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

From: David Laight <David.Laight@ACULAB.COM>
To: 'Alexei Starovoitov' <alexei.starovoitov@gmail.com>,
	"davem@davemloft.net" <davem@davemloft.net>
Cc: "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 13:10:03 +0000	[thread overview]
Message-ID: <dae5886a8b3b4d9f869e4f8ab3cefa96@AcuMS.aculab.com> (raw)
In-Reply-To: <20230718234021.43640-1-alexei.starovoitov@gmail.com>

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've swapped the order because I prefer conditional to be
	if (variable op constant)
and in this case skb_headlen() is the more constant value.

	David

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