4 files changed, 273 insertions, 0 deletions
diff --git a/src/internal/safefilepath/path.go b/src/internal/safefilepath/path.go
new file mode 100644
index 0000000..0f0a270
--- /dev/null
+++ b/src/internal/safefilepath/path.go
@@ -0,0 +1,21 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package safefilepath manipulates operating-system file paths.
+package safefilepath
+
+import (
+	"errors"
+)
+
+var errInvalidPath = errors.New("invalid path")
+
+// FromFS converts a slash-separated path into an operating-system path.
+//
+// FromFS returns an error if the path cannot be represented by the operating
+// system. For example, paths containing '\' and ':' characters are rejected
+// on Windows.
+func FromFS(path string) (string, error) {
+	return fromFS(path)
+}
diff --git a/src/internal/safefilepath/path_other.go b/src/internal/safefilepath/path_other.go
new file mode 100644
index 0000000..974e775
--- /dev/null
+++ b/src/internal/safefilepath/path_other.go
@@ -0,0 +1,23 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !windows
+
+package safefilepath
+
+import "runtime"
+
+func fromFS(path string) (string, error) {
+	if runtime.GOOS == "plan9" {
+		if len(path) > 0 && path[0] == '#' {
+			return "", errInvalidPath
+		}
+	}
+	for i := range path {
+		if path[i] == 0 {
+			return "", errInvalidPath
+		}
+	}
+	return path, nil
+}
diff --git a/src/internal/safefilepath/path_test.go b/src/internal/safefilepath/path_test.go
new file mode 100644
index 0000000..dc662c1
--- /dev/null
+++ b/src/internal/safefilepath/path_test.go
@@ -0,0 +1,88 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package safefilepath_test
+
+import (
+	"internal/safefilepath"
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+)
+
+type PathTest struct {
+	path, result string
+}
+
+const invalid = ""
+
+var fspathtests = []PathTest{
+	{".", "."},
+	{"/a/b/c", "/a/b/c"},
+	{"a\x00b", invalid},
+}
+
+var winreservedpathtests = []PathTest{
+	{`a\b`, `a\b`},
+	{`a:b`, `a:b`},
+	{`a/b:c`, `a/b:c`},
+	{`NUL`, `NUL`},
+	{`./com1`, `./com1`},
+	{`a/nul/b`, `a/nul/b`},
+}
+
+// Whether a reserved name with an extension is reserved or not varies by
+// Windows version.
+var winreservedextpathtests = []PathTest{
+	{"nul.txt", "nul.txt"},
+	{"a/nul.txt/b", "a/nul.txt/b"},
+}
+
+var plan9reservedpathtests = []PathTest{
+	{`#c`, `#c`},
+}
+
+func TestFromFS(t *testing.T) {
+	switch runtime.GOOS {
+	case "windows":
+		if canWriteFile(t, "NUL") {
+			t.Errorf("can unexpectedly write a file named NUL on Windows")
+		}
+		if canWriteFile(t, "nul.txt") {
+			fspathtests = append(fspathtests, winreservedextpathtests...)
+		} else {
+			winreservedpathtests = append(winreservedpathtests, winreservedextpathtests...)
+		}
+		for i := range winreservedpathtests {
+			winreservedpathtests[i].result = invalid
+		}
+		for i := range fspathtests {
+			fspathtests[i].result = filepath.FromSlash(fspathtests[i].result)
+		}
+	case "plan9":
+		for i := range plan9reservedpathtests {
+			plan9reservedpathtests[i].result = invalid
+		}
+	}
+	tests := fspathtests
+	tests = append(tests, winreservedpathtests...)
+	tests = append(tests, plan9reservedpathtests...)
