syscall: use fcntl F_DUP2FD_CLOEXEC in forkAndExecInChild on illumos

Use fcntl(oldfd, F_DUP2FD_CLOEXEC, newfd) to duplicate the file
descriptor and mark is as close-on-exec instead of dup2 & fcntl.

Illumos implements dup3 like this in libc.

Change-Id: I9782bce553ffb832e9b1a12bbf3c0a40c821f56e
Reviewed-on: https://go-review.googlesource.com/c/go/+/358374


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/syscall/exec_libc.go

@@ -10,6 +10,7 @@
 package syscall
 
 import (
+	"runtime"
 	"unsafe"
 )
 
@@ -197,11 +198,19 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 	// Pass 1: look for fd[i] < i and move those up above len(fd)
 	// so that pass 2 won't stomp on an fd it needs later.
 	if pipe < nextfd {
-		_, err1 = dup2child(uintptr(pipe), uintptr(nextfd))
+		switch runtime.GOOS {
+		case "illumos":
+			_, err1 = fcntl1(uintptr(pipe), _F_DUP2FD_CLOEXEC, uintptr(nextfd))
+		default:
+			_, err1 = dup2child(uintptr(pipe), uintptr(nextfd))
+			if err1 != 0 {
+				goto childerror
+			}
+			_, err1 = fcntl1(uintptr(nextfd), F_SETFD, FD_CLOEXEC)
+		}
 		if err1 != 0 {
 			goto childerror
 		}
-		fcntl1(uintptr(nextfd), F_SETFD, FD_CLOEXEC)
 		pipe = nextfd
 		nextfd++
 	}
@@ -210,11 +219,16 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 			if nextfd == pipe { // don't stomp on pipe
 				nextfd++
 			}
-			_, err1 = dup2child(uintptr(fd[i]), uintptr(nextfd))
-			if err1 != 0 {
-				goto childerror
+			switch runtime.GOOS {
+			case "illumos":
+				_, err1 = fcntl1(uintptr(fd[i]), _F_DUP2FD_CLOEXEC, uintptr(nextfd))
+			default:
+				_, err1 = dup2child(uintptr(fd[i]), uintptr(nextfd))
+				if err1 != 0 {
+					goto childerror
+				}
+				_, err1 = fcntl1(uintptr(nextfd), F_SETFD, FD_CLOEXEC)
 			}
-			_, err1 = fcntl1(uintptr(nextfd), F_SETFD, FD_CLOEXEC)
 			if err1 != 0 {
 				goto childerror
 			}

src/syscall/syscall_aix.go

@@ -31,6 +31,8 @@ const (
 	F_DUPFD_CLOEXEC = 0
 	// AF_LOCAL doesn't exist on AIX
 	AF_LOCAL = AF_UNIX
+
+	_F_DUP2FD_CLOEXEC = 0
 )
 
 func (ts *StTimespec_t) Unix() (sec int64, nsec int64) {

src/syscall/syscall_solaris.go

@@ -14,6 +14,8 @@ package syscall
 
 import "unsafe"
 
+const _F_DUP2FD_CLOEXEC = F_DUP2FD_CLOEXEC
+
 // Implemented in asm_solaris_amd64.s.
 func rawSysvicall6(trap, nargs, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err Errno)
 func sysvicall6(trap, nargs, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err Errno)