diff mbox series

[bpf-next,V5,2/5] bpf: bpf_fib_lookup return MTU value as output when looked up

Message ID 160407665728.1525159.18300199766779492971.stgit@firesoul (mailing list archive)
State Changes Requested
Delegated to: BPF
Headers show
Series Subj: bpf: New approach for BPF MTU handling | expand

Commit Message

Jesper Dangaard Brouer Oct. 30, 2020, 4:50 p.m. UTC
The BPF-helpers for FIB lookup (bpf_xdp_fib_lookup and bpf_skb_fib_lookup)
can perform MTU check and return BPF_FIB_LKUP_RET_FRAG_NEEDED.  The BPF-prog
don't know the MTU value that caused this rejection.

If the BPF-prog wants to implement PMTU (Path MTU Discovery) (rfc1191) it
need to know this MTU value for the ICMP packet.

Patch change lookup and result struct bpf_fib_lookup, to contain this MTU
value as output via a union with 'tot_len' as this is the value used for
the MTU lookup.

V5:
 - Fixed uninit value spotted by Dan Carpenter.
 - Name struct output member mtu_result

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 include/uapi/linux/bpf.h       |   11 +++++++++--
 net/core/filter.c              |   22 +++++++++++++++-------
 tools/include/uapi/linux/bpf.h |   11 +++++++++--
 3 files changed, 33 insertions(+), 11 deletions(-)

Comments

John Fastabend Oct. 30, 2020, 7:40 p.m. UTC | #1
Jesper Dangaard Brouer wrote:
> The BPF-helpers for FIB lookup (bpf_xdp_fib_lookup and bpf_skb_fib_lookup)
> can perform MTU check and return BPF_FIB_LKUP_RET_FRAG_NEEDED.  The BPF-prog
> don't know the MTU value that caused this rejection.
> 
> If the BPF-prog wants to implement PMTU (Path MTU Discovery) (rfc1191) it
> need to know this MTU value for the ICMP packet.
> 
> Patch change lookup and result struct bpf_fib_lookup, to contain this MTU
> value as output via a union with 'tot_len' as this is the value used for
> the MTU lookup.
> 
> V5:
>  - Fixed uninit value spotted by Dan Carpenter.
>  - Name struct output member mtu_result
> 
> Reported-by: kernel test robot <lkp@intel.com>
> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> ---
>  include/uapi/linux/bpf.h       |   11 +++++++++--
>  net/core/filter.c              |   22 +++++++++++++++-------
>  tools/include/uapi/linux/bpf.h |   11 +++++++++--
>  3 files changed, 33 insertions(+), 11 deletions(-)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index e6ceac3f7d62..01b2b17c645a 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -2219,6 +2219,9 @@ union bpf_attr {
>   *		* > 0 one of **BPF_FIB_LKUP_RET_** codes explaining why the
>   *		  packet is not forwarded or needs assist from full stack
>   *
> + *		If lookup fails with BPF_FIB_LKUP_RET_FRAG_NEEDED, then the MTU
> + *		was exceeded and result params->mtu contains the MTU.
> + *

Do we need to hide this behind a flag? It seems otherwise you might confuse
users. I imagine on error we could reuse the params arg, but now we changed
the tot_len value underneath them?

>   * long bpf_sock_hash_update(struct bpf_sock_ops *skops, struct bpf_map *map, void *key, u64 flags)
>   *	Description
>   *		Add an entry to, or update a sockhash *map* referencing sockets.
> @@ -4872,9 +4875,13 @@ struct bpf_fib_lookup {
>  	__be16	sport;
>  	__be16	dport;
David Ahern Oct. 31, 2020, 3:52 p.m. UTC | #2
On 10/30/20 10:50 AM, Jesper Dangaard Brouer wrote:
> The BPF-helpers for FIB lookup (bpf_xdp_fib_lookup and bpf_skb_fib_lookup)
> can perform MTU check and return BPF_FIB_LKUP_RET_FRAG_NEEDED.  The BPF-prog
> don't know the MTU value that caused this rejection.
> 
> If the BPF-prog wants to implement PMTU (Path MTU Discovery) (rfc1191) it
> need to know this MTU value for the ICMP packet.
> 
> Patch change lookup and result struct bpf_fib_lookup, to contain this MTU
> value as output via a union with 'tot_len' as this is the value used for
> the MTU lookup.
> 
> V5:
>  - Fixed uninit value spotted by Dan Carpenter.
>  - Name struct output member mtu_result
> 
> Reported-by: kernel test robot <lkp@intel.com>
> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> ---
>  include/uapi/linux/bpf.h       |   11 +++++++++--
>  net/core/filter.c              |   22 +++++++++++++++-------
>  tools/include/uapi/linux/bpf.h |   11 +++++++++--
>  3 files changed, 33 insertions(+), 11 deletions(-)
> 

