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| 1 | //go:build linux |
| 2 | |
| 3 | package instanceexec |
| 4 | |
| 5 | import ( |
| 6 | "errors" |
| 7 | "net" |
| 8 | "syscall" |
| 9 | "time" |
| 10 | |
| 11 | "golang.org/x/sys/unix" |
| 12 | ) |
| 13 | |
| 14 | // accessWritable asks the kernel whether path could be opened for writing |
| 15 | // without opening it: on a read-only mount access(2) answers EROFS, which |
| 16 | // is exactly the property ProtectControlGroups= must give a tenant. |
| 17 | func accessWritable(path string) error { |
| 18 | return unix.Access(path, unix.W_OK) |
| 19 | } |
| 20 | |
| 21 | // sendOneDatagram sends one empty UDP datagram. With a cgroup BPF egress |
| 22 | // filter (IPAddressDeny=) the filter's verdict is sendmsg's return value, |
| 23 | // EPERM; with no route it is ENETUNREACH; on a live path it is nil. |
| 24 | func sendOneDatagram(network, addr string) error { |
| 25 | c, err := net.Dial(network, addr) |
| 26 | if err != nil { |
| 27 | return unwrapErrno(err) |
| 28 | } |
| 29 | defer c.Close() |
| 30 | _, err = c.Write([]byte{}) |
| 31 | return unwrapErrno(err) |
| 32 | } |
| 33 | |
| 34 | func connectWithTimeout(addr string, timeout time.Duration) error { |
| 35 | c, err := net.DialTimeout("tcp", addr, timeout) |
| 36 | if err != nil { |
| 37 | return unwrapErrno(err) |
| 38 | } |
| 39 | c.Close() |
| 40 | return nil |
| 41 | } |
| 42 | |
| 43 | // unwrapErrno returns the syscall.Errno inside err so callers can |
| 44 | // errors.Is against EPERM and ENETUNREACH; net.OpError and os.SyscallError |
| 45 | // both unwrap to the Errno, so errors.As finds it. Anything else is |
| 46 | // returned as is. (unix.Errno is an alias of syscall.Errno.) |
| 47 | func unwrapErrno(err error) error { |
| 48 | var errno syscall.Errno |
| 49 | if errors.As(err, &errno) { |
| 50 | return errno |
| 51 | } |
| 52 | return err |
| 53 | } |
| 54 | |
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