internal/sys: add VerifyInode helper

This will be used for a few security patches in later patches in this
patchset. The need to verify what kind of inode we are operating on in a
race-free way turns out to be quite a common pattern...

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

internal/sys/doc.go

@@ -0,0 +1,5 @@
+// Package sys is an internal package that contains helper methods for dealing
+// with Linux that are more complicated than basic wrappers. Basic wrappers
+// usually belong in internal/linux. If you feel something belongs in
+// libcontainer/utils or libcontainer/system, it probably belongs here instead.
+package sys

internal/sys/verify_inode_unix.go

@@ -0,0 +1,30 @@
+package sys
+
+import (
+	"fmt"
+	"os"
+	"runtime"
+
+	"golang.org/x/sys/unix"
+)
+
+// VerifyInodeFunc is the callback passed to [VerifyInode] to check if the
+// inode is the expected type (and on the correct filesystem type, in the case
+// of filesystem-specific inodes).
+type VerifyInodeFunc func(stat *unix.Stat_t, statfs *unix.Statfs_t) error
+
+// VerifyInode verifies that the underlying inode for the given file matches an
+// expected inode type (possibly on a particular kind of filesystem). This is
+// mainly a wrapper around [VerifyInodeFunc].
+func VerifyInode(file *os.File, checkFunc VerifyInodeFunc) error {
+	var stat unix.Stat_t
+	if err := unix.Fstat(int(file.Fd()), &stat); err != nil {
+		return fmt.Errorf("fstat %q: %w", file.Name(), err)
+	}
+	var statfs unix.Statfs_t
+	if err := unix.Fstatfs(int(file.Fd()), &statfs); err != nil {
+		return fmt.Errorf("fstatfs %q: %w", file.Name(), err)
+	}
+	runtime.KeepAlive(file)
+	return checkFunc(&stat, &statfs)
+}