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Date:   Wed, 14 Oct 2020 12:47:59 -0700
From:   Xie He <xie.he.0141@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...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 8:12 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> 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?

It doesn't address any issue visible when using. Just to solve the
problem of the same dev->type having different hard_header_len values
which you mentioned. Making this change will not affect the user in
any way. So I think it is valuable to make this change.

> 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.

I thought we agreed that ideally GRE devices would not have
header_ops. Currently GRE devices (in normal situations) indeed do not
use header_ops (and use ARHPHRD_IPGRE as dev->type). I think we should
keep this behavior.

To solve the problem of the same dev->type having different
hard_header_len values which you mentioned. I think we should create a
new dev->type (ARPHRD_IPGRE_SPECIAL) for GRE devices that use
header_ops.

Also, for collect_md, I think we should use ARHPHRD_IPGRE. I see no
reason to use ARPHRD_NONE.

> > 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.

We can't always keep the interface to the user space unchanged when
fixing problems. When we fix drivers by adding hard_header_len or
removing hard_header_len, we ARE changing the interface. I did these
fixes a lot. I also changed skb->protocol when sending skbs for some
drivers, which in fact was also changing the interface. It is not
possible to keep the interface strictly unchanged, otherwise a lot of
problems will be impossible to fix.

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