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Message-ID: <87frhw7k9u.fsf@toke.dk>
Date: Fri, 25 Apr 2025 09:49:49 +0200
From: Toke Høiland-Jørgensen <toke@...nel.org>
To: Lorenzo Bianconi <lorenzo@...nel.org>
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann
<daniel@...earbox.net>, John Fastabend <john.fastabend@...il.com>, Andrii
Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>,
Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, Yonghong
Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, Stanislav
Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa
<jolsa@...nel.org>, "David S. Miller" <davem@...emloft.net>, Jakub
Kicinski <kuba@...nel.org>, Jesper Dangaard Brouer <hawk@...nel.org>,
Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
bpf@...r.kernel.org, netdev@...r.kernel.org,
linux-kselftest@...r.kernel.org
Subject: Re: [PATCH bpf-next v2] bpf: Allow XDP dev-bound programs to
perform XDP_REDIRECT into maps
Lorenzo Bianconi <lorenzo@...nel.org> writes:
>> Lorenzo Bianconi <lorenzo@...nel.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@...nel.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.
>
> According to my understanding the following path will be executed just for
> dev-bound program that performs XDP_REDIRECT into a BPF_MAP_TYPE_PROG_ARRAY:
>
> bpf_prog_select_runtime() -> bpf_check_tail_call() -> __bpf_prog_map_compatible()
>
> while for XDP program inserted into BPF_MAP_TYPE_PROG_ARRAY we will continue
> running bpf_prog_map_compatible() so we will forbid inserting ev-bound programs.
> This is even tested into xdp_metadata selftest:
>
> https://github.com/torvalds/linux/blob/master/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c#L416
>
> It seems to me v2 is not more relaxed than v1. Am I missing something?
No, you're right; see my reply to Stanislav - I misremembered the logic :)
-Toke
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