Re: [PATCH bpf-next 1/2] bpf: Relax KF_ACQUIRE kfuncs strict type matching constraint on non-zero offset pointers

[Juntong Deng]

On 8/21/24 19:02, Alexei Starovoitov wrote:
> On Fri, Aug 16, 2024 at 6:24 AM Juntong Deng <juntong.deng@xxxxxxxxxxx> wrote:
> > Currently the non-zero offset pointer constraint for KF_TRUSTED_ARGS
> > kfuncs has been relaxed in commit 605c96997d89 ("bpf: relax zero fixed
> > offset constraint on KF_TRUSTED_ARGS/KF_RCU"), which means that non-zero
> > offset does not affect whether a pointer is valid.
> >
> > But currently we still cannot pass non-zero offset pointers to
> > KF_ACQUIRE kfuncs. This is because KF_ACQUIRE kfuncs requires strict
> > type matching, but non-zero offset does not change the type of pointer,
> > which causes the ebpf program to be rejected by the verifier.
> >
> > This can cause some problems, one example is that bpf_skb_peek_tail
> > kfunc [0] cannot be implemented by just passing in non-zero offset
> > pointers.
> >
> > This patch makes KF_ACQUIRE kfuncs not require strict type matching
> > on non-zero offset pointers.
> >
> > [0]: https://lore.kernel.org/bpf/AM6PR03MB5848CA39CB4B7A4397D380B099B12@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> >
> > Signed-off-by: Juntong Deng <juntong.deng@xxxxxxxxxxx>
> > ---
> >   kernel/bpf/verifier.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index ebec74c28ae3..3a14002d24a0 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -11484,7 +11484,7 @@ static int process_kf_arg_ptr_to_btf_id(struct bpf_verifier_env *env,
> >           * btf_struct_ids_match() to walk the struct at the 0th offset, and
> >           * resolve types.
> >           */
> > -       if (is_kfunc_acquire(meta) ||
> > +       if ((is_kfunc_acquire(meta) && !reg->off) ||
>
> Agree that relaxing is fine and calling acquire kfunc like:
>    bpf_kfunc_nested_acquire_test(&sk->sk_write_queue);
>
> should be allowed,
> but above check is strange, since
> if offsetof(&sk_write_queue) == 0
> it will disallow calling a kfunc.
> I mean if the field is the first in the outer struct this
> condition will force strict type match which will fail, right?
>
> So should we remove the above is_kfunc_acquire() check instead?
>
> pw-bot: cr

I agree that if strict type matching is not required for both zero
offset and non-zero pointers, then we can remove the strict type
matching for KF_ACQUIRE kfuncs.

I sent the version 2 patch set:
https://lore.kernel.org/bpf/AM6PR03MB5848FD2BD89BF0B6B5AA3B4C99952@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/T/#t