Re: [PATCH bpf-next] bpf, net: Introduce skb_pointer_if_linear().

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: David Laight <David.Laight@aculab.com>
Cc: "davem@davemloft.net" <davem@davemloft.net>,
	"kuba@kernel.org" <kuba@kernel.org>,
	 "edumazet@google.com" <edumazet@google.com>,
	"pabeni@redhat.com" <pabeni@redhat.com>,
	 "daniel@iogearbox.net" <daniel@iogearbox.net>,
	"andrii@kernel.org" <andrii@kernel.org>,
	 "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"bpf@vger.kernel.org" <bpf@vger.kernel.org>,
	 "kernel-team@fb.com" <kernel-team@fb.com>
Subject: Re: [PATCH bpf-next] bpf, net: Introduce skb_pointer_if_linear().
Date: Wed, 19 Jul 2023 09:30:22 -0700	[thread overview]
Message-ID: <CAADnVQJ+NSeVOJROKHYY1VVRDwK8Gm8jgSoRTCmbrN8XG=y3JA@mail.gmail.com> (raw)
In-Reply-To: <dae5886a8b3b4d9f869e4f8ab3cefa96@AcuMS.aculab.com>

On Wed, Jul 19, 2023 at 6:10 AM David Laight <David.Laight@aculab.com> wrote:
>
> From: Alexei Starovoitov
> > Sent: 19 July 2023 00:40
> >
> > Network drivers always call skb_header_pointer() with non-null buffer.
> > Remove !buffer check to prevent accidental misuse of skb_header_pointer().
> > Introduce skb_pointer_if_linear() instead.
> >
> ...
> > +static inline void * __must_check
> > +skb_pointer_if_linear(const struct sk_buff *skb, int offset, int len)
> > +{
> > +     if (likely(skb_headlen(skb) - offset >= len))
> > +             return skb->data + offset;
> > +     return NULL;
> > +}
>
> Shouldn't both 'offset' and 'len' be 'unsigned int' ?
>
> The check should probably be written:
>                 offset + len <= skb_headlen(skb)
> so that it fails if 'offset' is also large.
> (Provided 'offset + len' itself doesn't wrap.)

I agree that this style is easier to read, but
consistency with skb_header_pointer() trumps all such considerations.