Subject: Re: [PATCH bpf-next v2 07/11] bpf, sockmap: Return socket cookie on lookup from syscall

From: Jakub Sitnicki <[email-masked]>
Date: Tue, 14 Jan 2020 16:48:23 +0100

On Tue, Jan 14, 2020 at 04:16 AM CET, John Fastabend wrote:
> Martin Lau wrote:
>> On Fri, Jan 10, 2020 at 11:50:23AM +0100, Jakub Sitnicki wrote:
>> > Tooling that populates the SOCKMAP with sockets from user-space needs a way
>> > to inspect its contents. Returning the struct sock * that SOCKMAP holds to
>> > user-space is neither safe nor useful. An approach established by
>> > REUSEPORT_SOCKARRAY is to return a socket cookie (a unique identifier)
>> > instead.
>> >
>> > Since socket cookies are u64 values SOCKMAP needs to support such a value
>> > size for lookup to be possible. This requires special handling on update,
>> > though. Attempts to do a lookup on SOCKMAP holding u32 values will be met
>> > with ENOSPC error.
>> >
>> > Signed-off-by: Jakub Sitnicki <[email-masked]>
>> > ---
>
> [...]
>
>> > +static void *sock_map_lookup_sys(struct bpf_map *map, void *key)
>> > +{
>> > +	struct sock *sk;
>> > +
>> > +	WARN_ON_ONCE(!rcu_read_lock_held());
>> It seems unnecessary.  It is only called by syscall.c which
>> holds the rcu_read_lock().  Other than that,
>>
>
> +1 drop it. The normal rcu annotations/splats should catch anything
> here.

Oh, okay. Thanks for pointing it out.

I noticed __sock_map_lookup_elem called from sock_map_lookup_sys has the
same WARN_ON_ONCE check. Looks like it can be cleaned up.

Granted, __sock_map_lookup_elem also gets invoked by sockmap BPF helpers
for redirecting (bpf_msg_redirect_map, bpf_sk_redirect_map). But we
always run sk_skb and sk_msg progs RCU read lock held.