runtime: support long paths without fixup on Windows 10 >= 1607

Windows 10 >= 1607 allows CreateFile and friends to use long paths if
bit 0x80 of the PEB's BitField member is set.

In time this means we'll be able to entirely drop our long path hacks,
which have never really worked right (see bugs below). Until that point,
we'll simply have things working well on recent Windows.

Updates #41734.
Updates #21782.
Updates #36375.

Change-Id: I765de6ea4859dd4e4b8ca80af7f337994734118e
Reviewed-on: https://go-review.googlesource.com/c/go/+/291291
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/export_windows_test.go

@@ -8,6 +8,7 @@ package os
 
 var (
 	FixLongPath       = fixLongPath
+	CanUseLongPaths   = canUseLongPaths
 	NewConsoleFile    = newConsoleFile
 	CommandLineToArgv = commandLineToArgv
 )

src/os/path_windows.go

@@ -128,6 +128,10 @@ func dirname(path string) string {
 	return vol + dir
 }
 
+// This is set via go:linkname on runtime.canUseLongPaths, and is true when the OS
+// supports opting into proper long path handling without the need for fixups.
+var canUseLongPaths bool
+
 // fixLongPath returns the extended-length (\\?\-prefixed) form of
 // path when needed, in order to avoid the default 260 character file
 // path limit imposed by Windows. If path is not easily converted to
@@ -137,6 +141,9 @@ func dirname(path string) string {
 //
 // See https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx#maxpath
 func fixLongPath(path string) string {
+	if canUseLongPaths {
+		return path
+	}
 	// Do nothing (and don't allocate) if the path is "short".
 	// Empirically (at least on the Windows Server 2013 builder),
 	// the kernel is arbitrarily okay with < 248 bytes. That

src/os/path_windows_test.go

@@ -12,6 +12,9 @@ import (
 )
 
 func TestFixLongPath(t *testing.T) {
+	if os.CanUseLongPaths {
+		return
+	}
 	// 248 is long enough to trigger the longer-than-248 checks in
 	// fixLongPath, but short enough not to make a path component
 	// longer than 255, which is illegal on Windows. (which
@@ -46,6 +49,26 @@ func TestFixLongPath(t *testing.T) {
 	}
 }
 
+func TestMkdirAllLongPath(t *testing.T) {
+	tmpDir, err := os.MkdirTemp("", "TestMkdirAllLongPath")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpDir)
+	path := tmpDir
+	for i := 0; i < 100; i++ {
+		path += `\another-path-component`
+	}
+	err = os.MkdirAll(path, 0777)
+	if err != nil {
+		t.Fatalf("MkdirAll(%q) failed; %v", path, err)
+	}
+	err = os.RemoveAll(tmpDir)
+	if err != nil {
+		t.Fatalf("RemoveAll(%q) failed; %v", tmpDir, err)
+	}
+}
+
 func TestMkdirAllExtendedLength(t *testing.T) {
 	tmpDir, err := os.MkdirTemp("", "TestMkdirAllExtendedLength")
 	if err != nil {

src/runtime/os_windows.go

@@ -18,6 +18,7 @@ const (
 //go:cgo_import_dynamic runtime._AddVectoredExceptionHandler AddVectoredExceptionHandler%2 "kernel32.dll"
 //go:cgo_import_dynamic runtime._CloseHandle CloseHandle%1 "kernel32.dll"
 //go:cgo_import_dynamic runtime._CreateEventA CreateEventA%4 "kernel32.dll"
+//go:cgo_import_dynamic runtime._CreateFileA CreateFileA%7 "kernel32.dll"
 //go:cgo_import_dynamic runtime._CreateIoCompletionPort CreateIoCompletionPort%4 "kernel32.dll"
 //go:cgo_import_dynamic runtime._CreateThread CreateThread%6 "kernel32.dll"
 //go:cgo_import_dynamic runtime._CreateWaitableTimerA CreateWaitableTimerA%3 "kernel32.dll"
@@ -67,6 +68,7 @@ var (
 	_AddVectoredExceptionHandler,
 	_CloseHandle,
 	_CreateEventA,
+	_CreateFileA,
 	_CreateIoCompletionPort,
 	_CreateThread,
 	_CreateWaitableTimerA,
@@ -132,7 +134,9 @@ var (
 	// Load ntdll.dll manually during startup, otherwise Mingw
 	// links wrong printf function to cgo executable (see issue
 	// 12030 for details).
-	_NtWaitForSingleObject stdFunction
+	_NtWaitForSingleObject  stdFunction
+	_RtlGetCurrentPeb       stdFunction
+	_RtlGetNtVersionNumbers stdFunction
 
 	// These are from non-kernel32.dll, so we prefer to LoadLibraryEx them.
 	_timeBeginPeriod,
@@ -219,21 +223,22 @@ func windowsFindfunc(lib uintptr, name []byte) stdFunction {
 	return stdFunction(unsafe.Pointer(f))
 }
 
