Subject: Re: [xdp-hints] [PATCH bpf-next 03/11] bpf: Support inlined/unrolled kfuncs for xdp metadata

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Tue, 15 Nov 2022 17:16:21 +0100

Stanislav Fomichev <[email-masked]> writes:

> Kfuncs have to be defined with KF_UNROLL for an attempted unroll.
> For now, only XDP programs can have their kfuncs unrolled, but
> we can extend this later on if more programs would like to use it.
>
> For XDP, we define a new kfunc set (xdp_metadata_kfunc_ids) which
> implements all possible metatada kfuncs. Not all devices have to
> implement them. If unrolling is not supported by the target device,
> the default implementation is called instead. The default
> implementation is unconditionally unrolled to 'return false/0/NULL'
> for now.
>
> Upon loading, if BPF_F_XDP_HAS_METADATA is passed via prog_flags,
> we treat prog_index as target device for kfunc unrolling.
> net_device_ops gains new ndo_unroll_kfunc which does the actual
> dirty work per device.
>
> The kfunc unrolling itself largely follows the existing map_gen_lookup
> unrolling example, so there is nothing new here.
>
> Cc: John Fastabend <[email-masked]>
> Cc: David Ahern <[email-masked]>
> Cc: Martin KaFai Lau <[email-masked]>
> Cc: Jakub Kicinski <[email-masked]>
> Cc: Willem de Bruijn <[email-masked]>
> Cc: Jesper Dangaard Brouer <[email-masked]>
> Cc: Anatoly Burakov <[email-masked]>
> Cc: Alexander Lobakin <[email-masked]>
> Cc: Magnus Karlsson <[email-masked]>
> Cc: Maryam Tahhan <[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]
> Signed-off-by: Stanislav Fomichev <[email-masked]>
> ---
>  Documentation/bpf/kfuncs.rst   |  8 +++++
>  include/linux/bpf.h            |  1 +
>  include/linux/btf.h            |  1 +
>  include/linux/btf_ids.h        |  4 +++
>  include/linux/netdevice.h      |  5 +++
>  include/net/xdp.h              | 24 +++++++++++++
>  include/uapi/linux/bpf.h       |  5 +++
>  kernel/bpf/syscall.c           | 28 ++++++++++++++-
>  kernel/bpf/verifier.c          | 65 ++++++++++++++++++++++++++++++++++
>  net/core/dev.c                 |  7 ++++
>  net/core/xdp.c                 | 39 ++++++++++++++++++++
>  tools/include/uapi/linux/bpf.h |  5 +++
>  12 files changed, 191 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
> index 0f858156371d..1723de2720bb 100644
> --- a/Documentation/bpf/kfuncs.rst
> +++ b/Documentation/bpf/kfuncs.rst
> @@ -169,6 +169,14 @@ rebooting or panicking. Due to this additional restrictions apply to these
>  calls. At the moment they only require CAP_SYS_BOOT capability, but more can be
>  added later.
>  
> +2.4.8 KF_UNROLL flag
> +-----------------------
> +
> +The KF_UNROLL flag is used for kfuncs that the verifier can attempt to unroll.
> +Unrolling is currently implemented only for XDP programs' metadata kfuncs.
> +The main motivation behind unrolling is to remove function call overhead
> +and allow efficient inlined kfuncs to be generated.
> +
>  2.5 Registering the kfuncs
>  --------------------------
>  
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 798aec816970..bf8936522dd9 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1240,6 +1240,7 @@ struct bpf_prog_aux {
>  		struct work_struct work;
>  		struct rcu_head	rcu;
>  	};
> +	const struct net_device_ops *xdp_kfunc_ndo;
>  };
>  
>  struct bpf_prog {
> diff --git a/include/linux/btf.h b/include/linux/btf.h
> index d80345fa566b..950cca997a5a 100644
> --- a/include/linux/btf.h
> +++ b/include/linux/btf.h
> @@ -51,6 +51,7 @@
>  #define KF_TRUSTED_ARGS (1 << 4) /* kfunc only takes trusted pointer arguments */
>  #define KF_SLEEPABLE    (1 << 5) /* kfunc may sleep */
>  #define KF_DESTRUCTIVE  (1 << 6) /* kfunc performs destructive actions */
