new hooks for sys_sendmsg.
> 
> It leverages existing BPF program type `BPF_PROG_TYPE_CGROUP_SOCK_ADDR`
> that provides access to socket itlself (properties like family, type,
> protocol) and user-passed `struct sockaddr *` so that BPF program can
> override destination IP and port for system calls such as sendto(2) or
> sendmsg(2) and/or assign source IP to the socket.
> 
> The hooks are implemented as two new attach types:
> `BPF_CGROUP_UDP4_SENDMSG` and `BPF_CGROUP_UDP6_SENDMSG` for UDPv4 and
> UDPv6 correspondingly.
> 
> UDPv4 and UDPv6 separate attach types for same reason as sys_bind and
> sys_connect hooks, i.e. to prevent reading from / writing to e.g.
> user_ip6 fields when user passes sockaddr_in since it'd be out-of-bound.
> 
> The difference with already existing hooks is sys_sendmsg are
> implemented only for unconnected UDP.
> 
> For TCP it doesn't make sense to change user-provided `struct sockaddr *`
> at sendto(2)/sendmsg(2) time since socket either was already connected
> and has source/destination set or wasn't connected and call to
> sendto(2)/sendmsg(2) would lead to ENOTCONN anyway.
> 
> Connected UDP is already handled by sys_connect hooks that can override
> source/destination at connect time and use fast-path later, i.e. these
> hooks don't affect UDP fast-path.
> 
> Rewriting source IP is implemented differently than that in sys_connect
> hooks. When sys_sendmsg is used with unconnected UDP it doesn't work to
> just bind socket to desired local IP address since source IP can be set
> on per-packet basis by using ancillary data (cmsg(3)). So no matter if
> socket is bound or not, source IP has to be rewritten on every call to
> sys_sendmsg.
> 
> To do so two new fields are added to UAPI `struct bpf_sock_addr`;
> * `msg_src_ip4` to set source IPv4 for UDPv4;
> * `msg_src_ip6` to set source IPv6 for UDPv6.
> 
> Signed-off-by: Andrey Ignatov <rdna@fb.com>
> Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>