netdev - Re: [PATCH bpf-next 14/17] bpf: Change bpf_getsockopt(SOL_TCP) to reuse do_tcp

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Date:   Fri, 26 Aug 2022 12:24:56 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     sdf@...gle.com
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>, kernel-team@...com,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH bpf-next 14/17] bpf: Change bpf_getsockopt(SOL_TCP) to
 reuse do_tcp_getsockopt()

On Thu, Aug 25, 2022 at 11:36:36AM -0700, sdf@...gle.com wrote:
> On 08/24, Martin KaFai Lau wrote:
> > This patch changes bpf_getsockopt(SOL_TCP) to reuse
> > do_tcp_getsockopt().  It removes the duplicated code from
> > bpf_getsockopt(SOL_TCP).
> 
> > Before this patch, there were some optnames available to
> > bpf_setsockopt(SOL_TCP) but missing in bpf_getsockopt(SOL_TCP).
> > For example, TCP_NODELAY, TCP_MAXSEG, TCP_KEEPIDLE, TCP_KEEPINTVL,
> > and a few more.  It surprises users from time to time.  This patch
> > automatically closes this gap without duplicating more code.
> 
> > bpf_getsockopt(TCP_SAVED_SYN) does not free the saved_syn,
> > so it stays in sol_tcp_sockopt().
> 
> > For string name value like TCP_CONGESTION, bpf expects it
> > is always null terminated, so sol_tcp_sockopt() decrements
> > optlen by one before calling do_tcp_getsockopt() and
> > the 'if (optlen < saved_optlen) memset(..,0,..);'
> > in __bpf_getsockopt() will always do a null termination.
> 
> > Signed-off-by: Martin KaFai Lau <kafai@...com>
> > ---
> >   include/net/tcp.h |  2 ++
> >   net/core/filter.c | 70 ++++++++++++++++++++++++++---------------------
> >   net/ipv4/tcp.c    |  4 +--
> >   3 files changed, 43 insertions(+), 33 deletions(-)
> 
> > diff --git a/include/net/tcp.h b/include/net/tcp.h
> > index c03a50c72f40..735e957f7f4b 100644
> > --- a/include/net/tcp.h
> > +++ b/include/net/tcp.h
> > @@ -402,6 +402,8 @@ void tcp_init_sock(struct sock *sk);
> >   void tcp_init_transfer(struct sock *sk, int bpf_op, struct sk_buff *skb);
> >   __poll_t tcp_poll(struct file *file, struct socket *sock,
> >   		      struct poll_table_struct *wait);
> > +int do_tcp_getsockopt(struct sock *sk, int level,
> > +		      int optname, sockptr_t optval, sockptr_t optlen);
> >   int tcp_getsockopt(struct sock *sk, int level, int optname,
> >   		   char __user *optval, int __user *optlen);
> >   bool tcp_bpf_bypass_getsockopt(int level, int optname);
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 68b52243b306..cdbbcec46e8b 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -5096,8 +5096,9 @@ static int bpf_sol_tcp_setsockopt(struct sock *sk,
> > int optname,
> >   	return 0;
> >   }
> 
> > -static int sol_tcp_setsockopt(struct sock *sk, int optname,
> > -			      char *optval, int optlen)
> > +static int sol_tcp_sockopt(struct sock *sk, int optname,
> > +			   char *optval, int *optlen,
> > +			   bool getopt)
> >   {
> >   	if (sk->sk_prot->setsockopt != tcp_setsockopt)
> >   		return -EINVAL;
> > @@ -5114,17 +5115,47 @@ static int sol_tcp_setsockopt(struct sock *sk,
> > int optname,
> >   	case TCP_USER_TIMEOUT:
> >   	case TCP_NOTSENT_LOWAT:
> >   	case TCP_SAVE_SYN:
> > -		if (optlen != sizeof(int))
> > +		if (*optlen != sizeof(int))
> >   			return -EINVAL;
> >   		break;
> 
> [..]
> 
> >   	case TCP_CONGESTION:
> > +		if (*optlen < 2)
> > +			return -EINVAL;
> > +		break;
> > +	case TCP_SAVED_SYN:
> > +		if (*optlen < 1)
> > +			return -EINVAL;
> >   		break;
> 
> This looks a bit inconsistent vs '*optlen != sizeof(int)' above. Maybe
> 
> if (*optlen < sizeof(u16))
> if (*optlen < sizeof(u8))
TCP_CONGESTION (name string) and TCP_SAVED_SYN (raw binary)
are not expecting integer optval, so I think it is
better to stay away from using integer u16 or u8.

> 
> ?
> 
> >   	default:
> > -		return bpf_sol_tcp_setsockopt(sk, optname, optval, optlen);
> > +		if (getopt)
> > +			return -EINVAL;
> > +		return bpf_sol_tcp_setsockopt(sk, optname, optval, *optlen);
> > +	}
> > +
> > +	if (getopt) {
> > +		if (optname == TCP_SAVED_SYN) {
> > +			struct tcp_sock *tp = tcp_sk(sk);
> > +
> > +			if (!tp->saved_syn ||
> > +			    *optlen > tcp_saved_syn_len(tp->saved_syn))
> > +				return -EINVAL;
> 
> You mention in the description that bpf doesn't doesn't free saved_syn,
> maybe worth putting a comment with the rationale here as well?
> I'm assuming we don't free from bpf because we want userspace to
> have an opportunity to read it as well?
Yes, it is the reason.  I will add a comment.

> 
> > +			memcpy(optval, tp->saved_syn->data, *optlen);
> > +			return 0;
> > +		}
> > +
> > +		if (optname == TCP_CONGESTION) {
> > +			if (!inet_csk(sk)->icsk_ca_ops)
> > +				return -EINVAL;
> 
> Is it worth it doing null termination more explicitly here?
> For readability sake:
> 			/* BPF always expects NULL-terminated strings. */
> 			optval[*optlen-1] = '\0';
Yep.  will do in v2.

> > +			(*optlen)--;
> > +		}
> > +
> > +		return do_tcp_getsockopt(sk, SOL_TCP, optname,
> > +					 KERNEL_SOCKPTR(optval),
> > +					 KERNEL_SOCKPTR(optlen));
> >   	}
> 
> >   	return do_tcp_setsockopt(sk, SOL_TCP, optname,
> > -				 KERNEL_SOCKPTR(optval), optlen);
> > +				 KERNEL_SOCKPTR(optval), *optlen);
> >   }