> +#define KF_UNROLL       (1 << 7) /* kfunc unrolling can be attempted */
>  
>  /*
>   * Return the name of the passed struct, if exists, or halt the build if for
> diff --git a/include/linux/btf_ids.h b/include/linux/btf_ids.h
> index c9744efd202f..eb448e9c79bb 100644
> --- a/include/linux/btf_ids.h
> +++ b/include/linux/btf_ids.h
> @@ -195,6 +195,10 @@ asm(							\
>  __BTF_ID_LIST(name, local)				\
>  __BTF_SET8_START(name, local)
>  
> +#define BTF_SET8_START_GLOBAL(name)			\
> +__BTF_ID_LIST(name, global)				\
> +__BTF_SET8_START(name, global)
> +
>  #define BTF_SET8_END(name)				\
>  asm(							\
>  ".pushsection " BTF_IDS_SECTION ",\"a\";      \n"	\
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 02a2318da7c7..2096b4f00e4b 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -73,6 +73,8 @@ struct udp_tunnel_info;
>  struct udp_tunnel_nic_info;
>  struct udp_tunnel_nic;
>  struct bpf_prog;
> +struct bpf_insn;
> +struct bpf_patch;
>  struct xdp_buff;
>  
>  void synchronize_net(void);
> @@ -1604,6 +1606,9 @@ struct net_device_ops {
>  	ktime_t			(*ndo_get_tstamp)(struct net_device *dev,
>  						  const struct skb_shared_hwtstamps *hwtstamps,
>  						  bool cycles);
> +	void			(*ndo_unroll_kfunc)(const struct bpf_prog *prog,
> +						    u32 func_id,
> +						    struct bpf_patch *patch);
>  };
>  
>  /**
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index 55dbc68bfffc..2a82a98f2f9f 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -7,6 +7,7 @@
>  #define __LINUX_NET_XDP_H__
>  
>  #include <linux/skbuff.h> /* skb_shared_info */
> +#include <linux/btf_ids.h> /* btf_id_set8 */
>  
>  /**
>   * DOC: XDP RX-queue information
> @@ -409,4 +410,27 @@ void xdp_attachment_setup(struct xdp_attachment_info *info,
>  
>  #define DEV_MAP_BULK_SIZE XDP_BULK_QUEUE_SIZE
>  
> +#define XDP_METADATA_KFUNC_xxx	\
> +	XDP_METADATA_KFUNC(XDP_METADATA_KFUNC_RX_TIMESTAMP_SUPPORTED, \
> +			   bpf_xdp_metadata_rx_timestamp_supported) \
> +	XDP_METADATA_KFUNC(XDP_METADATA_KFUNC_RX_TIMESTAMP, \
> +			   bpf_xdp_metadata_rx_timestamp) \
> +
> +enum {
> +#define XDP_METADATA_KFUNC(name, str) name,
> +XDP_METADATA_KFUNC_xxx
> +#undef XDP_METADATA_KFUNC
> +MAX_XDP_METADATA_KFUNC,
> +};
> +
> +#ifdef CONFIG_DEBUG_INFO_BTF
> +extern struct btf_id_set8 xdp_metadata_kfunc_ids;
> +static inline u32 xdp_metadata_kfunc_id(int id)
> +{
> +	return xdp_metadata_kfunc_ids.pairs[id].id;
> +}
> +#else
> +static inline u32 xdp_metadata_kfunc_id(int id) { return 0; }
> +#endif
> +
>  #endif /* __LINUX_NET_XDP_H__ */
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index fb4c911d2a03..b444b1118c4f 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -1156,6 +1156,11 @@ enum bpf_link_type {
>   */
>  #define BPF_F_XDP_HAS_FRAGS	(1U << 5)
>  
> +/* If BPF_F_XDP_HAS_METADATA is used in BPF_PROG_LOAD command, the loaded
> + * program becomes device-bound but can access it's XDP metadata.
> + */
> +#define BPF_F_XDP_HAS_METADATA	(1U << 6)
> +
>  /* link_create.kprobe_multi.flags used in LINK_CREATE command for
>   * BPF_TRACE_KPROBE_MULTI attach type to create return probe.
>   */
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 85532d301124..597c41949910 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -2426,6 +2426,20 @@ static bool is_perfmon_prog_type(enum bpf_prog_type prog_type)
>  /* last field in 'union bpf_attr' used by this command */
>  #define	BPF_PROG_LOAD_LAST_FIELD core_relo_rec_size
>  
> +static int xdp_resolve_netdev(struct bpf_prog *prog, int ifindex)
> +{
> +	struct net *net = current->nsproxy->net_ns;
> +	struct net_device *dev;
> +
> +	for_each_netdev(net, dev) {
> +		if (dev->ifindex == ifindex) {

