[net-next,1/2] net: retrieve netns cookie via getsocketopt

It's getting more common to run nested container environments for
testing cloud software. One of such examples is Kind [1] which runs a
Kubernetes cluster in Docker containers on a single host. Each container
acts as a Kubernetes node, and thus can run any Pod (aka container)
inside the former. This approach simplifies testing a lot, as it
eliminates complicated VM setups.

Unfortunately, such a setup breaks some functionality when cgroupv2 BPF
programs are used for load-balancing. The load-balancer BPF program
needs to detect whether a request originates from the host netns or a
container netns in order to allow some access, e.g. to a service via a
loopback IP address. Typically, the programs detect this by comparing
netns cookies with the one of the init ns via a call to
bpf_get_netns_cookie(NULL). However, in nested environments the latter
cannot be used given the Kubernetes node's netns is outside the init ns.
To fix this, we need to pass the Kubernetes node netns cookie to the
program in a different way: by extending getsockopt() with a
SO_NETNS_COOKIE option, the orchestrator which runs in the Kubernetes
node netns can retrieve the cookie and pass it to the program instead.

Thus, this is following up on Eric's commit 3d368ab87cf6 ("net:
initialize net->net_cookie at netns setup") to allow retrieval via
SO_NETNS_COOKIE.  This is also in line in how we retrieve socket cookie
via SO_COOKIE.

  [1] https://kind.sigs.k8s.io/

Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Martynas Pumputis <m@lambda.lt>
Cc: Eric Dumazet <edumazet@google.com>
---
 arch/alpha/include/uapi/asm/socket.h  | 2 ++
 arch/mips/include/uapi/asm/socket.h   | 2 ++
 arch/parisc/include/uapi/asm/socket.h | 2 ++
 arch/sparc/include/uapi/asm/socket.h  | 2 ++
 include/uapi/asm-generic/socket.h     | 2 ++
 net/core/sock.c                       | 9 +++++++++
 6 files changed, 19 insertions(+)

On Tue, 22 Jun 2021 at 21:24, Martynas Pumputis <m@lambda.lt> wrote:
>
> It's getting more common to run nested container environments for
> testing cloud software. One of such examples is Kind [1] which runs a
> Kubernetes cluster in Docker containers on a single host. Each container
> acts as a Kubernetes node, and thus can run any Pod (aka container)
> inside the former. This approach simplifies testing a lot, as it
> eliminates complicated VM setups.
>
> Unfortunately, such a setup breaks some functionality when cgroupv2 BPF
> programs are used for load-balancing. The load-balancer BPF program
> needs to detect whether a request originates from the host netns or a
> container netns in order to allow some access, e.g. to a service via a
> loopback IP address. Typically, the programs detect this by comparing
> netns cookies with the one of the init ns via a call to
> bpf_get_netns_cookie(NULL). However, in nested environments the latter
> cannot be used given the Kubernetes node's netns is outside the init ns.
> To fix this, we need to pass the Kubernetes node netns cookie to the
> program in a different way: by extending getsockopt() with a
> SO_NETNS_COOKIE option, the orchestrator which runs in the Kubernetes
> node netns can retrieve the cookie and pass it to the program instead.
>
> Thus, this is following up on Eric's commit 3d368ab87cf6 ("net:
> initialize net->net_cookie at netns setup") to allow retrieval via
> SO_NETNS_COOKIE.  This is also in line in how we retrieve socket cookie
> via SO_COOKIE.
>
>   [1] https://kind.sigs.k8s.io/
>
> Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
> Signed-off-by: Martynas Pumputis <m@lambda.lt>
> Cc: Eric Dumazet <edumazet@google.com>
> ---
>  arch/alpha/include/uapi/asm/socket.h  | 2 ++
>  arch/mips/include/uapi/asm/socket.h   | 2 ++
>  arch/parisc/include/uapi/asm/socket.h | 2 ++
>  arch/sparc/include/uapi/asm/socket.h  | 2 ++
>  include/uapi/asm-generic/socket.h     | 2 ++
>  net/core/sock.c                       | 9 +++++++++
>  6 files changed, 19 insertions(+)
>
> diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
> index 57420356ce4c..6b3daba60987 100644
> --- a/arch/alpha/include/uapi/asm/socket.h
> +++ b/arch/alpha/include/uapi/asm/socket.h
> @@ -127,6 +127,8 @@
>  #define SO_PREFER_BUSY_POLL    69
>  #define SO_BUSY_POLL_BUDGET    70
>
> +#define SO_NETNS_COOKIE                71
> +
>  #if !defined(__KERNEL__)
>
>  #if __BITS_PER_LONG == 64
> diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h
> index 2d949969313b..cdf404a831b2 100644
> --- a/arch/mips/include/uapi/asm/socket.h
> +++ b/arch/mips/include/uapi/asm/socket.h
> @@ -138,6 +138,8 @@
>  #define SO_PREFER_BUSY_POLL    69
>  #define SO_BUSY_POLL_BUDGET    70
>
> +#define SO_NETNS_COOKIE                71
> +
>  #if !defined(__KERNEL__)
>
>  #if __BITS_PER_LONG == 64
> diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h
> index f60904329bbc..5b5351cdcb33 100644
> --- a/arch/parisc/include/uapi/asm/socket.h
> +++ b/arch/parisc/include/uapi/asm/socket.h
> @@ -119,6 +119,8 @@
>  #define SO_PREFER_BUSY_POLL    0x4043
>  #define SO_BUSY_POLL_BUDGET    0x4044
>
> +#define SO_NETNS_COOKIE                0x4045
> +
>  #if !defined(__KERNEL__)
>
>  #if __BITS_PER_LONG == 64
> diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h
> index 848a22fbac20..92675dc380fa 100644
> --- a/arch/sparc/include/uapi/asm/socket.h
> +++ b/arch/sparc/include/uapi/asm/socket.h
> @@ -120,6 +120,8 @@
>  #define SO_PREFER_BUSY_POLL     0x0048
>  #define SO_BUSY_POLL_BUDGET     0x0049
>
> +#define SO_NETNS_COOKIE          0x0050
> +
>  #if !defined(__KERNEL__)
>
>
> diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
> index 4dcd13d097a9..d588c244ec2f 100644
> --- a/include/uapi/asm-generic/socket.h
> +++ b/include/uapi/asm-generic/socket.h
> @@ -122,6 +122,8 @@
>  #define SO_PREFER_BUSY_POLL    69
>  #define SO_BUSY_POLL_BUDGET    70
>
> +#define SO_NETNS_COOKIE                71
> +
>  #if !defined(__KERNEL__)
>
>  #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__))
> diff --git a/net/core/sock.c b/net/core/sock.c
> index ddfa88082a2b..462fe1fb2056 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -1635,6 +1635,15 @@ int sock_getsockopt(struct socket *sock, int level, int optname,
>                 v.val = sk->sk_bound_dev_if;
>                 break;
>
> +#ifdef CONFIG_NET_NS

Nit: sock_net already takes care of CONFIG_NET_NS and returns the root
ns if !NET_NS, so this define is not necessary. I think this behaviour
is nicer: uapi stays consistent even if kernel config changes.

> +       case SO_NETNS_COOKIE:
> +               lv = sizeof(u64);
> +               if (len != lv)
> +                       return -EINVAL;
> +               v.val64 = sock_net(sk)->net_cookie;
> +               break;
> +#endif
> +
>         default:
>                 /* We implement the SO_SNDLOWAT etc to not be settable
>                  * (1003.1g 7).
> --
> 2.32.0
>