unix: add O_CLOFORK and related constants, support it in Dup3

unix/dup3_clofork_test.go

@@ -0,0 +1,66 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build dragonfly || freebsd
+
+package unix_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"golang.org/x/sys/unix"
+)
+
+func TestDup3Flags(t *testing.T) {
+	f, err := os.Create(filepath.Join(t.TempDir(), "TestDup3Flags"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	oldFd := int(f.Fd())
+
+	tests := []struct {
+		name   string
+		flags  int
+		fdflag int
+	}{
+		{"none", 0, 0},
+		{"O_CLOEXEC", unix.O_CLOEXEC, unix.FD_CLOEXEC},
+		{"O_CLOFORK", unix.O_CLOFORK, unix.FD_CLOFORK},
+		{"O_CLOEXEC|O_CLOFORK", unix.O_CLOEXEC | unix.O_CLOFORK, unix.FD_CLOEXEC | unix.FD_CLOFORK},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			newFd, err := unix.Open(os.DevNull, unix.O_RDONLY, 0)
+			if err != nil {
+				t.Fatal(err)
+			}
+			defer unix.Close(newFd)
+
+			err = unix.Dup3(oldFd, newFd, tt.flags)
+			if err == unix.EINVAL && tt.flags&unix.O_CLOFORK != 0 {
+				t.Skip("kernel does not support F_DUP3FD")
+			}
+			if err != nil {
+				t.Fatalf("Dup3(%d, %d, %#x): %v", oldFd, newFd, tt.flags, err)
+			}
+			got, err := unix.FcntlInt(uintptr(newFd), unix.F_GETFD, 0)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if want := tt.fdflag; got != want {
+				t.Errorf("F_GETFD = %#x, want %#x", got, want)
+			}
+		})
+	}
+
+	if got, want := unix.Dup3(oldFd, oldFd, 0), unix.EINVAL; got != want {
+		t.Errorf("Dup3 with equal fds = %v, want %v", got, want)
+	}
+	if got, want := unix.Dup3(oldFd, 1000, unix.O_APPEND), unix.EINVAL; got != want {
+		t.Errorf("Dup3 with invalid flags = %v, want %v", got, want)
+	}
+}

unix/syscall_dragonfly.go

@@ -247,14 +247,24 @@ func Sendfile(outfd int, infd int, offset *int64, count int) (written int, err e
 }
 
 func Dup3(oldfd, newfd, flags int) error {
-	if oldfd == newfd || flags&^O_CLOEXEC != 0 {
+	if oldfd == newfd || flags&^(O_CLOEXEC|O_CLOFORK) != 0 {
 		return EINVAL
 	}
-	how := F_DUP2FD
+	if flags&O_CLOFORK == 0 {
+		// F_DUP3FD is not available on older kernels, so avoid it
+		// when possible.
+		how := F_DUP2FD
+		if flags&O_CLOEXEC != 0 {
+			how = F_DUP2FD_CLOEXEC
+		}
+		_, err := fcntl(oldfd, how, newfd)
+		return err
+	}
+	fdflags := FD_CLOFORK
 	if flags&O_CLOEXEC != 0 {
-		how = F_DUP2FD_CLOEXEC
+		fdflags |= FD_CLOEXEC
 	}
-	_, err := fcntl(oldfd, how, newfd)
+	_, err := fcntl(oldfd, F_DUP3FD|fdflags<<F_DUP3FD_SHIFT, newfd)
 	return err
 }
 

unix/syscall_freebsd.go

@@ -328,14 +328,23 @@ func PtraceSingleStep(pid int) (err error) {
 }
 
 func Dup3(oldfd, newfd, flags int) error {
-	if oldfd == newfd || flags&^O_CLOEXEC != 0 {
+	if oldfd == newfd || flags&^(O_CLOEXEC|O_CLOFORK) != 0 {
 		return EINVAL
 	}
-	how := F_DUP2FD
+	if flags&O_CLOFORK == 0 {
+		// F_DUP3FD requires FreeBSD 15.0, so avoid it when possible.
+		how := F_DUP2FD
+		if flags&O_CLOEXEC != 0 {
+			how = F_DUP2FD_CLOEXEC
+		}
+		_, err := fcntl(oldfd, how, newfd)
+		return err
+	}
+	fdflags := FD_CLOFORK
 	if flags&O_CLOEXEC != 0 {
-		how = F_DUP2FD_CLOEXEC
+		fdflags |= FD_CLOEXEC
 	}
-	_, err := fcntl(oldfd, how, newfd)
+	_, err := fcntl(oldfd, F_DUP3FD|fdflags<<F_DUP3FD_SHIFT, newfd)
 	return err
 }