From 50da2656089880061db06727512cfdaeda4b29e8 Mon Sep 17 00:00:00 2001 From: Andrew Dunham Date: Fri, 27 Jan 2023 16:05:10 -0500 Subject: [PATCH] net/netns: add post-review comments Follow-up to #7065 with some comments from Brad's review. Signed-off-by: Andrew Dunham Change-Id: Ia1219f4fa25479b2dada38ffe421065b408c5954 --- net/netns/netns_darwin.go | 25 ++++++++++++++++++++++++- 1 file changed, 24 insertions(+), 1 deletion(-) diff --git a/net/netns/netns_darwin.go b/net/netns/netns_darwin.go index 7083c89bd..92ad266e9 100644 --- a/net/netns/netns_darwin.go +++ b/net/netns/netns_darwin.go @@ -115,7 +115,11 @@ func interfaceIndexFor(addr netip.Addr, canRecurse bool) (int, error) { } rm := route.RouteMessage{ - Version: unix.RTM_VERSION, + // NOTE: This is unix.RTM_VERSION, but we want to pin this to a + // particular constant so that it doesn't change under us if + // the x/sys/unix package changes down the road. Currently this + // is 0x5 on both Darwin x86 and ARM64. + Version: 0x5, Type: unix.RTM_GET, Flags: unix.RTF_UP, ID: uintptr(os.Getpid()), @@ -132,6 +136,25 @@ func interfaceIndexFor(addr netip.Addr, canRecurse bool) (int, error) { if err != nil { return 0, fmt.Errorf("writing message: %w", err) } + + // On macOS, the RTM_GET call should return exactly one route message. + // Given the following sizes and constants: + // - sizeof(struct rt_msghdr) = 92 + // - RTAX_MAX = 8 + // - sizeof(struct sockaddr_in6) = 28 + // - sizeof(struct sockaddr_in) = 16 + // - sizeof(struct sockaddr_dl) = 20 + // + // The maximum buffer size should be: + // sizeof(struct rt_msghdr) + RTAX_MAX*sizeof(struct sockaddr_in6) + // = 92 + 8*28 + // = 316 + // + // During my testing, responses are typically ~120 bytes. + // + // We provide a much larger buffer just in case we're off by a bit, or + // the kernel decides to return more than one message; 2048 bytes + // should be plenty here. This also means we can do a single Read. var buf [2048]byte n, err := unix.Read(fd, buf[:]) if err != nil {