Re: [bpf-next PATCH 1/2] bpf, sockmap: add skb_adjust_room to pop bytes off ingress payload

[John Fastabend <john.fastabend@gmail.com>]

Daniel Borkmann wrote:
> On 9/26/20 6:27 AM, John Fastabend wrote:
> > This implements a new helper skb_adjust_room() so users can push/pop
> > extra bytes from a BPF_SK_SKB_STREAM_VERDICT program.
> > 
> > Some protocols may include headers and other information that we may
> > not want to include when doing a redirect from a BPF_SK_SKB_STREAM_VERDICT
> > program. One use case is to redirect TLS packets into a receive socket
> > that doesn't expect TLS data. In TLS case the first 13B or so contain the
> > protocol header. With KTLS the payload is decrypted so we should be able
> > to redirect this to a receiving socket, but the receiving socket may not
> > be expecting to receive a TLS header and discard the data. Using the
> > above helper we can pop the header off and put an appropriate header on
> > the payload. This allows for creating a proxy between protocols without
> > extra hops through the stack or userspace.
> > 
> > So in order to fix this case add skb_adjust_room() so users can strip the
> > header. After this the user can strip the header and an unmodified receiver
> > thread will work correctly when data is redirected into the ingress path
> > of a sock.
> > 
> > Signed-off-by: John Fastabend <john.fastabend@gmail.com>
> > ---
> >   net/core/filter.c |   51 +++++++++++++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 51 insertions(+)
> > 
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 4d8dc7a31a78..d232358f1dcd 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -76,6 +76,7 @@
> >   #include <net/bpf_sk_storage.h>
> >   #include <net/transp_v6.h>
> >   #include <linux/btf_ids.h>
> > +#include <net/tls.h>
> >   
> >   static const struct bpf_func_proto *
> >   bpf_sk_base_func_proto(enum bpf_func_id func_id);
> > @@ -3218,6 +3219,53 @@ static u32 __bpf_skb_max_len(const struct sk_buff *skb)
> >   			  SKB_MAX_ALLOC;
> >   }
> >   
> > +BPF_CALL_4(sk_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
> > +	   u32, mode, u64, flags)
> > +{
> > +	unsigned int len_diff_abs = abs(len_diff);
> 
> small nit: u32

Sure.

> 
> > +	bool shrink = len_diff < 0;
> > +	int ret = 0;
> > +
> > +	if (unlikely(flags))
> > +		return -EINVAL;
> 
> Parameter 'mode' is not used here, I guess we need to reject anything non-zero?

Probably its not used.

> 
> Similarly, any interaction wrt bpf_csum_level() that was needed back then for the
> bpf_skb_adjust_room()?

I don't believe so because we are above csum checks at this point.
Either we will put the skb data in the receive_queue for the socket
or redirect it into sendpage.

> 
> > +	if (unlikely(len_diff_abs > 0xfffU))
> > +		return -EFAULT;
> > +
> > +	if (!shrink) {
> > +		unsigned int grow = len_diff;
> 
> nit: u32 or just directly len_diff?

Just use len_diff missed when I cleaned this up.

> 
> > +		ret = skb_cow(skb, grow);
> > +		if (likely(!ret)) {
> > +			__skb_push(skb, len_diff_abs);
> > +			memset(skb->data, 0, len_diff_abs);
> > +		}
> > +	} else {
> > +		/* skb_ensure_writable() is not needed here, as we're
> > +		 * already working on an uncloned skb.
> > +		 */
> > +		if (unlikely(!pskb_may_pull(skb, len_diff_abs)))
> > +			return -ENOMEM;
> > +		__skb_pull(skb, len_diff_abs);
> > +	}
> > +	bpf_compute_data_end_sk_skb(skb);
> > +	if (tls_sw_has_ctx_rx(skb->sk)) {
> > +		struct strp_msg *rxm = strp_msg(skb);
> > +
> > +		rxm->full_len += len_diff;
> 
> If skb_cow() failed, we still adjust rxm->full_len?

Thanks. Will just return above on error like in the else
branch. I'll send a v2 shortly.