os: set FILE_FLAG_BACKUP_SEMANTICS when opening without I/O access

FILE_FLAG_BACKUP_SEMANTICS is necessary to open directories on Windows,
and to enable backup applications do extended operations on files if
they hold the SE_BACKUP_NAME and SE_RESTORE_NAME privileges.

os.OpenFile currently sets FILE_FLAG_BACKUP_SEMANTICS for all supported
cases except when the file is opened with O_WRONLY | O_RDWR (that is,
access mode 3). This access mode doesn't correspond to any of the
standard POSIX access modes, but some OSes special case it to mean
different things. For example, on Linux, O_WRONLY | O_RDWR means check
for read and write permission on the file and return a file descriptor
that can't be used for reading or writing.

On Windows, os.OpenFile has historically mapped O_WRONLY | O_RDWR to a
0 access mode, which Windows internally interprets as
FILE_READ_ATTRIBUTES. Additionally, it doesn't prepare the file for I/O,
given that the read attributes permission doesn't allow reading or
writing (not that this is similar to what happens on Linux). This
makes opening the file around 50% faster, and one can still use the
handle to stat it, so some projects have been using this behavior
to open files without I/O access.

This CL updates os.OpenFile so that directories can also be opened
without I/O access. This effectively closes #23312, as all the remaining
cases where we don't set FILE_FLAG_BACKUP_SEMANTICS imply opening
with O_WRONLY or O_RDWR, and that's not allowed by Unix's open.

Closes #23312.

Change-Id: I77c4f55e1ca377789aef75bd8a9bce2b7499f91d
Reviewed-on: https://go-review.googlesource.com/c/go/+/673035
Reviewed-by: Damien Neil <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: qmuntal

src/syscall/syscall_windows.go

@@ -370,8 +370,9 @@ func Open(name string, flag int, perm uint32) (fd Handle, err error) {
 	if err != nil {
 		return InvalidHandle, err
 	}
+	accessFlags := flag & (O_RDONLY | O_WRONLY | O_RDWR)
 	var access uint32
-	switch flag & (O_RDONLY | O_WRONLY | O_RDWR) {
+	switch accessFlags {
 	case O_RDONLY:
 		access = GENERIC_READ
 	case O_WRONLY:
@@ -416,9 +417,15 @@ func Open(name string, flag int, perm uint32) (fd Handle, err error) {
 	if perm&S_IWRITE == 0 {
 		attrs = FILE_ATTRIBUTE_READONLY
 	}
-	if flag&O_WRONLY == 0 && flag&O_RDWR == 0 {
-		// We might be opening or creating a directory.
-		// CreateFile requires FILE_FLAG_BACKUP_SEMANTICS
+	switch accessFlags {
+	case O_WRONLY, O_RDWR:
+		// Unix doesn't allow opening a directory with O_WRONLY
+		// or O_RDWR, so we don't set the flag in that case,
+		// which will make CreateFile fail with ERROR_ACCESS_DENIED.
+		// We will map that to EISDIR if the file is a directory.
+	default:
+		// We might be opening a directory for reading,
+		// and CreateFile requires FILE_FLAG_BACKUP_SEMANTICS
 		// to work with directories.
 		attrs |= FILE_FLAG_BACKUP_SEMANTICS
 	}
@@ -428,7 +435,7 @@ func Open(name string, flag int, perm uint32) (fd Handle, err error) {
 	}
 	h, err := createFile(namep, access, sharemode, sa, createmode, attrs, 0)
 	if h == InvalidHandle {
-		if err == ERROR_ACCESS_DENIED && (flag&O_WRONLY != 0 || flag&O_RDWR != 0) {
+		if err == ERROR_ACCESS_DENIED && (attrs&FILE_FLAG_BACKUP_SEMANTICS == 0) {
 			// We should return EISDIR when we are trying to open a directory with write access.
 			fa, e1 := GetFileAttributes(namep)
 			if e1 == nil && fa&FILE_ATTRIBUTE_DIRECTORY != 0 {

src/syscall/syscall_windows_test.go

@@ -36,8 +36,9 @@ func TestOpen(t *testing.T) {
 		{dir, syscall.O_RDONLY | syscall.O_CREAT, nil},
 		{file, syscall.O_APPEND | syscall.O_WRONLY | os.O_CREATE, nil},
 		{file, syscall.O_APPEND | syscall.O_WRONLY | os.O_CREATE | os.O_TRUNC, nil},
+		{file, syscall.O_WRONLY | syscall.O_RDWR, nil},
+		{dir, syscall.O_WRONLY | syscall.O_RDWR, nil},
 		{dir, syscall.O_RDONLY | syscall.O_TRUNC, syscall.ERROR_ACCESS_DENIED},
-		{dir, syscall.O_WRONLY | syscall.O_RDWR, syscall.EISDIR},
 		{dir, syscall.O_WRONLY, syscall.EISDIR},
 		{dir, syscall.O_RDWR, syscall.EISDIR},
 	}