Re: [PATCH bpf-next v9 2/5] bpf: Allow helpers to accept pointers with a fixed size

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, May 3, 2022 at 10:15 AM Maxim Mikityanskiy <maximmi@nvidia.com> wrote:
>
> Before this commit, the BPF verifier required ARG_PTR_TO_MEM arguments
> to be followed by ARG_CONST_SIZE holding the size of the memory region.
> The helpers had to check that size in runtime.
>
> There are cases where the size expected by a helper is a compile-time
> constant. Checking it in runtime is an unnecessary overhead and waste of
> BPF registers.
>
> This commit allows helpers to accept ARG_PTR_TO_MEM arguments without
> the corresponding ARG_CONST_SIZE, given that they define the memory
> region size in struct bpf_func_proto.

I think it's much less confusing and cleaner to have
ARG_PTR_TO_FIXED_SIZE_MEM (or whatever similar name), instead of
adding special casing to ARG_PTR_TO_MEM.

>
> Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
> Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
> ---
>  include/linux/bpf.h   | 10 ++++++++++
>  kernel/bpf/verifier.c | 26 +++++++++++++++-----------
>  2 files changed, 25 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index be94833d390a..255ae3652225 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -514,6 +514,16 @@ struct bpf_func_proto {
>                 };
>                 u32 *arg_btf_id[5];
>         };
> +       union {
> +               struct {
> +                       size_t arg1_size;
> +                       size_t arg2_size;
> +                       size_t arg3_size;
> +                       size_t arg4_size;
> +                       size_t arg5_size;
> +               };
> +               size_t arg_size[5];
> +       };

We have almost 250 instances of struct bpf_func_proto variables:

$ rg 'const struct bpf_func_proto.* = \{' | wc -l
244

You are adding 40 bytes to it just to use it for 1-2 special
prototypes. It is quite expensive, IMHO, to increase vmlinux image
size by 10KB just for this.

Should we reuse arg_btf_id union (same argument won't be PTR_TO_BTF_ID
and PTR_TO_FIXED_SIZE_MEM, right)? Or alternatively special-case those
few prototypes in verifier code directly when trying to fetch memory
size?


>         int *ret_btf_id; /* return value btf_id */
>         bool (*allowed)(const struct bpf_prog *prog);
>  };

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