linux-kernel - Re: [PATCH bpf-next v9 02/23] bpf/verifier: do not clear meta in check_mem

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Message-ID: <CAP01T76tie9dpjacCLxCcAjtra12GxfmeO9f_mYnUU6pO4otzQ@mail.gmail.com>
Date:   Fri, 26 Aug 2022 03:54:45 +0200
From:   Kumar Kartikeya Dwivedi <memxor@...il.com>
To:     Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Jiri Kosina <jikos@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Shuah Khan <shuah@...nel.org>,
        Dave Marchevsky <davemarchevsky@...com>,
        Joe Stringer <joe@...ium.io>, Jonathan Corbet <corbet@....net>,
        Tero Kristo <tero.kristo@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-input@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH bpf-next v9 02/23] bpf/verifier: do not clear meta in check_mem_size

On Wed, 24 Aug 2022 at 15:41, Benjamin Tissoires
<benjamin.tissoires@...hat.com> wrote:
>
> The purpose of this clear is to prevent meta->raw_mode to be evaluated
> at true, but this also prevents to forward any other data to the other
> callees.
>
> Only switch back raw_mode to false so we don't entirely clear meta.
>
> Acked-by: Yonghong Song <yhs@...com>
> Signed-off-by: Benjamin Tissoires <benjamin.tissoires@...hat.com>
>
> ---
>
> no changes in v9
>
> no changes in v8
>
> no changes in v7
>
> new in v6
> ---
>  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 d694f43ab911..13190487fb12 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -5287,7 +5287,7 @@ static int check_mem_size_reg(struct bpf_verifier_env *env,
>                  * initialize all the memory that the helper could
>                  * just partially fill up.
>                  */
> -               meta = NULL;
> +               meta->raw_mode = false;

But this is adding a side effect, the caller's meta->raw_mode becomes
false, which the caller may not expect...

>
>         if (reg->smin_value < 0) {
>                 verbose(env, "R%d min value is negative, either use unsigned or 'var &= const'\n",
> --
> 2.36.1
>