-var sysDirectory [521]byte
+const _MAX_PATH = 260 // https://docs.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation
+var sysDirectory [_MAX_PATH + 1]byte
 var sysDirectoryLen uintptr
 
 func windowsLoadSystemLib(name []byte) uintptr {
+	if sysDirectoryLen == 0 {
+		l := stdcall2(_GetSystemDirectoryA, uintptr(unsafe.Pointer(&sysDirectory[0])), uintptr(len(sysDirectory)-1))
+		if l == 0 || l > uintptr(len(sysDirectory)-1) {
+			throw("Unable to determine system directory")
+		}
+		sysDirectory[l] = '\\'
+		sysDirectoryLen = l + 1
+	}
 	if useLoadLibraryEx {
 		return stdcall3(_LoadLibraryExA, uintptr(unsafe.Pointer(&name[0])), 0, _LOAD_LIBRARY_SEARCH_SYSTEM32)
 	} else {
-		if sysDirectoryLen == 0 {
-			l := stdcall2(_GetSystemDirectoryA, uintptr(unsafe.Pointer(&sysDirectory[0])), uintptr(len(sysDirectory)-1))
-			if l == 0 || l > uintptr(len(sysDirectory)-1) {
-				throw("Unable to determine system directory")
-			}
-			sysDirectory[l] = '\\'
-			sysDirectoryLen = l + 1
-		}
 		absName := append(sysDirectory[:sysDirectoryLen], name...)
 		return stdcall1(_LoadLibraryA, uintptr(unsafe.Pointer(&absName[0])))
 	}
@@ -266,6 +271,8 @@ func loadOptionalSyscalls() {
 		throw("ntdll.dll not found")
 	}
 	_NtWaitForSingleObject = windowsFindfunc(n32, []byte("NtWaitForSingleObject\000"))
+	_RtlGetCurrentPeb = windowsFindfunc(n32, []byte("RtlGetCurrentPeb\000"))
+	_RtlGetNtVersionNumbers = windowsFindfunc(n32, []byte("RtlGetNtVersionNumbers\000"))
 
 	if !haveCputicksAsm {
 		_QueryPerformanceCounter = windowsFindfunc(k32, []byte("QueryPerformanceCounter\000"))
@@ -471,6 +478,74 @@ func initHighResTimer() {
 	}
 }
 
+//go:linkname canUseLongPaths os.canUseLongPaths
+var canUseLongPaths bool
+
+// We want this to be large enough to hold the contents of sysDirectory, *plus*
+// a slash and another component that itself is greater than MAX_PATH.
+var longFileName [(_MAX_PATH+1)*2 + 1]byte
+
+// initLongPathSupport initializes the canUseLongPaths variable, which is
+// linked into os.canUseLongPaths for determining whether or not long paths
+// need to be fixed up. In the best case, this function is running on newer
+// Windows 10 builds, which have a bit field member of the PEB called
+// "IsLongPathAwareProcess." When this is set, we don't need to go through the
+// error-prone fixup function in order to access long paths. So this init
+// function first checks the Windows build number, sets the flag, and then
+// tests to see if it's actually working. If everything checks out, then
+// canUseLongPaths is set to true, and later when called, os.fixLongPath
+// returns early without doing work.
+func initLongPathSupport() {
+	const (
+		IsLongPathAwareProcess = 0x80
+		PebBitFieldOffset      = 3
+		OPEN_EXISTING          = 3
+		ERROR_PATH_NOT_FOUND   = 3
+	)
+
+	// Check that we're ≥ 10.0.15063.
+	var maj, min, build uint32
+	stdcall3(_RtlGetNtVersionNumbers, uintptr(unsafe.Pointer(&maj)), uintptr(unsafe.Pointer(&min)), uintptr(unsafe.Pointer(&build)))
+	if maj < 10 || (maj == 10 && min == 0 && build&0xffff < 15063) {
+		return
+	}
+
+	// Set the IsLongPathAwareProcess flag of the PEB's bit field.
+	bitField := (*byte)(unsafe.Pointer(stdcall0(_RtlGetCurrentPeb) + PebBitFieldOffset))
+	originalBitField := *bitField
+	*bitField |= IsLongPathAwareProcess
+
+	// Check that this actually has an effect, by constructing a large file
+	// path and seeing whether we get ERROR_PATH_NOT_FOUND, rather than
+	// some other error, which would indicate the path is too long, and
+	// hence long path support is not successful. This whole section is NOT
+	// strictly necessary, but is a nice validity check for the near to
+	// medium term, when this functionality is still relatively new in
+	// Windows.
+	getRandomData(longFileName[len(longFileName)-33 : len(longFileName)-1])
+	start := copy(longFileName[:], sysDirectory[:sysDirectoryLen])
+	const dig = "0123456789abcdef"
+	for i := 0; i < 32; i++ {
+		longFileName[start+i*2] = dig[longFileName[len(longFileName)-33+i]>>4]
+		longFileName[start+i*2+1] = dig[longFileName[len(longFileName)-33+i]&0xf]
+	}
+	start += 64
+	for i := start; i < len(longFileName)-1; i++ {
+		longFileName[i] = 'A'
+	}
+	stdcall7(_CreateFileA, uintptr(unsafe.Pointer(&longFileName[0])), 0, 0, 0, OPEN_EXISTING, 0, 0)
+	// The ERROR_PATH_NOT_FOUND error value is distinct from
+	// ERROR_FILE_NOT_FOUND or ERROR_INVALID_NAME, the latter of which we
+	// expect here due to the final component being too long.
+	if getlasterror() == ERROR_PATH_NOT_FOUND {
+		*bitField = originalBitField
+		println("runtime: warning: IsLongPathAwareProcess failed to enable long paths; proceeding in fixup mode")
+		return
+	}
+
+	canUseLongPaths = true
+}
+
 func osinit() {
 	asmstdcallAddr = unsafe.Pointer(funcPC(asmstdcall))
 
@@ -487,6 +562,8 @@ func osinit() {
 	initHighResTimer()
 	timeBeginPeriodRetValue = osRelax(false)
 
+	initLongPathSupport()
+
 	ncpu = getproccount()
 
 	physPageSize = getPageSize()