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Date:   Wed, 14 Oct 2020 11:12:17 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Xie He <xie.he.0141@...il.com>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        syzbot <syzbot+4a2c52677a8a1aa283cb@...kaller.appspotmail.com>,
        William Tu <u9012063@...il.com>
Subject: Re: [Patch net v2] ip_gre: set dev->hard_header_len and
 dev->needed_headroom properly

On Wed, Oct 14, 2020 at 4:52 AM Xie He <xie.he.0141@...il.com> wrote:
>
> On Sun, Oct 11, 2020 at 2:01 PM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > There is agreement that hard_header_len should be the length of link
> > layer headers visible to the upper layers, needed_headroom the
> > additional room required for headers that are not exposed, i.e., those
> > pushed inside ndo_start_xmit.
> >
> > The link layer header length also has to agree with the interface
> > hardware type (ARPHRD_..).
> >
> > Tunnel devices have not always been consistent in this, but today
> > "bare" ip tunnel devices without additional headers (ipip, sit, ..) do
> > match this and advertise 0 byte hard_header_len. Bareudp, vxlan and
> > geneve also conform to this. Known exception that probably needs to be
> > addressed is sit, which still advertises LL_MAX_HEADER and so has
> > exposed quite a few syzkaller issues. Side note, it is not entirely
> > clear to me what sets ARPHRD_TUNNEL et al apart from ARPHRD_NONE and
> > why they are needed.
> >
> > GRE devices advertise ARPHRD_IPGRE and GRETAP advertise ARPHRD_ETHER.
> > The second makes sense, as it appears as an Ethernet device. The first
> > should match "bare" ip tunnel devices, if following the above logic.
> > Indeed, this is what commit e271c7b4420d ("gre: do not keep the GRE
> > header around in collect medata mode") implements. It changes
> > dev->type to ARPHRD_NONE in collect_md mode.
> >
> > Some of the inconsistency comes from the various modes of the GRE
> > driver. Which brings us to ipgre_header_ops. It is set only in two
> > special cases.
> >
> > Commit 6a5f44d7a048 ("[IPV4] ip_gre: sendto/recvfrom NBMA address")
> > added ipgre_header_ops.parse to be able to receive the inner ip source
> > address with PF_PACKET recvfrom. And apparently relies on
> > ipgre_header_ops.create to be able to set an address, which implies
> > SOCK_DGRAM.
> >
> > The other special case, CONFIG_NET_IPGRE_BROADCAST, predates git. Its
> > implementation starts with the beautiful comment "/* Nice toy.
> > Unfortunately, useless in real life :-)". From the rest of that
> > detailed comment, it is not clear to me why it would need to expose
> > the headers. The example does not use packet sockets.
> >
> > A packet socket cannot know devices details such as which configurable
> > mode a device may be in. And different modes conflict with the basic
> > rule that for a given well defined link layer type, i.e., dev->type,
> > header length can be expected to be consistent. In an ideal world
> > these exceptions would not exist, therefore.
> >
> > Unfortunately, this is legacy behavior that will have to continue to
> > be supported.
>
> Thanks for your explanation. So header_ops for GRE devices is only
> used in 2 special situations. In normal situations, header_ops is not
> used for GRE devices. And we consider not using header_ops should be
> the ideal arrangement for GRE devices.
>
> Can we create a new dev->type (like ARPHRD_IPGRE_SPECIAL) for GRE
> devices that use header_ops? I guess changing dev->type will not
> affect the interface to the user space? This way we can solve the
> problem of the same dev->type having different hard_header_len values.

But does that address any real issue?

If anything, it would make sense to keep ARHPHRD_IPGRE for tunnels
that expect headers and switch to ARPHRD_NONE for those that do not.
As the collect_md commit I mentioned above does.

> Also, for the second special situation, if there's no obvious reason
> to use header_ops, maybe we can consider removing header_ops for this
> situation.

Unfortunately, there's no knowing if some application is using this
broadcast mode *with* a process using packet sockets.

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