+	for _, test := range tests {
+		got, err := safefilepath.FromFS(test.path)
+		if (got == "") != (err != nil) {
+			t.Errorf(`FromFS(%q) = %q, %v; want "" only if err != nil`, test.path, got, err)
+		}
+		if got != test.result {
+			t.Errorf("FromFS(%q) = %q, %v; want %q", test.path, got, err, test.result)
+		}
+	}
+}
+
+func canWriteFile(t *testing.T, name string) bool {
+	path := filepath.Join(t.TempDir(), name)
+	os.WriteFile(path, []byte("ok"), 0666)
+	b, _ := os.ReadFile(path)
+	return string(b) == "ok"
+}
diff --git a/src/internal/safefilepath/path_windows.go b/src/internal/safefilepath/path_windows.go
new file mode 100644
index 0000000..7cfd6ce
--- /dev/null
+++ b/src/internal/safefilepath/path_windows.go
@@ -0,0 +1,141 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package safefilepath
+
+import (
+	"syscall"
+	"unicode/utf8"
+)
+
+func fromFS(path string) (string, error) {
+	if !utf8.ValidString(path) {
+		return "", errInvalidPath
+	}
+	for len(path) > 1 && path[0] == '/' && path[1] == '/' {
+		path = path[1:]
+	}
+	containsSlash := false
+	for p := path; p != ""; {
+		// Find the next path element.
+		i := 0
+		for i < len(p) && p[i] != '/' {
+			switch p[i] {
+			case 0, '\\', ':':
+				return "", errInvalidPath
+			}
+			i++
+		}
+		part := p[:i]
+		if i < len(p) {
+			containsSlash = true
+			p = p[i+1:]
+		} else {
+			p = ""
+		}
+		if IsReservedName(part) {
+			return "", errInvalidPath
+		}
+	}
+	if containsSlash {
+		// We can't depend on strings, so substitute \ for / manually.
+		buf := []byte(path)
+		for i, b := range buf {
+			if b == '/' {
+				buf[i] = '\\'
+			}
+		}
+		path = string(buf)
+	}
+	return path, nil
+}
+
+// IsReservedName reports if name is a Windows reserved device name.
+// It does not detect names with an extension, which are also reserved on some Windows versions.
+//
+// For details, search for PRN in
+// https://docs.microsoft.com/en-us/windows/desktop/fileio/naming-a-file.
+func IsReservedName(name string) bool {
+	// Device names can have arbitrary trailing characters following a dot or colon.
+	base := name
+	for i := 0; i < len(base); i++ {
+		switch base[i] {
+		case ':', '.':
+			base = base[:i]
+		}
+	}
+	// Trailing spaces in the last path element are ignored.
+	for len(base) > 0 && base[len(base)-1] == ' ' {
+		base = base[:len(base)-1]
+	}
+	if !isReservedBaseName(base) {
+		return false
+	}
+	if len(base) == len(name) {
+		return true
+	}
+	// The path element is a reserved name with an extension.
+	// Some Windows versions consider this a reserved name,
+	// while others do not. Use FullPath to see if the name is
+	// reserved.
+	if p, _ := syscall.FullPath(name); len(p) >= 4 && p[:4] == `\\.\` {
+		return true
+	}
+	return false
+}
+
+func isReservedBaseName(name string) bool {
+	if len(name) == 3 {
+		switch string([]byte{toUpper(name[0]), toUpper(name[1]), toUpper(name[2])}) {
+		case "CON", "PRN", "AUX", "NUL":
+			return true
+		}
+	}
+	if len(name) >= 4 {
+		switch string([]byte{toUpper(name[0]), toUpper(name[1]), toUpper(name[2])}) {
+		case "COM", "LPT":
+			if len(name) == 4 && '1' <= name[3] && name[3] <= '9' {
+				return true
+			}
+			// Superscript ¹, ², and ³ are considered numbers as well.
+			switch name[3:] {
+			case "\u00b2", "\u00b3", "\u00b9":
+				return true
+			}
+			return false
+		}
+	}
+
+	// Passing CONIN$ or CONOUT$ to CreateFile opens a console handle.
+	// https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilea#consoles
+	//
+	// While CONIN$ and CONOUT$ aren't documented as being files,
+	// they behave the same as CON. For example, ./CONIN$ also opens the console input.
+	if len(name) == 6 && name[5] == '$' && equalFold(name, "CONIN$") {
+		return true
+	}
+	if len(name) == 7 && name[6] == '$' && equalFold(name, "CONOUT$") {
+		return true
+	}
+	return false
+}
+
+func equalFold(a, b string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := 0; i < len(a); i++ {
+		if toUpper(a[i]) != toUpper(b[i]) {
+			return false
+		}
+	}
+	return true
+}
+
+func toUpper(c byte) byte {
+	if 'a' <= c && c <= 'z' {
+		return c - ('a' - 'A')
+	}
+	return c
+}