Subject: Re: [PATCH ipsec-next,v2 2/3] xfrm: interface: Add unstable helpers for setting/getting XFRM metadata from TC-BPF

From: Martin KaFai Lau <[email-masked]>
Date: Wed, 30 Nov 2022 10:14:56 -0800

On 11/29/22 5:20 AM, Eyal Birger wrote:
> diff --git a/net/xfrm/xfrm_interface_bpf.c b/net/xfrm/xfrm_interface_bpf.c
> new file mode 100644
> index 000000000000..757e15857dbf
> --- /dev/null
> +++ b/net/xfrm/xfrm_interface_bpf.c
> @@ -0,0 +1,100 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/* Unstable XFRM Helpers for TC-BPF hook
> + *
> + * These are called from SCHED_CLS BPF programs. Note that it is
> + * allowed to break compatibility for these functions since the interface they
> + * are exposed through to BPF programs is explicitly unstable.
> + */
> +
> +#include <linux/bpf.h>
> +#include <linux/btf_ids.h>
> +
> +#include <net/dst_metadata.h>
> +#include <net/xfrm.h>
> +
> +struct bpf_xfrm_info {
No need to introduce a bpf variant of the "struct xfrm_md_info" (more on this 
later).

> +	u32 if_id;
> +	int link;
> +};
> +
> +static struct metadata_dst __percpu *xfrm_md_dst;
> +__diag_push();
> +__diag_ignore_all("-Wmissing-prototypes",
> +		  "Global functions as their definitions will be in xfrm_interface BTF");
> +
> +__used noinline
> +int bpf_skb_get_xfrm_info(struct __sk_buff *skb_ctx, struct bpf_xfrm_info *to)

This kfunc is not needed.  It only reads the skb->_skb_refdst.  The new kfunc 
bpf_rdonly_cast() can be used.  Take a look at the bpf_rdonly_cast() usages in 
the selftests/bpf/progs/type_cast.c.  It was in bpf-next only but should also be 
in net-next now.

> +{
> +	struct sk_buff *skb = (struct sk_buff *)skb_ctx;
> +	struct xfrm_md_info *info;
> +
> +	memset(to, 0, sizeof(*to));
> +
> +	info = skb_xfrm_md_info(skb);
> +	if (!info)
> +		return -EINVAL;
> +
> +	to->if_id = info->if_id;
> +	to->link = info->link;
> +	return 0;
> +}
> +
> +__used noinline
> +int bpf_skb_set_xfrm_info(struct __sk_buff *skb_ctx,
> +			  const struct bpf_xfrm_info *from)

Directly use "const struct xfrm_md_info *from" instead.  This kfunc can check 
from->dst_orig != NULL and return -EINVAL.  It will then have a consistent API 
with the bpf_rdonly_cast() mentioned above.

> +{
> +	struct sk_buff *skb = (struct sk_buff *)skb_ctx;
> +	struct metadata_dst *md_dst;
> +	struct xfrm_md_info *info;
> +
> +	if (unlikely(skb_metadata_dst(skb)))
> +		return -EINVAL;
> +
> +	md_dst = this_cpu_ptr(xfrm_md_dst);
> +
> +	info = &md_dst->u.xfrm_info;
> +	memset(info, 0, sizeof(*info));

Unnecessary memset here.  Everything should have been initialized below. 
bpf_skb_set_tunnel_key() needs memset but not here.

> +
> +	info->if_id = from->if_id;
> +	info->link = from->link;
> +	skb_dst_force(skb);
> +	info->dst_orig = skb_dst(skb);
> +
> +	dst_hold((struct dst_entry *)md_dst);
> +	skb_dst_set(skb, (struct dst_entry *)md_dst);
> +	return 0;
> +}
> +
> +__diag_pop()
> +
> +BTF_SET8_START(xfrm_ifc_kfunc_set)
> +BTF_ID_FLAGS(func, bpf_skb_get_xfrm_info)
> +BTF_ID_FLAGS(func, bpf_skb_set_xfrm_info)
> +BTF_SET8_END(xfrm_ifc_kfunc_set)
> +
> +static const struct btf_kfunc_id_set xfrm_interface_kfunc_set = {
> +	.owner = THIS_MODULE,
> +	.set   = &xfrm_ifc_kfunc_set,
> +};
> +
> +int __init register_xfrm_interface_bpf(void)
> +{
> +	int err;
> +
> +	xfrm_md_dst = metadata_dst_alloc_percpu(0, METADATA_XFRM,
> +						GFP_KERNEL);

May be DEFINE_PER_CPU() instead?

> +	if (!xfrm_md_dst)
> +		return -ENOMEM;
> +	err = register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS,
> +					&xfrm_interface_kfunc_set);
> +	if (err < 0) {
> +		cleanup_xfrm_interface_bpf();
> +		return err;
> +	}
> +	return 0;
> +}
> +
> +void __exit cleanup_xfrm_interface_bpf(void)
> +{
> +	metadata_dst_free_percpu(xfrm_md_dst);
> +}