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

[Alexei Starovoitov]

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