Commit 43e74c02 authored by Toke Høiland-Jørgensen's avatar Toke Høiland-Jørgensen Committed by Daniel Borkmann
Browse files

bpf_xdp_redirect_map: Perform map lookup in eBPF helper 



The bpf_redirect_map() helper used by XDP programs doesn't return any
indication of whether it can successfully redirect to the map index it was
given. Instead, BPF programs have to track this themselves, leading to
programs using duplicate maps to track which entries are populated in the
devmap.

This patch fixes this by moving the map lookup into the bpf_redirect_map()
helper, which makes it possible to return failure to the eBPF program. The
lower bits of the flags argument is used as the return code, which means
that existing users who pass a '0' flag argument will get XDP_ABORTED.

With this, a BPF program can check the return code from the helper call and
react by, for instance, substituting a different redirect. This works for
any type of map used for redirect.

Signed-off-by: default avatarToke Høiland-Jørgensen <toke@redhat.com>
Acked-by: default avatarJonathan Lemon <jonathan.lemon@gmail.com>
Acked-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
parent 4b55cf29

include/linux/filter.h

+1 −0
Original line number Diff line number Diff line
@@ -580,6 +580,7 @@ struct bpf_skb_data_end { 
struct bpf_redirect_info {

	u32 flags;

	u32 tgt_index;

	void *tgt_value;

	struct bpf_map *map;

	struct bpf_map *map_to_flush;

	u32 kern_flags;

include/trace/events/xdp.h

+2 −3
Original line number Diff line number Diff line
@@ -175,9 +175,8 @@ struct _bpf_dtab_netdev { 
#endif /* __DEVMAP_OBJ_TYPE */



#define devmap_ifindex(fwd, map)				\

	(!fwd ? 0 :						\

	((map->map_type == BPF_MAP_TYPE_DEVMAP) ?		\

	  ((struct _bpf_dtab_netdev *)fwd)->dev->ifindex : 0))

	  ((struct _bpf_dtab_netdev *)fwd)->dev->ifindex : 0)



#define _trace_xdp_redirect_map(dev, xdp, fwd, map, idx)		\

	 trace_xdp_redirect_map(dev, xdp, devmap_ifindex(fwd, map),	\

include/uapi/linux/bpf.h

+5 −2
Original line number Diff line number Diff line
@@ -1571,8 +1571,11 @@ union bpf_attr { 
 * 		but this is only implemented for native XDP (with driver

 * 		support) as of this writing).

 *

 * 		All values for *flags* are reserved for future usage, and must

 * 		be left at zero.

 * 		The lower two bits of *flags* are used as the return code if

 * 		the map lookup fails. This is so that the return value can be

 * 		one of the XDP program return codes up to XDP_TX, as chosen by

 * 		the caller. Any higher bits in the *flags* argument must be

 * 		unset.

 *

 * 		When used to redirect packets to net devices, this helper

 * 		provides a high performance increase over **bpf_redirect**\ ().

net/core/filter.c

+18 −14
Original line number Diff line number Diff line
@@ -3605,17 +3605,13 @@ static int xdp_do_redirect_map(struct net_device *dev, struct xdp_buff *xdp, 
			       struct bpf_redirect_info *ri)

{

	u32 index = ri->tgt_index;

	void *fwd = NULL;

	void *fwd = ri->tgt_value;

	int err;



	ri->tgt_index = 0;

	ri->tgt_value = NULL;

	WRITE_ONCE(ri->map, NULL);



	fwd = __xdp_map_lookup_elem(map, index);

	if (unlikely(!fwd)) {

		err = -EINVAL;

		goto err;

	}

	if (ri->map_to_flush && unlikely(ri->map_to_flush != map))

		xdp_do_flush_map();
@@ -3652,18 +3648,13 @@ static int xdp_do_generic_redirect_map(struct net_device *dev, 
{

	struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);

	u32 index = ri->tgt_index;

	void *fwd = NULL;

	void *fwd = ri->tgt_value;

	int err = 0;



	ri->tgt_index = 0;

	ri->tgt_value = NULL;

	WRITE_ONCE(ri->map, NULL);



	fwd = __xdp_map_lookup_elem(map, index);

	if (unlikely(!fwd)) {

		err = -EINVAL;

		goto err;

	}



	if (map->map_type == BPF_MAP_TYPE_DEVMAP) {

		struct bpf_dtab_netdev *dst = fwd;
@@ -3732,6 +3723,7 @@ BPF_CALL_2(bpf_xdp_redirect, u32, ifindex, u64, flags) 


	ri->flags = flags;

	ri->tgt_index = ifindex;

	ri->tgt_value = NULL;

	WRITE_ONCE(ri->map, NULL);



	return XDP_REDIRECT;
@@ -3750,9 +3742,21 @@ BPF_CALL_3(bpf_xdp_redirect_map, struct bpf_map *, map, u32, ifindex, 
{

	struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);



	if (unlikely(flags))

	/* Lower bits of the flags are used as return code on lookup failure */

	if (unlikely(flags > XDP_TX))

		return XDP_ABORTED;



	ri->tgt_value = __xdp_map_lookup_elem(map, ifindex);

	if (unlikely(!ri->tgt_value)) {

		/* If the lookup fails we want to clear out the state in the

		 * redirect_info struct completely, so that if an eBPF program

		 * performs multiple lookups, the last one always takes

		 * precedence.

		 */

		WRITE_ONCE(ri->map, NULL);

		return flags;

	}



	ri->flags = flags;

	ri->tgt_index = ifindex;

	WRITE_ONCE(ri->map, map);

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