netdev - Re: [PATCH next 0/3] blackhole device to invalidate dst

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Date:   Sun, 23 Jun 2019 16:01:07 -0700
From:   Mahesh Bandewar (महेश बंडेवार) 
        <maheshb@...gle.com>
To:     David Ahern <dsahern@...il.com>
Cc:     Netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>,
        Michael Chan <michael.chan@...adcom.com>,
        Daniel Axtens <dja@...ens.net>,
        Mahesh Bandewar <mahesh@...dewar.net>
Subject: Re: [PATCH next 0/3] blackhole device to invalidate dst

On Sat, Jun 22, 2019 at 8:35 AM David Ahern <dsahern@...il.com> wrote:
>
> On 6/21/19 6:45 PM, Mahesh Bandewar wrote:
> > When we invalidate dst or mark it "dead", we assign 'lo' to
> > dst->dev. First of all this assignment is racy and more over,
> > it has MTU implications.
> >
> > The standard dev MTU is 1500 while the Loopback MTU is 64k. TCP
> > code when dereferencing the dst don't check if the dst is valid
> > or not. TCP when dereferencing a dead-dst while negotiating a
> > new connection, may use dst device which is 'lo' instead of
> > using the correct device. Consider the following scenario:
> >
>
> Why doesn't the TCP code (or any code) check if a cached dst is valid?
> That's the whole point of marking it dead - to tell users not to rely on
> it.

Well, I'm not an expert in TCP, but I could guess. Eric, please help
me being honest with my guess.
The code that marks it dead (control path) and the code that need to
check the validity (data-path) need to have some sort of
synchronization and that could be costly in the data-path. Also what
is the worst case scenario? ... that packet is going to get dropped
since the under-lying device or route has issues (minus this corner
case). May be that was the reason.

As I mentioned in the log, we could  remove the racy-ness  with
barriers or locks but then that would be an additional cost in the
data-path. having a dummy/backhole dev is the cheapest solution with
no changes to the data-path.