utils: use safe procfs for /proc/self/fd loop code

From a safety perspective this might not be strictly required, but it
paves the way for us to remove utils.ProcThreadSelf.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

internal/linux/linux.go

@@ -6,6 +6,23 @@ import (
 	"golang.org/x/sys/unix"
 )
 
+// Readlinkat wraps [unix.Readlinkat].
+func Readlinkat(dir *os.File, path string) (string, error) {
+	size := 4096
+	for {
+		linkBuf := make([]byte, size)
+		n, err := unix.Readlinkat(int(dir.Fd()), path, linkBuf)
+		if err != nil {
+			return "", &os.PathError{Op: "readlinkat", Path: dir.Name() + "/" + path, Err: err}
+		}
+		if n != size {
+			return string(linkBuf[:n]), nil
+		}
+		// Possible truncation, resize the buffer.
+		size *= 2
+	}
+}
+
 // GetPtyPeer is a wrapper for ioctl(TIOCGPTPEER).
 func GetPtyPeer(ptyFd uintptr, unsafePeerPath string, flags int) (*os.File, error) {
 	// Make sure O_NOCTTY is always set -- otherwise runc might accidentally

libcontainer/integration/exec_test.go

@@ -14,12 +14,13 @@ import (
 	"syscall"
 	"testing"
 
+	"github.com/opencontainers/runc/internal/linux"
+	"github.com/opencontainers/runc/internal/pathrs"
 	"github.com/opencontainers/runc/libcontainer"
 	"github.com/opencontainers/runc/libcontainer/cgroups"
 	"github.com/opencontainers/runc/libcontainer/cgroups/systemd"
 	"github.com/opencontainers/runc/libcontainer/configs"
 	"github.com/opencontainers/runc/libcontainer/internal/userns"
-	"github.com/opencontainers/runc/libcontainer/utils"
 	"github.com/opencontainers/runtime-spec/specs-go"
 
 	"golang.org/x/sys/unix"
@@ -1695,11 +1696,9 @@ func TestFdLeaksSystemd(t *testing.T) {
 }
 
 func fdList(t *testing.T) []string {
-	procSelfFd, closer := utils.ProcThreadSelf("fd")
-	defer closer()
-
-	fdDir, err := os.Open(procSelfFd)
+	fdDir, closer, err := pathrs.ProcThreadSelfOpen("fd/", unix.O_DIRECTORY|unix.O_CLOEXEC)
 	ok(t, err)
+	defer closer()
 	defer fdDir.Close()
 
 	fds, err := fdDir.Readdirnames(-1)
@@ -1738,8 +1737,10 @@ func testFdLeaks(t *testing.T, systemd bool) {
 
 	count := 0
 
-	procSelfFd, closer := utils.ProcThreadSelf("fd/")
+	procSelfFd, closer, err := pathrs.ProcThreadSelfOpen("fd/", unix.O_DIRECTORY|unix.O_CLOEXEC)
+	ok(t, err)
 	defer closer()
+	defer procSelfFd.Close()
 
 next_fd:
 	for _, fd1 := range fds1 {
@@ -1748,7 +1749,7 @@ next_fd:
 				continue next_fd
 			}
 		}
-		dst, _ := os.Readlink(filepath.Join(procSelfFd, fd1))
+		dst, _ := linux.Readlinkat(procSelfFd, fd1)
 		for _, ex := range excludedPaths {
 			if ex == dst {
 				continue next_fd

libcontainer/utils/utils_unix.go

@@ -13,6 +13,7 @@ import (
 	_ "unsafe" // for go:linkname
 
 	securejoin "github.com/cyphar/filepath-securejoin"
+	"github.com/opencontainers/runc/internal/pathrs"
 	"github.com/sirupsen/logrus"
 	"golang.org/x/sys/unix"
 )
@@ -58,15 +59,15 @@ type fdFunc func(fd int)
 // fdRangeFrom calls the passed fdFunc for each file descriptor that is open in
 // the current process.
 func fdRangeFrom(minFd int, fn fdFunc) error {
-	procSelfFd, closer := ProcThreadSelf("fd")
-	defer closer()
-
-	fdDir, err := os.Open(procSelfFd)
+	fdDir, closer, err := pathrs.ProcThreadSelfOpen("fd/", unix.O_DIRECTORY|unix.O_CLOEXEC)
 	if err != nil {
-		return err
+		return fmt.Errorf("get handle to /proc/thread-self/fd: %w", err)
 	}
+	defer closer()
 	defer fdDir.Close()
 
+	// NOTE: This is not really necessary since securejoin.ProcThreadSelf
+	// verifies this in a far stricter sense than EnsureProcHandle.
 	if err := EnsureProcHandle(fdDir); err != nil {
 		return err
 	}

utils_linux.go

@@ -16,6 +16,7 @@ import (
 	"github.com/urfave/cli"
 	"golang.org/x/sys/unix"
 
+	"github.com/opencontainers/runc/internal/pathrs"
 	"github.com/opencontainers/runc/libcontainer"
 	"github.com/opencontainers/runc/libcontainer/configs"
 	"github.com/opencontainers/runc/libcontainer/specconv"
@@ -229,10 +230,14 @@ func (r *runner) run(config *specs.Process) (int, error) {
 		process.ExtraFiles = append(process.ExtraFiles, r.listenFDs...)
 	}
 	baseFd := 3 + len(process.ExtraFiles)
-	procSelfFd, closer := utils.ProcThreadSelf("fd/")
+	procSelfFd, closer, err := pathrs.ProcThreadSelfOpen("fd/", unix.O_DIRECTORY|unix.O_CLOEXEC)
+	if err != nil {
+		return -1, err
+	}
 	defer closer()
+	defer procSelfFd.Close()
 	for i := baseFd; i < baseFd+r.preserveFDs; i++ {
-		_, err = os.Stat(filepath.Join(procSelfFd, strconv.Itoa(i)))
+		err := unix.Faccessat(int(procSelfFd.Fd()), strconv.Itoa(i), unix.F_OK, 0)
 		if err != nil {
 			return -1, fmt.Errorf("unable to stat preserved-fd %d (of %d): %w", i-baseFd, r.preserveFDs, err)
 		}