Re: [RFC bpf-next 1/6] bpf: xfrm: Add bpf_xdp_get_xfrm_state() kfunc

From: Daniel Xu
Date: Wed Nov 01 2023 - 13:51:55 EST

On Tue, Oct 31, 2023 at 03:38:26PM -0700, Alexei Starovoitov wrote:
> On Sun, Oct 29, 2023 at 3:55 PM Daniel Xu <dxu@xxxxxxxxx> wrote:
> >
> > Hi Alexei,
> >
> > On Sat, Oct 28, 2023 at 04:49:45PM -0700, Alexei Starovoitov wrote:
> > > On Fri, Oct 27, 2023 at 11:46 AM Daniel Xu <dxu@xxxxxxxxx> wrote:
> > > >
> > > > This commit adds an unstable kfunc helper to access internal xfrm_state
> > > > associated with an SA. This is intended to be used for the upcoming
> > > > IPsec pcpu work to assign special pcpu SAs to a particular CPU. In other
> > > > words: for custom software RSS.
> > > >
> > > > That being said, the function that this kfunc wraps is fairly generic
> > > > and used for a lot of xfrm tasks. I'm sure people will find uses
> > > > elsewhere over time.
> > > >
> > > > Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> > > > ---
> > > > include/net/xfrm.h | 9 ++++
> > > > net/xfrm/Makefile | 1 +
> > > > net/xfrm/xfrm_policy.c | 2 +
> > > > net/xfrm/xfrm_state_bpf.c | 105 ++++++++++++++++++++++++++++++++++++++
> > > > 4 files changed, 117 insertions(+)
> > > > create mode 100644 net/xfrm/xfrm_state_bpf.c
> > > >
> > > > diff --git a/include/net/xfrm.h b/include/net/xfrm.h
> > > > index 98d7aa78adda..ab4cf66480f3 100644
> > > > --- a/include/net/xfrm.h
> > > > +++ b/include/net/xfrm.h
> > > > @@ -2188,4 +2188,13 @@ static inline int register_xfrm_interface_bpf(void)
> > > >
> > > > #endif
> > > >
> > > > +#if IS_ENABLED(CONFIG_DEBUG_INFO_BTF)
> > > > +int register_xfrm_state_bpf(void);
> > > > +#else
> > > > +static inline int register_xfrm_state_bpf(void)
> > > > +{
> > > > + return 0;
> > > > +}
> > > > +#endif
> > > > +
> > > > #endif /* _NET_XFRM_H */
> > > > diff --git a/net/xfrm/Makefile b/net/xfrm/Makefile
> > > > index cd47f88921f5..547cec77ba03 100644
> > > > --- a/net/xfrm/Makefile
> > > > +++ b/net/xfrm/Makefile
> > > > @@ -21,3 +21,4 @@ obj-$(CONFIG_XFRM_USER_COMPAT) += xfrm_compat.o
> > > > obj-$(CONFIG_XFRM_IPCOMP) += xfrm_ipcomp.o
> > > > obj-$(CONFIG_XFRM_INTERFACE) += xfrm_interface.o
> > > > obj-$(CONFIG_XFRM_ESPINTCP) += espintcp.o
> > > > +obj-$(CONFIG_DEBUG_INFO_BTF) += xfrm_state_bpf.o
> > > > diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
> > > > index 5cdd3bca3637..62e64fa7ae5c 100644
> > > > --- a/net/xfrm/xfrm_policy.c
> > > > +++ b/net/xfrm/xfrm_policy.c
> > > > @@ -4267,6 +4267,8 @@ void __init xfrm_init(void)
> > > > #ifdef CONFIG_XFRM_ESPINTCP
> > > > espintcp_init();
> > > > #endif
> > > > +
> > > > + register_xfrm_state_bpf();
> > > > }
> > > >
> > > > #ifdef CONFIG_AUDITSYSCALL
> > > > diff --git a/net/xfrm/xfrm_state_bpf.c b/net/xfrm/xfrm_state_bpf.c
> > > > new file mode 100644
> > > > index 000000000000..a73a17a6497b
> > > > --- /dev/null
> > > > +++ b/net/xfrm/xfrm_state_bpf.c
> > > > @@ -0,0 +1,105 @@
> > > > +// SPDX-License-Identifier: GPL-2.0-only
> > > > +/* Unstable XFRM state BPF helpers.
> > > > + *
> > > > + * 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/xdp.h>
> > > > +#include <net/xfrm.h>
> > > > +
> > > > +/* bpf_xfrm_state_opts - Options for XFRM state lookup helpers
> > > > + *
> > > > + * Members:
> > > > + * @error - Out parameter, set for any errors encountered
> > > > + * Values:
> > > > + * -EINVAL - netns_id is less than -1
> > > > + * -EINVAL - Passed NULL for opts
> > > > + * -EINVAL - opts__sz isn't BPF_XFRM_STATE_OPTS_SZ
> > > > + * -ENONET - No network namespace found for netns_id
> > > > + * @netns_id - Specify the network namespace for lookup
> > > > + * Values:
> > > > + * BPF_F_CURRENT_NETNS (-1)
> > > > + * Use namespace associated with ctx
> > > > + * [0, S32_MAX]
> > > > + * Network Namespace ID
> > > > + * @mark - XFRM mark to match on
