procfs: make procSelfFdReadlink more generic with generics

In order to add a reopen primitive, it'll be necessary to do very
similar checks to procSelfFdReadlink but with slightly different types.
By refactoring procSelfFdReadlink with generics we can use the same
codepath for both operations.

Signed-off-by: Aleksa Sarai <[email protected]>

procfs_linux.go

@@ -10,6 +10,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"path/filepath"
 	"runtime"
 	"strconv"
 	"sync"
@@ -380,12 +381,17 @@ func checkSymlinkOvermount(dir *os.File, path string) error {
 	return nil
 }
 
-func doRawProcSelfFdReadlink(procRoot *os.File, fd int) (string, error) {
-	procSelfFd, closer, err := procThreadSelf(procRoot, "fd/")
+func doProcSelfMagiclink[T any](procRoot *os.File, subPath string, fn func(procDirHandle *os.File, base string) (T, error)) (T, error) {
+	// We cannot operate on the magic-link directly with a handle, we need to
+	// create a handle to the parent of the magic-link and then do
+	// single-component operations on it.
+	dir, base := filepath.Dir(subPath), filepath.Base(subPath)
+
+	procDirHandle, closer, err := procThreadSelf(procRoot, dir)
 	if err != nil {
-		return "", fmt.Errorf("get safe /proc/thread-self/fd handle: %w", err)
+		return *new(T), fmt.Errorf("get safe /proc/thread-self/%s handle: %w", dir, err)
 	}
-	defer procSelfFd.Close()
+	defer procDirHandle.Close()
 	defer closer()
 
 	// Try to detect if there is a mount on top of the symlink we are about to
@@ -397,12 +403,15 @@ func doRawProcSelfFdReadlink(procRoot *os.File, fd int) (string, error) {
 	//
 	// [1]: Linux commit ee2e3f50629f ("mount: fix mounting of detached mounts
 	// onto targets that reside on shared mounts").
-	fdStr := strconv.Itoa(fd)
-	if err := checkSymlinkOvermount(procSelfFd, fdStr); err != nil {
-		return "", fmt.Errorf("check safety of fd %d proc magiclink: %w", fd, err)
+	if err := checkSymlinkOvermount(procDirHandle, base); err != nil {
+		return *new(T), fmt.Errorf("check safety of %s proc magiclink: %w", subPath, err)
 	}
-	// Get the contents of the link.
-	return readlinkatFile(procSelfFd, fdStr)
+	return fn(procDirHandle, base)
+}
+
+func doRawProcSelfFdReadlink(procRoot *os.File, fd int) (string, error) {
+	fdPath := fmt.Sprintf("fd/%d", fd)
+	return doProcSelfMagiclink(procRoot, fdPath, readlinkatFile)
 }
 
 func rawProcSelfFdReadlink(fd int) (string, error) {