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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Fri, 25 Apr 2025 09:48:35 +0200

Stanislav Fomichev <[email-masked]> writes:

> On 04/24, Toke Høiland-Jørgensen wrote:
>> Lorenzo Bianconi <[email-masked]> 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 <[email-masked]>
>> > ---
>> > 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/[email-masked]
>> > ---
>> >  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.
>
> Won't this (devbound progs in tail call maps) be still prohibited
> by a bpf_prog_map_compatible check in prog_fd_array_get_ptr?

Yeah, you're right, I misremembered the check and somehow convinced
myself that the check in bpf_check_tail_call() was the one that
prevented dev-bound programs from being loaded into tail call maps...

-Toke