[release-branch.go1.23] os: don't follow symlinks on Windows when O_CREATE|O_EXCL

(This cherry-pick includes both CL 672396 and CL 676655.)

Match standard Unix behavior: Symlinks are not followed when
O_CREATE|O_EXCL is passed to open.

Thanks to Junyoung Park and Dong-uk Kim of KAIST Hacking Lab
for discovering this issue.

For #73702
Fixes #73719
Fixes CVE-2025-0913

Change-Id: Ieb46a6780c5e9a6090b09cd34290f04a8e3b0ca5
Reviewed-on: https://go-review.googlesource.com/c/go/+/672396
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/677195
TryBot-Bypass: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/os/os_test.go

@@ -2174,6 +2174,24 @@ func TestAppend(t *testing.T) {
 	}
 }
 
+func TestOpenFileCreateExclDanglingSymlink(t *testing.T) {
+	defer chtmpdir(t)()
+	const link = "link"
+	if err := Symlink("does_not_exist", link); err != nil {
+		t.Fatal(err)
+	}
+	f, err := OpenFile(link, O_WRONLY|O_CREATE|O_EXCL, 0o666)
+	if err == nil {
+		f.Close()
+	}
+	if !errors.Is(err, ErrExist) {
+		t.Errorf("OpenFile of a dangling symlink with O_CREATE|O_EXCL = %v, want ErrExist", err)
+	}
+	if _, err := Stat(link); err == nil {
+		t.Errorf("OpenFile of a dangling symlink with O_CREATE|O_EXCL created a file")
+	}
+}
+
 func TestStatDirWithTrailingSlash(t *testing.T) {
 	t.Parallel()
 

src/syscall/syscall_windows.go

@@ -406,6 +406,9 @@ func Open(path string, mode int, perm uint32) (fd Handle, err error) {
 			}
 		}
 	}
+	if createmode == CREATE_NEW {
+		attrs |= FILE_FLAG_OPEN_REPARSE_POINT // don't follow symlinks
+	}
 	if createmode == OPEN_EXISTING && access == GENERIC_READ {
 		// Necessary for opening directory handles.
 		attrs |= FILE_FLAG_BACKUP_SEMANTICS