netdev - Re: [PATCH v10 12/12] bpf: add sample for xdp forwarding and rewrite

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Date:	Wed, 03 Aug 2016 11:31:20 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	bblanco@...mgrid.com
Cc:	tom@...bertland.com, alexei.starovoitov@...il.com,
	netdev@...r.kernel.org, jhs@...atatu.com,
	saeedm@....mellanox.co.il, kafai@...com, brouer@...hat.com,
	as754m@....com, gerlitz.or@...il.com, john.fastabend@...il.com,
	hannes@...essinduktion.org, tgraf@...g.ch, daniel@...earbox.net,
	ttoukan.linux@...il.com, haoxuany@...com
Subject: Re: [PATCH v10 12/12] bpf: add sample for xdp forwarding and
 rewrite

From: Brenden Blanco <bblanco@...mgrid.com>
Date: Wed, 3 Aug 2016 11:29:52 -0700

> On Wed, Aug 03, 2016 at 10:29:58AM -0700, Tom Herbert wrote:
>> On Wed, Aug 3, 2016 at 10:11 AM, Alexei Starovoitov
>> <alexei.starovoitov@...il.com> wrote:
>> > On Wed, Aug 03, 2016 at 10:01:54AM -0700, Tom Herbert wrote:
>> >> On Tue, Jul 19, 2016 at 12:16 PM, Brenden Blanco <bblanco@...mgrid.com> wrote:
> [...]
>> >> > +SEC("xdp1")
>> >> > +int xdp_prog1(struct xdp_md *ctx)
>> >> > +{
>> >> > +       void *data_end = (void *)(long)ctx->data_end;
>> >> > +       void *data = (void *)(long)ctx->data;
>> >>
>> >> Brendan,
>> >>
>> >> It seems that the cast to long here is done because data_end and data
>> >> are u32s in xdp_md. So the effect is that we are upcasting a
>> >> thirty-bit integer into a sixty-four bit pointer (in fact without the
>> >> cast we see compiler warnings). I don't understand how this can be
>> >> correct. Can you shed some light on this?
>> >
>> > please see:
>> > http://lists.iovisor.org/pipermail/iovisor-dev/2016-August/000355.html
>> >
>> That doesn't explain it. The only thing I can figure is that there is
>> an implicit assumption somewhere that even though the pointer size may
>> be 64 bits, only the low order thirty-two bits are relevant in this
>> environment (i.e. upper bit are always zero for any pointers)-- so
>> then it would safe store pointers as u32 and to upcast them to void *.
> No, the actual pointer storage is always void* sized (see struct
> xdp_buff). The mangling is cosmetic. The verifier converts the
> underlying bpf load instruction to the right sized operation.

And this is what Alexei meant by "meta".  Tom this stuff works just
fine.