atomicfile: reject overwriting irregular files

The intent of atomicfile is to overwrite regular files. Most use cases
that would overwrite irregular files, unix sockets, named pipes,
devices, and so on are more than likely misuse, so disallow them.

Fixes #7658

Signed-off-by: James Tucker <james@tailscale.com>

atomicfile/atomicfile.go

@@ -8,14 +8,20 @@
 package atomicfile // import "tailscale.com/atomicfile"
 
 import (
+	"fmt"
 	"os"
 	"path/filepath"
 	"runtime"
 )
 
-// WriteFile writes data to filename+some suffix, then renames it
-// into filename. The perm argument is ignored on Windows.
+// WriteFile writes data to filename+some suffix, then renames it into filename.
+// The perm argument is ignored on Windows. If the target filename already
+// exists but is not a regular file, WriteFile returns an error.
 func WriteFile(filename string, data []byte, perm os.FileMode) (err error) {
+	fi, err := os.Stat(filename)
+	if err == nil && !fi.Mode().IsRegular() {
+		return fmt.Errorf("%s already exists and is not a regular file", filename)
+	}
 	f, err := os.CreateTemp(filepath.Dir(filename), filepath.Base(filename)+".tmp")
 	if err != nil {
 		return err