Subject: Re: [PATCH bpf-next v2 09/11] bpf: Allow selecting reuseport socket from a SOCKMAP

From: Jakub Sitnicki <[email-masked]>
Date: Wed, 15 Jan 2020 13:57:56 +0100

On Tue, Jan 14, 2020 at 12:51 AM CET, Martin Lau wrote:
> On Fri, Jan 10, 2020 at 11:50:25AM +0100, Jakub Sitnicki wrote:
>> SOCKMAP now supports storing references to listening sockets. Nothing keeps
>> us from using it as an array of sockets to select from in SK_REUSEPORT
>> programs.
>>
>> Whitelist the map type with the BPF helper for selecting socket.
>>
>> The restriction that the socket has to be a member of a reuseport group
>> still applies. Socket from a SOCKMAP that does not have sk_reuseport_cb set
>> is not a valid target and we signal it with -EINVAL.
>>
>> Signed-off-by: Jakub Sitnicki <[email-masked]>
>> ---
>>  kernel/bpf/verifier.c |  6 ++++--
>>  net/core/filter.c     | 15 ++++++++++-----
>>  2 files changed, 14 insertions(+), 7 deletions(-)
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index f5af759a8a5f..0ee5f1594b5c 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -3697,7 +3697,8 @@ static int check_map_func_compatibility(struct bpf_verifier_env *env,
>>  		if (func_id != BPF_FUNC_sk_redirect_map &&
>>  		    func_id != BPF_FUNC_sock_map_update &&
>>  		    func_id != BPF_FUNC_map_delete_elem &&
>> -		    func_id != BPF_FUNC_msg_redirect_map)
>> +		    func_id != BPF_FUNC_msg_redirect_map &&
>> +		    func_id != BPF_FUNC_sk_select_reuseport)
>>  			goto error;
>>  		break;
>>  	case BPF_MAP_TYPE_SOCKHASH:
>> @@ -3778,7 +3779,8 @@ static int check_map_func_compatibility(struct bpf_verifier_env *env,
>>  			goto error;
>>  		break;
>>  	case BPF_FUNC_sk_select_reuseport:
>> -		if (map->map_type != BPF_MAP_TYPE_REUSEPORT_SOCKARRAY)
>> +		if (map->map_type != BPF_MAP_TYPE_REUSEPORT_SOCKARRAY &&
>> +		    map->map_type != BPF_MAP_TYPE_SOCKMAP)
>>  			goto error;
>>  		break;
>>  	case BPF_FUNC_map_peek_elem:
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index a702761ef369..c79c62a54167 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -8677,6 +8677,7 @@ struct sock *bpf_run_sk_reuseport(struct sock_reuseport *reuse, struct sock *sk,
>>  BPF_CALL_4(sk_select_reuseport, struct sk_reuseport_kern *, reuse_kern,
>>  	   struct bpf_map *, map, void *, key, u32, flags)
>>  {
>> +	bool is_sockarray = map->map_type == BPF_MAP_TYPE_REUSEPORT_SOCKARRAY;
> A nit.
> Since map_type is tested, reuseport_array_lookup_elem() or sock_map_lookup()
> can directly be called also.  mostly for consideration.  will not
> insist.

sock_map_lookup() isn't global currently.

If I'm following your thinking, you're suggesting an optimization
against retpoline overhead along the lines of INDIRECT_CALL_$n wrappers:

/*
 * INDIRECT_CALL_$NR - wrapper for indirect calls with $NR known builtin
 *  @f: function pointer
 *  @f$NR: builtin functions names, up to $NR of them
 *  @__VA_ARGS__: arguments for @f
 *
 * Avoid retpoline overhead for known builtin, checking @f vs each of them and
 * eventually invoking directly the builtin function. The functions are check
 * in the given order. Fallback to the indirect call.
 */
#define INDIRECT_CALL_1(f, f1, ...)					\
	({								\
		likely(f == f1) ? f1(__VA_ARGS__) : f(__VA_ARGS__);	\
	})
#define INDIRECT_CALL_2(f, f2, f1, ...)					\
	({								\
		likely(f == f2) ? f2(__VA_ARGS__) :			\
				  INDIRECT_CALL_1(f, f1, __VA_ARGS__);	\
	})

Will resist the temptation to optimize it as part of this series,
because the indirect call is already there.