Don't read forwarding from netstack in sentry

https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt:
  /proc/sys/net/ipv4/* Variables:

  ip_forward - BOOLEAN
    0 - disabled (default)
    not 0 - enabled

    Forward Packets between interfaces.

    This variable is special, its change resets all configuration
    parameters to their default state (RFC1122 for hosts, RFC1812
    for routers)

/proc/sys/net/ipv4/ip_forward only does work when its value is changed
and always returns the last written value. The last written value may
not reflect the current state of the netstack (e.g. when `ip_forward`
was written a value of "1" then disable forwarding on an interface)
so there is no need for sentry to probe netstack to get the current
forwarding state of interfaces.

```
~$ cat /proc/sys/net/ipv4/ip_forward
0
~$ sudo bash -c "echo 1 > /proc/sys/net/ipv4/ip_forward"
~$ cat /proc/sys/net/ipv4/ip_forward
1
~$ sudo sysctl -a | grep ipv4 | grep forward
net.ipv4.conf.all.forwarding = 1
net.ipv4.conf.default.forwarding = 1
net.ipv4.conf.eno1.forwarding = 1
net.ipv4.conf.lo.forwarding = 1
net.ipv4.conf.wlp1s0.forwarding = 1
net.ipv4.ip_forward = 1
net.ipv4.ip_forward_update_priority = 1
net.ipv4.ip_forward_use_pmtu = 0
~$ sudo sysctl -w net.ipv4.conf.wlp1s0.forwarding=0
net.ipv4.conf.wlp1s0.forwarding = 0
~$ sudo sysctl -a | grep ipv4 | grep forward
net.ipv4.conf.all.forwarding = 1
net.ipv4.conf.default.forwarding = 1
net.ipv4.conf.eno1.forwarding = 1
net.ipv4.conf.lo.forwarding = 1
net.ipv4.conf.wlp1s0.forwarding = 0
net.ipv4.ip_forward = 1
net.ipv4.ip_forward_update_priority = 1
net.ipv4.ip_forward_use_pmtu = 0
~$ cat /proc/sys/net/ipv4/ip_forward
1
~$ sudo bash -c "echo 1 > /proc/sys/net/ipv4/ip_forward"
~$ sudo sysctl -a | grep ipv4 | grep forward
net.ipv4.conf.all.forwarding = 1
net.ipv4.conf.default.forwarding = 1
net.ipv4.conf.eno1.forwarding = 1
net.ipv4.conf.lo.forwarding = 1
net.ipv4.conf.wlp1s0.forwarding = 0
net.ipv4.ip_forward = 1
net.ipv4.ip_forward_update_priority = 1
net.ipv4.ip_forward_use_pmtu = 0
~$ sudo bash -c "echo 0 > /proc/sys/net/ipv4/ip_forward"
~$ sudo sysctl -a | grep ipv4 | grep forward
sysctl: unable to open directory "/proc/sys/fs/binfmt_misc/"
net.ipv4.conf.all.forwarding = 0
net.ipv4.conf.default.forwarding = 0
net.ipv4.conf.eno1.forwarding = 0
net.ipv4.conf.lo.forwarding = 0
net.ipv4.conf.wlp1s0.forwarding = 0
net.ipv4.ip_forward = 0
net.ipv4.ip_forward_update_priority = 1
net.ipv4.ip_forward_use_pmtu = 0
~$ cat /proc/sys/net/ipv4/ip_forward
0
```

In the above example we can see that writing "1" to
/proc/sys/net/ipv4/ip_forward configures the stack to be a router (all
interfaces are configured to enable forwarding). However, if we manually
update an interace (`wlp1s0`) to not forward packets,
/proc/sys/net/ipv4/ip_forward continues to return the last written value
of "1", even though not all interfaces will forward packets.

Also note that writing the same value twice has no effect; work is
performed iff the value changes.

This change also removes the 'unset' state from sentry's ip forwarding
data structures as an 'unset' ip forwarding value is the same as leaving
forwarding disabled as the stack is always brought up with forwarding
initially disabled; disabling forwarding on a newly created stack is a
no-op.

PiperOrigin-RevId: 373853106

2 files changed, 0 insertions, 8 deletions

diff --git a/pkg/sentry/inet/inet.go b/pkg/sentry/inet/inet.go
index 6b71bd3a9..80dda1559 100644
--- a/pkg/sentry/inet/inet.go
+++ b/pkg/sentry/inet/inet.go
@@ -88,9 +88,6 @@ type Stack interface {
 	// for restoring a stack after a save.
 	RestoreCleanupEndpoints([]stack.TransportEndpoint)
 
-	// Forwarding returns if packet forwarding between NICs is enabled.
-	Forwarding(protocol tcpip.NetworkProtocolNumber) bool
-
 	// SetForwarding enables or disables packet forwarding between NICs.
 	SetForwarding(protocol tcpip.NetworkProtocolNumber, enable bool) error
 
diff --git a/pkg/sentry/inet/test_stack.go b/pkg/sentry/inet/test_stack.go
index 03e2608c2..218d9dafc 100644
--- a/pkg/sentry/inet/test_stack.go
+++ b/pkg/sentry/inet/test_stack.go
@@ -154,11 +154,6 @@ func (s *TestStack) CleanupEndpoints() []stack.TransportEndpoint {
 // RestoreCleanupEndpoints implements inet.Stack.RestoreCleanupEndpoints.
 func (s *TestStack) RestoreCleanupEndpoints([]stack.TransportEndpoint) {}
 
-// Forwarding implements inet.Stack.Forwarding.
-func (s *TestStack) Forwarding(protocol tcpip.NetworkProtocolNumber) bool {
-	return s.IPForwarding
-}
-
 // SetForwarding implements inet.Stack.SetForwarding.
 func (s *TestStack) SetForwarding(protocol tcpip.NetworkProtocolNumber, enable bool) error {
 	s.IPForwarding = enable