Re: [RFC PATCH bpf-next 7/8] bpf: Call bpf_run_sk_reuseport() for socket migration.

[Martin KaFai Lau]

On Tue, Nov 17, 2020 at 06:40:22PM +0900, Kuniyuki Iwashima wrote:
> This patch makes it possible to select a new listener for socket migration
> by eBPF.
> 
> The noteworthy point is that we select a listening socket in
> reuseport_detach_sock() and reuseport_select_sock(), but we do not have
> struct skb in the unhash path.
> 
> Since we cannot pass skb to the eBPF program, we run only the
> BPF_PROG_TYPE_SK_REUSEPORT program by calling bpf_run_sk_reuseport() with
> skb NULL. So, some fields derived from skb are also NULL in the eBPF
> program.
More things need to be considered here when skb is NULL.

Some helpers are probably assuming skb is not NULL.

Also, the sk_lookup in filter.c is actually passing a NULL skb to avoid
doing the reuseport select.

> 
> Moreover, we can cancel migration by returning SK_DROP. This feature is
> useful when listeners have different settings at the socket API level or
> when we want to free resources as soon as possible.
> 
> Reviewed-by: Benjamin Herrenschmidt <benh@xxxxxxxxxx>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxxxx>
> ---
>  net/core/filter.c          | 26 +++++++++++++++++++++-----
>  net/core/sock_reuseport.c  | 23 ++++++++++++++++++++---
>  net/ipv4/inet_hashtables.c |  2 +-
>  3 files changed, 42 insertions(+), 9 deletions(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 01e28f283962..ffc4591878b8 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -8914,6 +8914,22 @@ static u32 xdp_convert_ctx_access(enum bpf_access_type type,
>  	SOCK_ADDR_LOAD_NESTED_FIELD_SIZE_OFF(S, NS, F, NF,		       \
>  					     BPF_FIELD_SIZEOF(NS, NF), 0)
>  
> +#define SOCK_ADDR_LOAD_NESTED_FIELD_SIZE_OFF_OR_NULL(S, NS, F, NF, SIZE, OFF)	\
> +	do {									\
> +		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(S, F), si->dst_reg,	\
> +				      si->src_reg, offsetof(S, F));		\
> +		*insn++ = BPF_JMP_IMM(BPF_JEQ, si->dst_reg, 0, 1);		\
Although it may not matter much, always doing this check seems not very ideal
considering the fast path will always have skb and only the slow
path (accept-queue migrate) has skb is NULL.  I think the req_sk usually
has the skb also except the timer one.

First thought is to create a temp skb but it has its own issues.
or it may actually belong to a new prog type.  However, lets keep
exploring possible options (including NULL skb).

> +		*insn++ = BPF_LDX_MEM(						\
> +			SIZE, si->dst_reg, si->dst_reg,				\
> +			bpf_target_off(NS, NF, sizeof_field(NS, NF),		\
> +				       target_size)				\
> +			+ OFF);							\
> +	} while (0)