FileBasicInfo alignment is incorrect, leading to winio.SetFileBasicInfo returning "Invalid access to memory location."

[dagood]

Code that calls SetFileBasicInfo(f *os.File, bi *FileBasicInfo) error can in some cases get an error like this:

SetFileInformationByHandle \\?\C:\Windows\SystemTemp\hcs2001018820\Files.$wcidirs$: Invalid access to memory location.

This is ERROR_NOACCESS, which can indicate a STATUS_DATATYPE_MISALIGNMENT error.

The reason is that winio's FileBasicInfo uses windows.Filetime, causing the struct alignment to be wrong. I can't directly prove the cause for sure (it's not trivial to repro with specific functions), but I opened golang/go#65069 to get some help with it, and I don't think there's any doubt.

Here's FileBasicInfo for reference:

go-winio/fileinfo.go

Lines 14 to 19 in bc421d9

	// FileBasicInfo contains file access time and file attributes information.
	type FileBasicInfo struct {
	CreationTime, LastAccessTime, LastWriteTime, ChangeTime windows.Filetime
	FileAttributes uint32
	_ uint32 // padding
	}

type Filetime struct {
	LowDateTime  uint32
	HighDateTime uint32
}

The biggest field of FileBasicInfo is uint32, so the struct as a whole is aligned on 32 bits.

But the input to SetFileInformationByHandle needs to be 64-bit aligned to be compatible with Windows' struct:

FileBasicInfo defines the time properties using LARGE_INTEGER, which is a uint64.

typedef struct _FILE_BASIC_INFORMATION {
  LARGE_INTEGER CreationTime;
  LARGE_INTEGER LastAccessTime;
  LARGE_INTEGER LastWriteTime;
  LARGE_INTEGER ChangeTime;
  ULONG         FileAttributes;
} FILE_BASIC_INFORMATION, *PFILE_BASIC_INFORMATION;

---

A workaround is to define a new struct with the right alignment and copy the data over before sending it to the syscall:

func SetFileBasicInfo(f *os.File, bi *FileBasicInfo) error {
	merge := func(t windows.Filetime) uint64 {
		return *(*uint64)(unsafe.Pointer(&t))
	}
	biAligned := struct {
		CreationTime, LastAccessTime, LastWriteTime, ChangeTime uint64
		FileAttributes                                          uint32
		_                                                       uint32 // padding
	}{
		merge(bi.CreationTime), merge(bi.LastAccessTime), merge(bi.LastWriteTime), merge(bi.ChangeTime),
		bi.FileAttributes,
		0,
	}

	if err := windows.SetFileInformationByHandle(
		windows.Handle(f.Fd()),
		windows.FileBasicInfo,
		(*byte)(unsafe.Pointer(&biAligned)),
		uint32(unsafe.Sizeof(biAligned)),
	); err != nil {
		return &os.PathError{Op: "SetFileInformationByHandle", Path: f.Name(), Err: err}
	}
	runtime.KeepAlive(f)
	return nil
}

It seems to me that something similar should also be added to GetFileBasicInfo. At least, I don't know any reason why the alignment problem wouldn't also eventually cause a problem for someone using that function, too.

---

This issue is making moby/moby's attempt to update to go1.22rc1 fail in dockerd: moby/moby#46982. I confirmed that applying the workaround above makes the update pass CI.

For some reason, go1.22rc1 aggravated this issue in moby's case, but I don't see any reason it wouldn't affect any Go version, depending on your luck. More info at golang/go#65069.

I wrote a test case that could make it easier to spot this kind of thing in PR review: https://gist.github.com/dagood/3cd4207c1f12608f1cca2183af143715.

[dagood]

Fixed by #312