Reviewed-by: David Ahern <dsahern@kernel.org>
Jesper Dangaard Brouer Nov. 2, 2020, 9:28 a.m. UTC | #3
On Fri, 30 Oct 2020 12:40:21 -0700
John Fastabend <john.fastabend@gmail.com> wrote:

> Jesper Dangaard Brouer wrote:
> > The BPF-helpers for FIB lookup (bpf_xdp_fib_lookup and bpf_skb_fib_lookup)
> > can perform MTU check and return BPF_FIB_LKUP_RET_FRAG_NEEDED.  The BPF-prog
> > don't know the MTU value that caused this rejection.
> > 
> > If the BPF-prog wants to implement PMTU (Path MTU Discovery) (rfc1191) it
> > need to know this MTU value for the ICMP packet.
> > 
> > Patch change lookup and result struct bpf_fib_lookup, to contain this MTU
> > value as output via a union with 'tot_len' as this is the value used for
> > the MTU lookup.
> > 
> > V5:
> >  - Fixed uninit value spotted by Dan Carpenter.
> >  - Name struct output member mtu_result
> > 
> > Reported-by: kernel test robot <lkp@intel.com>
> > Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> > Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> > ---
> >  include/uapi/linux/bpf.h       |   11 +++++++++--
> >  net/core/filter.c              |   22 +++++++++++++++-------
> >  tools/include/uapi/linux/bpf.h |   11 +++++++++--
> >  3 files changed, 33 insertions(+), 11 deletions(-)
> > 
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index e6ceac3f7d62..01b2b17c645a 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -2219,6 +2219,9 @@ union bpf_attr {
> >   *		* > 0 one of **BPF_FIB_LKUP_RET_** codes explaining why the
> >   *		  packet is not forwarded or needs assist from full stack
> >   *
> > + *		If lookup fails with BPF_FIB_LKUP_RET_FRAG_NEEDED, then the MTU
> > + *		was exceeded and result params->mtu contains the MTU.
> > + *  
> 
> Do we need to hide this behind a flag? It seems otherwise you might confuse
> users. I imagine on error we could reuse the params arg, but now we changed
> the tot_len value underneath them?

The principle behind this bpf_fib_lookup helper, is that params (struct
bpf_fib_lookup) is used for both input and output (results). Almost
every field is change after the lookup. (For performance reasons this
is kept at 64 bytes (cache-line))  Thus, users of this helper already
expect/knows the contents of params have changed.
David Ahern Nov. 2, 2020, 3:59 p.m. UTC | #4
On 11/2/20 2:28 AM, Jesper Dangaard Brouer wrote:
>>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>>> index e6ceac3f7d62..01b2b17c645a 100644
>>> --- a/include/uapi/linux/bpf.h
>>> +++ b/include/uapi/linux/bpf.h
>>> @@ -2219,6 +2219,9 @@ union bpf_attr {
>>>   *		* > 0 one of **BPF_FIB_LKUP_RET_** codes explaining why the
>>>   *		  packet is not forwarded or needs assist from full stack
>>>   *
>>> + *		If lookup fails with BPF_FIB_LKUP_RET_FRAG_NEEDED, then the MTU
>>> + *		was exceeded and result params->mtu contains the MTU.
>>> + *  
>>
>> Do we need to hide this behind a flag? It seems otherwise you might confuse
>> users. I imagine on error we could reuse the params arg, but now we changed
>> the tot_len value underneath them?
> 
> The principle behind this bpf_fib_lookup helper, is that params (struct
> bpf_fib_lookup) is used for both input and output (results). Almost
> every field is change after the lookup. (For performance reasons this
> is kept at 64 bytes (cache-line))  Thus, users of this helper already
> expect/knows the contents of params have changed.
> 

yes, that was done on purpose.

