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net/netns: add post-review comments 

Follow-up to #7065 with some comments from Brad's review.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ia1219f4fa25479b2dada38ffe421065b408c5954
This commit is contained in:
Andrew Dunham 2023-01-27 16:05:10 -05:00
parent 2fc8de485c
commit 50da265608
1 changed files with 24 additions and 1 deletions
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25
net/netns/netns_darwin.go
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@ -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 {