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>
pull/7799/head
James Tucker 2 years ago committed by James Tucker
parent 02582083d5
commit f4da995940

@ -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

@ -0,0 +1,38 @@
// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
//go:build !js && !windows
package atomicfile
import (
"net"
"path/filepath"
"strings"
"testing"
)
func TestDoesNotOverwriteIrregularFiles(t *testing.T) {
// Per tailscale/tailscale#7658 as one example, almost any imagined use of
// atomicfile.Write should likely not attempt to overwrite an irregular file
// such as a device node, socket, or named pipe.
d := t.TempDir()
special := filepath.Join(d, "special")
// The least troublesome thing to make that is not a file is a unix socket.
// Making a null device sadly requries root.
l, err := net.ListenUnix("unix", &net.UnixAddr{Name: special, Net: "unix"})
if err != nil {
t.Fatal(err)
}
defer l.Close()
err = WriteFile(special, []byte("hello"), 0644)
if err == nil {
t.Fatal("expected error, got nil")
}
if !strings.Contains(err.Error(), "is not a regular file") {
t.Fatalf("unexpected error: %v", err)
}
}
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