netdev - Re: [PATCH v3 bpf-next 2/3] bpf: Add array support to btf_struct

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Message-ID: <CAEf4BzaQOEjbJwV9Ycb1QdBVkFRQLB_3cyw1sfXTz-iV_pt4Yw@mail.gmail.com>
Date:   Wed, 6 Nov 2019 18:41:15 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v3 bpf-next 2/3] bpf: Add array support to btf_struct_access

On Wed, Nov 6, 2019 at 5:49 PM Martin KaFai Lau <kafai@...com> wrote:
>
> This patch adds array support to btf_struct_access().
> It supports array of int, array of struct and multidimensional
> array.
>
> It also allows using u8[] as a scratch space.  For example,
> it allows access the "char cb[48]" with size larger than
> the array's element "char".  Another potential use case is
> "u64 icsk_ca_priv[]" in the tcp congestion control.
>
> btf_resolve_size() is added to resolve the size of any type.
> It will follow the modifier if there is any.  Please
> see the function comment for details.
>
> This patch also adds the "off < moff" check at the beginning
> of the for loop.  It is to reject cases when "off" is pointing
> to a "hole" in a struct.
>
> Signed-off-by: Martin KaFai Lau <kafai@...com>
> ---

Looks good, just two small nits.

Acked-by: Andrii Nakryiko <andriin@...com>

>  kernel/bpf/btf.c | 187 +++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 157 insertions(+), 30 deletions(-)
>
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 128d89601d73..5c4b6aa7b9f0 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -1036,6 +1036,82 @@ static const struct resolve_vertex *env_stack_peak(struct btf_verifier_env *env)
>         return env->top_stack ? &env->stack[env->top_stack - 1] : NULL;
>  }
>

[...]

> -               if (off + size <= moff / 8)
> -                       /* won't find anything, field is already too far */
> +               /* offset of the field in bytes */
> +               moff = btf_member_bit_offset(t, member) / 8;
> +               if (off + size <= moff)

you dropped useful comment :(

>                         break;
> +               /* In case of "off" is pointing to holes of a struct */
> +               if (off < moff)
> +                       continue;
>

[...]

> +
> +               mtrue_end = moff + msize;

nit: there is no other _end, so might be just mend (in line with moff)

> +               if (off >= mtrue_end)
>                         /* no overlap with member, keep iterating */
>                         continue;
> +

[...]