Jesper: you should remove the '(if requested check_mtu)' comment in the
documentation. That is an internal flag only -- xdp is true, tc is false.
John Fastabend Nov. 2, 2020, 4:18 p.m. UTC | #5
David Ahern wrote:
> On 11/2/20 2:28 AM, Jesper Dangaard Brouer wrote:
> >>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> >>> index e6ceac3f7d62..01b2b17c645a 100644
> >>> --- a/include/uapi/linux/bpf.h
> >>> +++ b/include/uapi/linux/bpf.h
> >>> @@ -2219,6 +2219,9 @@ union bpf_attr {
> >>>   *		* > 0 one of **BPF_FIB_LKUP_RET_** codes explaining why the
> >>>   *		  packet is not forwarded or needs assist from full stack
> >>>   *
> >>> + *		If lookup fails with BPF_FIB_LKUP_RET_FRAG_NEEDED, then the MTU
> >>> + *		was exceeded and result params->mtu contains the MTU.
> >>> + *  
> >>
> >> Do we need to hide this behind a flag? It seems otherwise you might confuse
> >> users. I imagine on error we could reuse the params arg, but now we changed
> >> the tot_len value underneath them?
> > 
> > The principle behind this bpf_fib_lookup helper, is that params (struct
> > bpf_fib_lookup) is used for both input and output (results). Almost
> > every field is change after the lookup. (For performance reasons this
> > is kept at 64 bytes (cache-line))  Thus, users of this helper already
> > expect/knows the contents of params have changed.
> > 
> 
> yes, that was done on purpose.

OK sounds good then. Thanks.

> 
> Jesper: you should remove the '(if requested check_mtu)' comment in the
> documentation. That is an internal flag only -- xdp is true, tc is false.
diff mbox series

Patch

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index e6ceac3f7d62..01b2b17c645a 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -2219,6 +2219,9 @@  union bpf_attr {
  *		* > 0 one of **BPF_FIB_LKUP_RET_** codes explaining why the
  *		  packet is not forwarded or needs assist from full stack
  *
+ *		If lookup fails with BPF_FIB_LKUP_RET_FRAG_NEEDED, then the MTU
+ *		was exceeded and result params->mtu contains the MTU.
+ *
  * long bpf_sock_hash_update(struct bpf_sock_ops *skops, struct bpf_map *map, void *key, u64 flags)
  *	Description
  *		Add an entry to, or update a sockhash *map* referencing sockets.
@@ -4872,9 +4875,13 @@  struct bpf_fib_lookup {
 	__be16	sport;
 	__be16	dport;
 
-	/* total length of packet from network header - used for MTU check */
-	__u16	tot_len;
+	union {	/* used for MTU check */
+		/* input to lookup */
+		__u16	tot_len; /* total length of packet from network hdr */
 
+		/* output: MTU value (if requested check_mtu) */
+		__u16	mtu_result;
+	};
 	/* input: L3 device index for lookup
 	 * output: device index from FIB lookup
 	 */
diff --git a/net/core/filter.c b/net/core/filter.c
index 1ee97fdeea64..edb543c477b6 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5265,12 +5265,14 @@  static const struct bpf_func_proto bpf_skb_get_xfrm_state_proto = {
 #if IS_ENABLED(CONFIG_INET) || IS_ENABLED(CONFIG_IPV6)
 static int bpf_fib_set_fwd_params(struct bpf_fib_lookup *params,
 				  const struct neighbour *neigh,
-				  const struct net_device *dev)
+				  const struct net_device *dev, u32 mtu)
 {
 	memcpy(params->dmac, neigh->ha, ETH_ALEN);
 	memcpy(params->smac, dev->dev_addr, ETH_ALEN);
 	params->h_vlan_TCI = 0;
 	params->h_vlan_proto = 0;
+	if (mtu)
+		params->mtu_result = mtu; /* union with tot_len */
 
