Subject: Re: [PATCH bpf-next] bpf: Allow XDP dev bounded program to perform XDP_REDIRECT into maps

From: Stanislav Fomichev <[email-masked]>
Date: Wed, 23 Apr 2025 07:33:04 -0700

On 04/23, Lorenzo Bianconi wrote:
> 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 <[email-masked]>
> > > ---
> > >  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?

Thanks for the info! Yes, that should work. I wonder if you hit
bpf_prog_select_runtime->bpf_check_tail_call->bpf_prog_map_compatible
path? Looks like we should not do bpf_prog_is_dev_bound in that case (the rest
of the bpf_prog_map_compatible callers should).

When doing a follow up, can you also extend tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
to cover these conditions? (redirect to empty map -> nop, adding
dev-bound program to devmap is einval).