Re: [PATCH net v4 0/2] net: initialize fastreuse on inet_inherit_port

[David Miller]

From: Tim Froidcoeur <tim.froidcoeur@xxxxxxxxxxxx>
Date: Tue, 11 Aug 2020 20:33:22 +0200

> In the case of TPROXY, bind_conflict optimizations for SO_REUSEADDR or
> SO_REUSEPORT are broken, possibly resulting in O(n) instead of O(1) bind
> behaviour or in the incorrect reuse of a bind.
> 
> the kernel keeps track for each bind_bucket if all sockets in the
> bind_bucket support SO_REUSEADDR or SO_REUSEPORT in two fastreuse flags.
> These flags allow skipping the costly bind_conflict check when possible
> (meaning when all sockets have the proper SO_REUSE option).
> 
> For every socket added to a bind_bucket, these flags need to be updated.
> As soon as a socket that does not support reuse is added, the flag is
> set to false and will never go back to true, unless the bind_bucket is
> deleted.
> 
> Note that there is no mechanism to re-evaluate these flags when a socket
> is removed (this might make sense when removing a socket that would not
> allow reuse; this leaves room for a future patch).
> 
> For this optimization to work, it is mandatory that these flags are
> properly initialized and updated.
> 
> When a child socket is created from a listen socket in
> __inet_inherit_port, the TPROXY case could create a new bind bucket
> without properly initializing these flags, thus preventing the
> optimization to work. Alternatively, a socket not allowing reuse could
> be added to an existing bind bucket without updating the flags, causing
> bind_conflict to never be called as it should.
> 
> Patch 1/2 refactors the fastreuse update code in inet_csk_get_port into a
> small helper function, making the actual fix tiny and easier to understand.
> 
> Patch 2/2 calls this new helper when __inet_inherit_port decides to create
> a new bind_bucket or use a different bind_bucket than the one of the listen
> socket.
> 
> v4: - rebase on latest linux/net master branch
> v3: - remove company disclaimer from automatic signature
> v2: - remove unnecessary cast

Series applied and queued up for -stable, thank you.