linux-kernel - Re: [PATCH bpf-next 4/7] bpf: Attachment verification for BPF_MODIFY

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Message-ID: <20200304000326.nk7jmkgxazl3umbh@ast-mbp>
Date:   Tue, 3 Mar 2020 16:03:27 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     KP Singh <kpsingh@...omium.org>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        open list <linux-kernel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Paul Turner <pjt@...gle.com>,
        Florent Revest <revest@...omium.org>,
        Brendan Jackman <jackmanb@...omium.org>
Subject: Re: [PATCH bpf-next 4/7] bpf: Attachment verification for
 BPF_MODIFY_RETURN

On Wed, Mar 04, 2020 at 12:21:51AM +0100, KP Singh wrote:
> 
> > > +                       t = btf_type_skip_modifiers(btf, t->type, NULL);
> > > +                       if (!btf_type_is_int(t)) {
> > 
> > Should the size of int be verified here? E.g., if some function
> > returns u8, is that ok for BPF program to return, say, (1<<30) ?
> 
> Would this work?
> 
>        if (size != t->size) {
>                bpf_log(log,
>                        "size accessed = %d should be %d\n",
>                        size, t->size);
>                return false;
>        }

It will cause spurious failures later when llvm optimizes
if (ret & 0xff) into u8 load.
I think btf_type_is_int() is enough as-is.