On Fri, 30 Oct 2020 12:40:21 -0700
John Fastabend <john.fastabend@xxxxxxxxx> 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@xxxxxxxxx>
> > Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> > Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
> > ---
> > 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.
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
struct bpf_fib_lookup {
/* input: network family for lookup (AF_INET, AF_INET6)
* output: network family of egress nexthop
*/
__u8 family;
/* set if lookup is to consider L4 data - e.g., FIB rules */
__u8 l4_protocol;
__be16 sport;
__be16 dport;
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
*/
__u32 ifindex;
union {
/* inputs to lookup */
__u8 tos; /* AF_INET */
__be32 flowinfo; /* AF_INET6, flow_label + priority */
/* output: metric of fib result (IPv4/IPv6 only) */
__u32 rt_metric;
};
union {
__be32 ipv4_src;
__u32 ipv6_src[4]; /* in6_addr; network order */
};
/* input to bpf_fib_lookup, ipv{4,6}_dst is destination address in
* network header. output: bpf_fib_lookup sets to gateway address
* if FIB lookup returns gateway route
*/
union {
__be32 ipv4_dst;
__u32 ipv6_dst[4]; /* in6_addr; network order */
};
/* output */
__be16 h_vlan_proto;
__be16 h_vlan_TCI;
__u8 smac[6]; /* ETH_ALEN */
__u8 dmac[6]; /* ETH_ALEN */
};
