netdev - Re: [PATCH bpf-next] bpf: Allow XDP dev bounded program to perform XDP

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Message-ID: <aAi80as6PpOeuWJU@lore-desk>
Date: Wed, 23 Apr 2025 12:11:29 +0200
From: Lorenzo Bianconi <lorenzo@...nel.org>
To: Stanislav Fomichev <stfomichev@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	John Fastabend <john.fastabend@...il.com>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>,
	Jesper Dangaard Brouer <hawk@...nel.org>, bpf@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next] bpf: Allow XDP dev bounded program to perform
 XDP_REDIRECT into maps

On Apr 22, Stanislav Fomichev wrote:
> On 04/22, Lorenzo Bianconi wrote:
> > In the current implementation if the program is bounded to a specific
> > device, it will not be possible to perform XDP_REDIRECT into a DEVMAP
> > or CPUMAP even if the program is not attached to the map entry. This
> > seems in contrast with the explanation available in
> > bpf_prog_map_compatible routine. Fix the issue taking into account
> > even the attach program type and allow XDP dev bounded program to
> > perform XDP_REDIRECT into maps if the attach type is not BPF_XDP_DEVMAP
> > or BPF_XDP_CPUMAP.
> > 
> > Fixes: 3d76a4d3d4e59 ("bpf: XDP metadata RX kfuncs")
> > Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> > ---
> >  kernel/bpf/core.c | 22 +++++++++++++++++++++-
> >  1 file changed, 21 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > index ba6b6118cf504041278d05417c4212d57be6fca0..a33175efffc377edbfe281397017eb467bfbcce9 100644
> > --- a/kernel/bpf/core.c
> > +++ b/kernel/bpf/core.c
> > @@ -2358,6 +2358,26 @@ static unsigned int __bpf_prog_ret0_warn(const void *ctx,
> >  	return 0;
> >  }
> >  
> > +static bool bpf_prog_dev_bound_map_compatible(struct bpf_map *map,
> > +					      const struct bpf_prog *prog)
> > +{
> > +	if (!bpf_prog_is_dev_bound(prog->aux))
> > +		return true;
> > +
> > +	if (map->map_type == BPF_MAP_TYPE_PROG_ARRAY)
> > +		return false;
> 
> [..]
> 
> > +	if (map->map_type == BPF_MAP_TYPE_DEVMAP &&
> > +	    prog->expected_attach_type != BPF_XDP_DEVMAP)
> > +		return true;
> > +
> > +	if (map->map_type == BPF_MAP_TYPE_CPUMAP &&
> > +	    prog->expected_attach_type != BPF_XDP_CPUMAP)
> > +		return true;
> 
> Not sure I understand, what does it mean exactly? That it's ok to add
> a dev-bound program to the dev/cpumap if the program itself is gonna
> be attached only to the real device? Can you expand more on the specific
> use-case?
> 
> The existing check makes sure that the dev-bound programs run only in the
> contexts that have hw descriptors. devmap and cpumap don't satisfy
> this constraint afaiu.

My use-case is to use a hw-metadata kfunc like bpf_xdp_metadata_rx_timestamp()
to read hw timestamp from the NIC and then redirect the xdp_buff into a DEVMP
(please note there are no programs attached to any DEVMAP entries):

extern int bpf_xdp_metadata_rx_timestamp(const struct xdp_md *ctx,
					 __u64 *timestamp) __ksym;

struct {
	__uint(type, BPF_MAP_TYPE_DEVMAP);
	__uint(key_size, sizeof(__u32));
	__uint(value_size, sizeof(struct bpf_devmap_val));
	__uint(max_entries, 1);
} dev_map SEC(".maps");

SEC("xdp")
int xdp_meta_redirect(struct xdp_md *ctx)
{
	__u64 timestamp;

	...
	bpf_xdp_metadata_rx_timestamp(ctx, &timestamp);
	...

	return bpf_redirect_map(&dev_map, ctx->rx_queue_index, XDP_PASS);
}

According to my understanding this is feasible just if the "xdp_meta_redirect"
program is bounded to a device otherwise the program is reject with the following
error at load time:

libbpf: prog 'xdp_meta_redirect': BPF program load failed: -EINVAL
libbpf: prog 'xdp_meta_redirect': -- BEGIN PROG LOAD LOG --
metadata kfuncs require device-bound program
processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0
peak_states 0 mark_read 0
-- END PROG LOAD LOG --

in order to fix it:

	...
	index = if_nametoindex(DEV); 
	bpf_program__set_ifindex(prog, index);
	bpf_program__set_flags(prog, BPF_F_XDP_DEV_BOUND_ONLY);
	...

Doing so the program load still fails for the check in bpf_prog_map_compatible():

	bool bpf_prog_map_compatible()
	{
		...
		if (bpf_prog_is_dev_bound(aux))
			return false;
		...

In other words, a dev-bound XDP program can't interact with a DEVMAP (or
CPUMAP) even if it is not attached to a map entry.
I think if the XDP program is just running in the driver NAPI context
it should be doable to use a hw-metada kfunc and perform a redirect into
a DEVMAP or CPUMAP, right? Am I missing something?

Regards,
Lorenzo

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