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

On Apr 24, Jesper Dangaard Brouer wrote:

On 23/04/2025 19.44, Lorenzo Bianconi wrote:

...

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 Bianconilorenzo@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@kern...

---

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)) {
  ret = -EINVAL;
  goto out;
  

  }

Does this change allow us to have a dev_bound BPF-prog that have tail-call BPF-progs that are not dev_bound?

The use-case is a dev_bound BPF-prog that reads e.g. HW vlan, store this in data_meta (or a per CPU array), and then tail-calls another BPF-prog that reads the data stored (from data_meta area). Maybe this is already supported before?

I think this patch allows a dev-bound program to run hw-metadata kfuncs and perform XDP_REDIRECT into a prog_array but you will not be able to read these info via hw-metadata kfuncs in a tail-call program since just dev-bound programs are currently allowed to do that (and you can't insert a dev-bound programs in BPF_MAP_TYPE_PROG_ARRAY).

Regards, Lorenzo

--Jesper