 	return 0;
 }
@@ -5286,8 +5288,8 @@  static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 	struct net_device *dev;
 	struct fib_result res;
 	struct flowi4 fl4;
+	u32 mtu = 0;
 	int err;
-	u32 mtu;
 
 	dev = dev_get_by_index_rcu(net, params->ifindex);
 	if (unlikely(!dev))
@@ -5354,8 +5356,10 @@  static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 
 	if (check_mtu) {
 		mtu = ip_mtu_from_fib_result(&res, params->ipv4_dst);
-		if (params->tot_len > mtu)
+		if (params->tot_len > mtu) {
+			params->mtu_result = mtu; /* union with tot_len */
 			return BPF_FIB_LKUP_RET_FRAG_NEEDED;
+		}
 	}
 
 	nhc = res.nhc;
@@ -5389,7 +5393,7 @@  static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 	if (!neigh)
 		return BPF_FIB_LKUP_RET_NO_NEIGH;
 
-	return bpf_fib_set_fwd_params(params, neigh, dev);
+	return bpf_fib_set_fwd_params(params, neigh, dev, mtu);
 }
 #endif
 
@@ -5406,7 +5410,7 @@  static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 	struct flowi6 fl6;
 	int strict = 0;
 	int oif, err;
-	u32 mtu;
+	u32 mtu = 0;
 
 	/* link local addresses are never forwarded */
 	if (rt6_need_strict(dst) || rt6_need_strict(src))
@@ -5481,8 +5485,10 @@  static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 
 	if (check_mtu) {
 		mtu = ipv6_stub->ip6_mtu_from_fib6(&res, dst, src);
-		if (params->tot_len > mtu)
+		if (params->tot_len > mtu) {
+			params->mtu_result = mtu; /* union with tot_len */
 			return BPF_FIB_LKUP_RET_FRAG_NEEDED;
+		}
 	}
 
 	if (res.nh->fib_nh_lws)
@@ -5502,7 +5508,7 @@  static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 	if (!neigh)
 		return BPF_FIB_LKUP_RET_NO_NEIGH;
 
-	return bpf_fib_set_fwd_params(params, neigh, dev);
+	return bpf_fib_set_fwd_params(params, neigh, dev, mtu);
 }
 #endif
 
@@ -5571,6 +5577,8 @@  BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
 		dev = dev_get_by_index_rcu(net, params->ifindex);
 		if (!is_skb_forwardable(dev, skb))
 			rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
+
+		params->mtu_result = dev->mtu; /* union with tot_len */
 	}
 
 	return rc;
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index e6ceac3f7d62..01b2b17c645a 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -2219,6 +2219,9 @@  union bpf_attr {
  *		* > 0 one of **BPF_FIB_LKUP_RET_** codes explaining why the
  *		  packet is not forwarded or needs assist from full stack
  *
+ *		If lookup fails with BPF_FIB_LKUP_RET_FRAG_NEEDED, then the MTU
+ *		was exceeded and result params->mtu contains the MTU.
+ *
  * long bpf_sock_hash_update(struct bpf_sock_ops *skops, struct bpf_map *map, void *key, u64 flags)
  *	Description
  *		Add an entry to, or update a sockhash *map* referencing sockets.
@@ -4872,9 +4875,13 @@  struct bpf_fib_lookup {
 	__be16	sport;
 	__be16	dport;
 
-	/* total length of packet from network header - used for MTU check */
-	__u16	tot_len;
+	union {	/* used for MTU check */
+		/* input to lookup */
+		__u16	tot_len; /* total length of packet from network hdr */
 
+		/* output: MTU value (if requested check_mtu) */
+		__u16	mtu_result;
+	};
 	/* input: L3 device index for lookup
 	 * output: device index from FIB lookup
 	 */