> > > > + * @daddr - Destination address to match on
> > > > + * @spi - Security parameter index to match on
> > > > + * @proto - L3 protocol to match on
> > > > + * @family - L3 protocol family to match on
> > > > + */
> > > > +struct bpf_xfrm_state_opts {
> > > > + s32 error;
> > > > + s32 netns_id;
> > > > + u32 mark;
> > > > + xfrm_address_t daddr;
> > > > + __be32 spi;
> > > > + u8 proto;
> > > > + u16 family;
> > > > +};
> > > > +
> > > > +enum {
> > > > + BPF_XFRM_STATE_OPTS_SZ = sizeof(struct bpf_xfrm_state_opts),
> > > > +};
> > > > +
> > > > +__diag_push();
> > > > +__diag_ignore_all("-Wmissing-prototypes",
> > > > + "Global functions as their definitions will be in xfrm_state BTF");
> > > > +
> > > > +/* bpf_xdp_get_xfrm_state - Get XFRM state
> > > > + *
> > > > + * Parameters:
> > > > + * @ctx - Pointer to ctx (xdp_md) in XDP program
> > > > + * Cannot be NULL
> > > > + * @opts - Options for lookup (documented above)
> > > > + * Cannot be NULL
> > > > + * @opts__sz - Length of the bpf_xfrm_state_opts structure
> > > > + * Must be BPF_XFRM_STATE_OPTS_SZ
> > > > + */
> > > > +__bpf_kfunc struct xfrm_state *
> > > > +bpf_xdp_get_xfrm_state(struct xdp_md *ctx, struct bpf_xfrm_state_opts *opts, u32 opts__sz)
> > > > +{
> > > > + struct xdp_buff *xdp = (struct xdp_buff *)ctx;
> > > > + struct net *net = dev_net(xdp->rxq->dev);
> > > > +
> > > > + if (!opts || opts__sz != BPF_XFRM_STATE_OPTS_SZ) {
> > > > + opts->error = -EINVAL;
> > > > + return NULL;
> > > > + }
> > > > +
> > > > + if (unlikely(opts->netns_id < BPF_F_CURRENT_NETNS)) {
> > > > + opts->error = -EINVAL;
> > > > + return NULL;
> > > > + }
> > > > +
> > > > + if (opts->netns_id >= 0) {
> > > > + net = get_net_ns_by_id(net, opts->netns_id);
> > > > + if (unlikely(!net)) {
> > > > + opts->error = -ENONET;
> > > > + return NULL;
> > > > + }
> > > > + }
> > > > +
> > > > + return xfrm_state_lookup(net, opts->mark, &opts->daddr, opts->spi,
> > > > + opts->proto, opts->family);
> > > > +}
> > >
> > > Patch 6 example does little to explain how this kfunc can be used.
> > > Cover letter sounds promising, but no code to demonstrate the result.
> >
> > Part of the reason for that is this kfunc is intended to be used with a
> > not-yet-upstreamed xfrm patchset. The other is that the usage is quite
> > trivial. This is the code the experiments were run with:
> >
> > https://github.com/danobi/xdp-tools/blob/e89a1c617aba3b50d990f779357d6ce2863ecb27/xdp-bench/xdp_redirect_cpumap.bpf.c#L385-L406
> >
> > We intend to upstream that cpumap mode to xdp-tools as soon as the xfrm
> > patches are in. (Note the linked code is a little buggy but the
> > main idea is there).
>
> I don't understand how it survives anything, but sanity check.
> To measure perf gains it needs to be under traffic for some time,
> but
> x = bpf_xdp_get_xfrm_state(ctx, &opts, sizeof(opts));
> will keep refcnt++ that state for every packet.
> Minimum -> memory leak or refcnt overflow.

Yeah, I agree the code in this patchset is not correct. I have the fix
(a KF_RELEASE wrapper around xfrm_state_put()) ready to send. I think
Steffen was gonna chat w/ you about this at IETF next week. But I can
send it now if you'd like.

To answer your question why it doesn't blow up immediately:

* The test system only has ~33 inbound SAs and the test doesn't try to
delete any. So leak is not noticed in the test. Oddly enough I recall
`ip x s flush` working correctly... Could be misremembering.

* Refcnt overflow will indeed happen, but some rough math shows it'll
take about 12 hrs receiving at 100Gbps for that to happen. 100Gbps =
12.5 GB/s. 12.5GB / (32 CPUs) / (9000B) = 43k pps for each pcpu SA.
INT_MAX = 2 billion. 2B / 4k = 46k. 46k seconds to hours is ~12 hrs.
And I was only running traffic for ~1 hour.

At least I think that math is right.

Thanks,
Daniel