So this is basically dev_get_by_index(), except you're not doing
dev_hold()? Which also means there's no protection against the netdev
going away?

> +			prog->aux->xdp_kfunc_ndo = dev->netdev_ops;
> +			return 0;
> +		}
> +	}

> +	return -EINVAL;
> +}
> +
>  static int bpf_prog_load(union bpf_attr *attr, bpfptr_t uattr)
>  {
>  	enum bpf_prog_type type = attr->prog_type;
> @@ -2443,7 +2457,8 @@ static int bpf_prog_load(union bpf_attr *attr, bpfptr_t uattr)
>  				 BPF_F_TEST_STATE_FREQ |
>  				 BPF_F_SLEEPABLE |
>  				 BPF_F_TEST_RND_HI32 |
> -				 BPF_F_XDP_HAS_FRAGS))
> +				 BPF_F_XDP_HAS_FRAGS |
> +				 BPF_F_XDP_HAS_METADATA))
>  		return -EINVAL;
>  
>  	if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) &&
> @@ -2531,6 +2546,17 @@ static int bpf_prog_load(union bpf_attr *attr, bpfptr_t uattr)
>  	prog->aux->sleepable = attr->prog_flags & BPF_F_SLEEPABLE;
>  	prog->aux->xdp_has_frags = attr->prog_flags & BPF_F_XDP_HAS_FRAGS;
>  
> +	if (attr->prog_flags & BPF_F_XDP_HAS_METADATA) {
> +		/* Reuse prog_ifindex to carry request to unroll
> +		 * metadata kfuncs.
> +		 */
> +		prog->aux->offload_requested = false;
> +
> +		err = xdp_resolve_netdev(prog, attr->prog_ifindex);
> +		if (err < 0)
> +			goto free_prog;
> +	}
> +
>  	err = security_bpf_prog_alloc(prog->aux);
>  	if (err)
>  		goto free_prog;
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 07c0259dfc1a..b657ed6eb277 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9,6 +9,7 @@
>  #include <linux/types.h>
>  #include <linux/slab.h>
>  #include <linux/bpf.h>
> +#include <linux/bpf_patch.h>
>  #include <linux/btf.h>
>  #include <linux/bpf_verifier.h>
>  #include <linux/filter.h>
> @@ -14015,6 +14016,43 @@ static int fixup_call_args(struct bpf_verifier_env *env)
>  	return err;
>  }
>  
> +static int unroll_kfunc_call(struct bpf_verifier_env *env,
> +			     struct bpf_insn *insn,
> +			     struct bpf_patch *patch)
> +{
> +	enum bpf_prog_type prog_type;
> +	struct bpf_prog_aux *aux;
> +	struct btf *desc_btf;
> +	u32 *kfunc_flags;
> +	u32 func_id;
> +
> +	desc_btf = find_kfunc_desc_btf(env, insn->off);
> +	if (IS_ERR(desc_btf))
> +		return PTR_ERR(desc_btf);
> +
> +	prog_type = resolve_prog_type(env->prog);
> +	func_id = insn->imm;
> +
> +	kfunc_flags = btf_kfunc_id_set_contains(desc_btf, prog_type, func_id);
> +	if (!kfunc_flags)
> +		return 0;
> +	if (!(*kfunc_flags & KF_UNROLL))
> +		return 0;
> +	if (prog_type != BPF_PROG_TYPE_XDP)
> +		return 0;

Should this just handle XDP_METADATA_KFUNC_EXPORT_TO_SKB instead of
passing that into the driver (to avoid every driver having to
reimplement the same call to xdp_metadata_export_to_skb())?

-Toke