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

From: Stanislav Fomichev <[email-masked]>
Date: Tue, 22 Apr 2025 15:49:52 -0700

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.