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

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Message-ID: <CAADnVQ+4aehGYPJ2qT_HWWXmOSo4WXf69N=N9-dpzERKfzuSzQ@mail.gmail.com>
Date:   Tue, 18 Jul 2023 09:52:24 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
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, Jul 18, 2023 at 9:06 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> 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.

you're still missing the point. Pls read the whole patch series.
It is _not_ input validation.
skb_copy_bits is a slow path. One extra check doesn't affect
performance at all. So 'fast paths' isn't a valid argument here.
The code is reusing
        if (likely(hlen - offset >= len))
                return (void *)data + offset;
which _is_ the fast path.

What you're requesting is to copy paste
the whole __skb_header_pointer into __skb_header_pointer2.
Makes no sense.