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device: improve MTU change handling

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The old code silently accepted negative MTUs.
It also set MTUs above the maximum.
It also had hard to follow deeply nested conditionals.

Add more paranoid handling,
and make the code more straight-line.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>
This commit is contained in:
Josh Bleecher Snyder 2021-01-21 09:23:45 -08:00
parent 3516ccc1e2
commit 675ff32e6c
1 changed files with 15 additions and 8 deletions
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21
device/tun.go
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@ -6,6 +6,7 @@
package device
import (
"fmt"
"sync/atomic"
"golang.zx2c4.com/wireguard/tun"
@ -20,16 +21,22 @@ func (device *Device) RoutineTUNEventReader() {
for event := range device.tun.device.Events() {
if event&tun.EventMTUUpdate != 0 {
mtu, err := device.tun.device.MTU()
old := atomic.LoadInt32(&device.tun.mtu)
if err != nil {
device.log.Errorf("Failed to load updated MTU of device: %v", err)
} else if int(old) != mtu {
if mtu+MessageTransportSize > MaxMessageSize {
device.log.Verbosef("MTU updated: %v (too large)", mtu)
} else {
device.log.Verbosef("MTU updated: %v", mtu)
continue
}
atomic.StoreInt32(&device.tun.mtu, int32(mtu))
if mtu < 0 {
device.log.Errorf("MTU not updated to negative value: %v", mtu)
continue
}
var tooLarge string
if mtu > MaxContentSize {
tooLarge = fmt.Sprintf(" (too large, capped at %v)", MaxContentSize)
mtu = MaxContentSize
}
old := atomic.SwapInt32(&device.tun.mtu, int32(mtu))
if int(old) != mtu {
device.log.Verbosef("MTU updated: %v%s", mtu, tooLarge)
}
}