linux-kernel - Re: [PATCH v2 1/3] bpf: Allow NULL buffers in bpf_dynptr_slice(

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Date:   Tue, 18 Jul 2023 09:06:32 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Daniel Rosenberg <drosen@...gle.com>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Shuah Khan <shuah@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Joanne Koong <joannelkoong@...il.com>,
        Mykola Lysenko <mykolal@...com>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH v2 1/3] bpf: Allow NULL buffers in bpf_dynptr_slice(_rw)

On Tue, 18 Jul 2023 08:52:55 -0700 Alexei Starovoitov wrote:
> On Tue, Jul 18, 2023 at 8:26 AM Jakub Kicinski <kuba@...nel.org> wrote:
> > On Mon,  1 May 2023 17:52:16 -0700 Daniel Rosenberg wrote:  
> > > --- a/include/linux/skbuff.h
> > > +++ b/include/linux/skbuff.h
> > > @@ -4033,7 +4033,7 @@ __skb_header_pointer(const struct sk_buff *skb, int offset, int len,
> > >       if (likely(hlen - offset >= len))
> > >               return (void *)data + offset;
> > >
> > > -     if (!skb || unlikely(skb_copy_bits(skb, offset, buffer, len) < 0))
> > > +     if (!skb || !buffer || unlikely(skb_copy_bits(skb, offset, buffer, len) < 0))
> > >               return NULL;  
> >
> > First off - please make sure you CC netdev on changes to networking!
> >
> > Please do not add stupid error checks to core code for BPF safety.
> > Wrap the call if you can't guarantee that value is sane, this is
> > a very bad precedent.  
> 
> This is NOT for safety. You misread the code.

Doesn't matter, safety or optionality. skb_header_pointer() is used 
on the fast paths of the networking stack, adding heavy handed input
validation to it is not okay. No sane code should be passing NULL
buffer to skb_header_pointer(). Please move the NULL check to the BPF
code so the rest of the networking stack does not have to pay the cost.

This should be common sense. If one caller is doing something..
"special" the extra code should live in the caller, not the callee.
That's basic code hygiene.