linux-kernel - Re: [PATCH bpf-next] bpf: Fix a verifier message for alloc size helper arg

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Message-ID: <CACYkzJ5v6ANJnB0qk7hdEFf+44RH02G22+JFi0OUoJiFLe-RMA@mail.gmail.com>
Date:   Tue, 12 Jan 2021 14:14:33 +0100
From:   KP Singh <kpsingh@...nel.org>
To:     Brendan Jackman <jackmanb@...gle.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Florent Revest <revest@...omium.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next] bpf: Fix a verifier message for alloc size
 helper arg

On Tue, Jan 12, 2021 at 1:39 PM Brendan Jackman <jackmanb@...gle.com> wrote:
>
> The error message here is misleading, the argument will be rejected
> unless it is a known constant.
>
> Signed-off-by: Brendan Jackman <jackmanb@...gle.com>
> ---
>  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 17270b8404f1..5534e667bdb1 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -4319,7 +4319,7 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
>                         err = mark_chain_precision(env, regno);
>         } else if (arg_type_is_alloc_size(arg_type)) {
>                 if (!tnum_is_const(reg->var_off)) {
> -                       verbose(env, "R%d unbounded size, use 'var &= const' or 'if (var < const)'\n",

Can you check if:

int var = 1000;
var += 1;

if (var < 2000)
   // call helper

and then using var in the argument works? If so, the existing error
message would be correct.


> +                       verbose(env, "R%d is not a known constant'\n",
>                                 regno);
>                         return -EACCES;
>                 }
>
> base-commit: e22d7f05e445165e58feddb4e40cc9c0f94453bc
> --
> 2.30.0.284.gd98b1dd5eaa7-goog
>