Re: [PATCH bpf-next v2] bpf: Allow XDP dev-bound programs to perform XDP_REDIRECT into maps

["Toke Høiland-Jørgensen" <toke@kernel.org>]

Lorenzo Bianconi <lorenzo@kernel.org> writes:

> In the current implementation if the program is dev-bound to a specific
> device, it will not be possible to perform XDP_REDIRECT into a DEVMAP
> or CPUMAP even if the program is running in the driver NAPI context and
> it is not attached to any map entry. This seems in contrast with the
> explanation available in bpf_prog_map_compatible routine.
> Fix the issue introducing __bpf_prog_map_compatible utility routine in
> order to avoid bpf_prog_is_dev_bound() check running bpf_check_tail_call()
> at program load time (bpf_prog_select_runtime()).
> Continue forbidding to attach a dev-bound program to XDP maps
> (BPF_MAP_TYPE_PROG_ARRAY, BPF_MAP_TYPE_DEVMAP and BPF_MAP_TYPE_CPUMAP).
>
> Fixes: 3d76a4d3d4e59 ("bpf: XDP metadata RX kfuncs")
> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> ---
> Changes in v2:
> - Introduce __bpf_prog_map_compatible() utility routine in order to skip
>   bpf_prog_is_dev_bound check in bpf_check_tail_call()
> - Extend xdp_metadata selftest
> - Link to v1: https://lore.kernel.org/r/20250422-xdp-prog-bound-fix-v1-1-0b581fa186fe@kernel.org
> ---
>  kernel/bpf/core.c                                  | 27 +++++++++++++---------
>  .../selftests/bpf/prog_tests/xdp_metadata.c        | 22 +++++++++++++++++-
>  tools/testing/selftests/bpf/progs/xdp_metadata.c   | 13 +++++++++++
>  3 files changed, 50 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index ba6b6118cf504041278d05417c4212d57be6fca0..a3e571688421196c3ceaed62b3b59b62a0258a8c 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -2358,8 +2358,8 @@ static unsigned int __bpf_prog_ret0_warn(const void *ctx,
>  	return 0;
>  }
>  
> -bool bpf_prog_map_compatible(struct bpf_map *map,
> -			     const struct bpf_prog *fp)
> +static bool __bpf_prog_map_compatible(struct bpf_map *map,
> +				      const struct bpf_prog *fp)
>  {
>  	enum bpf_prog_type prog_type = resolve_prog_type(fp);
>  	bool ret;
> @@ -2368,14 +2368,6 @@ bool bpf_prog_map_compatible(struct bpf_map *map,
>  	if (fp->kprobe_override)
>  		return false;
>  
> -	/* XDP programs inserted into maps are not guaranteed to run on
> -	 * a particular netdev (and can run outside driver context entirely
> -	 * in the case of devmap and cpumap). Until device checks
> -	 * are implemented, prohibit adding dev-bound programs to program maps.
> -	 */
> -	if (bpf_prog_is_dev_bound(aux))
> -		return false;
> -
>  	spin_lock(&map->owner.lock);
>  	if (!map->owner.type) {
>  		/* There's no owner yet where we could check for
> @@ -2409,6 +2401,19 @@ bool bpf_prog_map_compatible(struct bpf_map *map,
>  	return ret;
>  }
>  
> +bool bpf_prog_map_compatible(struct bpf_map *map, const struct bpf_prog *fp)
> +{
> +	/* XDP programs inserted into maps are not guaranteed to run on
> +	 * a particular netdev (and can run outside driver context entirely
> +	 * in the case of devmap and cpumap). Until device checks
> +	 * are implemented, prohibit adding dev-bound programs to program maps.
> +	 */
> +	if (bpf_prog_is_dev_bound(fp->aux))
> +		return false;
> +
> +	return __bpf_prog_map_compatible(map, fp);
> +}
> +
>  static int bpf_check_tail_call(const struct bpf_prog *fp)
>  {
>  	struct bpf_prog_aux *aux = fp->aux;
> @@ -2421,7 +2426,7 @@ static int bpf_check_tail_call(const struct bpf_prog *fp)
>  		if (!map_type_contains_progs(map))
>  			continue;
>  
> -		if (!bpf_prog_map_compatible(map, fp)) {
> +		if (!__bpf_prog_map_compatible(map, fp)) {

Hmm, so this allows devbound programs in tail call maps, right? But
there's no guarantee that a tail call map will always be used for a
particular device, is there? For instance, it could be shared between
multiple XDP programs, bound to different devices, thus getting the
wrong kfunc.

Or you could even have dev-bound programs tail-called from non-dev-bound
programs with this change AFAICT?

In other words, I think this is too relaxed, your change in v1 that only
relaxed cpumap and devmap checks here was better.

In fact, I don't really see why bpf_check_tail_call() needs to look at
devmap/cpumap at all, so maybe just changing the
map_type_contains_progs() call to only match tail call maps is better?

-Toke