Error if dup'ing stdio FDs will clobber another FD

The loader dup's stdio FD into stable FD's starting at a fixed
number. During tests, it's possible that the target FD is already
in use. Added check to error early so it's easier to debug failures.
Also bumped up the starting FD number to prevent collisions.

PiperOrigin-RevId: 324917299

runsc/boot/loader.go

@@ -16,12 +16,12 @@
 package boot
 
 import (
+	"errors"
 	"fmt"
 	mrand "math/rand"
 	"os"
 	"runtime"
 	"sync/atomic"
-	"syscall"
 	gtime "time"
 
 	specs "github.com/opencontainers/runtime-spec/specs-go"
@@ -187,7 +187,7 @@ type Args struct {
 }
 
 // make sure stdioFDs are always the same on initial start and on restore
-const startingStdioFD = 64
+const startingStdioFD = 256
 
 // New initializes a new kernel loader configured by spec.
 // New also handles setting up a kernel for restoring a container.
@@ -360,15 +360,20 @@ func New(args Args) (*Loader, error) {
 	var stdioFDs []int
 	newfd := startingStdioFD
 	for _, fd := range args.StdioFDs {
-		err := syscall.Dup3(fd, newfd, syscall.O_CLOEXEC)
+		// Check that newfd is unused to avoid clobbering over it.
+		if _, err := unix.FcntlInt(uintptr(newfd), unix.F_GETFD, 0); !errors.Is(err, unix.EBADF) {
+			if err != nil {
+				return nil, fmt.Errorf("error checking for FD (%d) conflict: %w", newfd, err)
+			}
+			return nil, fmt.Errorf("unable to remap stdios, FD %d is already in use", newfd)
+		}
+
+		err := unix.Dup3(fd, newfd, unix.O_CLOEXEC)
 		if err != nil {
 			return nil, fmt.Errorf("dup3 of stdioFDs failed: %v", err)
 		}
 		stdioFDs = append(stdioFDs, newfd)
-		err = syscall.Close(fd)
-		if err != nil {
-			return nil, fmt.Errorf("close original stdioFDs failed: %v", err)
-		}
+		_ = unix.Close(fd)
 		newfd++
 	}
 
@@ -458,6 +463,11 @@ func (l *Loader) Destroy() {
 		l.stopSignalForwarding()
 	}
 	l.watchdog.Stop()
+
+	for i, fd := range l.root.stdioFDs {
+		_ = unix.Close(fd)
+		l.root.stdioFDs[i] = -1
+	}
 }
 
 func createPlatform(conf *Config, deviceFile *os.File) (platform.Platform, error) {
@@ -591,11 +601,9 @@ func (l *Loader) run() error {
 	// during restore, we can release l.stdioFDs now. VFS2 takes ownership of the
 	// passed FDs, so only close for VFS1.
 	if !kernel.VFS2Enabled {
-		for _, fd := range l.root.stdioFDs {
-			err := syscall.Close(fd)
-			if err != nil {
-				return fmt.Errorf("close dup()ed stdioFDs: %v", err)
-			}
+		for i, fd := range l.root.stdioFDs {
+			_ = unix.Close(fd)
+			l.root.stdioFDs[i] = -1
 		}
 	}
 
@@ -686,7 +694,7 @@ func (l *Loader) startContainer(spec *specs.Spec, conf *Config, cid string, file
 
 	// Can't take ownership away from os.File. dup them to get a new FDs.
 	for _, f := range files[3:] {
-		fd, err := syscall.Dup(int(f.Fd()))
+		fd, err := unix.Dup(int(f.Fd()))
 		if err != nil {
 			return fmt.Errorf("failed to dup file: %v", err)
 		}