device: add test to ensure Peer fields are safe for atomic access on 32-bit

Adds a test that will fail consistently on 32-bit platforms if the
struct ever changes again to violate the rules. This is likely not
needed because unaligned access crashes reliably, but this will reliably
fail even if tests accidentally pass due to lucky alignment.

Signed-Off-By: David Anderson <danderson@tailscale.com>
This commit is contained in:
David Anderson 2020-03-01 00:39:24 -08:00 committed by Jason A. Donenfeld
parent 224bc9e60c
commit 3dce460c88
2 changed files with 48 additions and 1 deletions

View File

@ -27,7 +27,11 @@ type Peer struct {
endpoint Endpoint
persistentKeepaliveInterval uint16
// This must be 64-bit aligned, so make sure the above members come out to even alignment and pad accordingly
// These fields are accessed with atomic operations, which must be
// 64-bit aligned even on 32-bit platforms. Go guarantees that an
// allocated struct will be 64-bit aligned. So we place
// atomically-accessed fields up front, so that they can share in
// this alignment before smaller fields throw it off.
stats struct {
txBytes uint64 // bytes send to peer (endpoint)
rxBytes uint64 // bytes received from peer

43
device/peer_test.go Normal file
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@ -0,0 +1,43 @@
/* SPDX-License-Identifier: MIT
*
* Copyright (C) 2017-2019 WireGuard LLC. All Rights Reserved.
*/
package device
import (
"reflect"
"testing"
"unsafe"
)
func checkAlignment(t *testing.T, name string, offset uintptr) {
t.Helper()
if offset%8 != 0 {
t.Errorf("offset of %q within struct is %d bytes, which does not align to 64-bit word boundaries (missing %d bytes). Atomic operations will crash on 32-bit systems.", name, offset, 8-(offset%8))
}
}
// TestPeerAlignment checks that atomically-accessed fields are
// aligned to 64-bit boundaries, as required by the atomic package.
//
// Unfortunately, violating this rule on 32-bit platforms results in a
// hard segfault at runtime.
func TestPeerAlignment(t *testing.T) {
var p Peer
typ := reflect.TypeOf(p)
t.Logf("Peer type size: %d, with fields:", typ.Size())
for i := 0; i < typ.NumField(); i++ {
field := typ.Field(i)
t.Logf("\t%30s\toffset=%3v\t(type size=%3d, align=%d)",
field.Name,
field.Offset,
field.Type.Size(),
field.Type.Align(),
)
}
checkAlignment(t, "Peer.stats", unsafe.Offsetof(p.stats))
checkAlignment(t, "Peer.isRunning", unsafe.Offsetof(p.isRunning))
}