mingw: special-case administrators even more (#712)

The check for dubious ownership has one particular quirk on Windows: if
running as an administrator, files owned by the Administrators _group_
are considered owned by the user.

The rationale for that is: When running in elevated mode, Git creates
files that aren't owned by the individual user but by the Administrators
group.

There is yet another quirk, though: The check I introduced to determine
whether the current user is an administrator uses the
`CheckTokenMembership()` function with the current process token. And
that check only succeeds when running in elevated mode!

Let's be a bit more lenient here and look harder whether the current
user is an administrator. We do this by looking for a so-called "linked
token". That token exists when administrators run in non-elevated mode,
and can be used to create a new process in elevated mode. And feeding
_that_ token to the `CheckTokenMembership()` function succeeds!

compat/mingw.c

@@ -3718,31 +3718,44 @@ static void setup_windows_environment(void)
 		has_symlinks = 0;
 }
 
-static PSID get_current_user_sid(void)
+static void get_current_user_sid(PSID *sid, HANDLE *linked_token)
 {
 	HANDLE token;
 	DWORD len = 0;
-	PSID result = NULL;
+	TOKEN_ELEVATION_TYPE elevationType;
+	DWORD size;
+
+	*sid = NULL;
+	*linked_token = NULL;
 
 	if (!OpenProcessToken(GetCurrentProcess(), TOKEN_QUERY, &token))
-		return NULL;
+		return;
 
 	if (!GetTokenInformation(token, TokenUser, NULL, 0, &len)) {
 		TOKEN_USER *info = xmalloc((size_t)len);
 		if (GetTokenInformation(token, TokenUser, info, len, &len)) {
 			len = GetLengthSid(info->User.Sid);
-			result = xmalloc(len);
-			if (!CopySid(len, result, info->User.Sid)) {
+			*sid = xmalloc(len);
+			if (!CopySid(len, *sid, info->User.Sid)) {
 				error(_("failed to copy SID (%ld)"),
 				      GetLastError());
-				FREE_AND_NULL(result);
+				FREE_AND_NULL(*sid);
 			}
 		}
 		FREE_AND_NULL(info);
 	}
-	CloseHandle(token);
 
-	return result;
+	if (GetTokenInformation(token, TokenElevationType, &elevationType, sizeof(elevationType), &size) &&
+	    elevationType == TokenElevationTypeLimited) {
+		/*
+		 * The current process is run by a member of the Administrators
+		 * group, but is not running elevated.
+		 */
+		if (!GetTokenInformation(token, TokenLinkedToken, linked_token, sizeof(*linked_token), &size))
+			linked_token = NULL; /* there is no linked token */
+	}
+
+	CloseHandle(token);
 }
 
 static BOOL user_sid_to_user_name(PSID sid, LPSTR *str)
@@ -3821,18 +3834,22 @@ int is_path_owned_by_current_sid(const char *path, struct strbuf *report)
 	if (err == ERROR_SUCCESS && sid && IsValidSid(sid)) {
 		/* Now, verify that the SID matches the current user's */
 		static PSID current_user_sid;
+		static HANDLE linked_token;
 		BOOL is_member;
 
 		if (!current_user_sid)
-			current_user_sid = get_current_user_sid();
+			get_current_user_sid(&current_user_sid, &linked_token);
 
 		if (current_user_sid &&
 		    IsValidSid(current_user_sid) &&
 		    EqualSid(sid, current_user_sid))
 			result = 1;
 		else if (IsWellKnownSid(sid, WinBuiltinAdministratorsSid) &&
-			 CheckTokenMembership(NULL, sid, &is_member) &&
-			 is_member)
+			 ((CheckTokenMembership(NULL, sid, &is_member) &&
+			   is_member) ||
+			  (linked_token &&
+			   CheckTokenMembership(linked_token, sid, &is_member) &&
+			   is_member)))
 			/*
 			 * If owned by the Administrators group, and the
 			 * current user is an administrator, we consider that

t/helper/test-path-utils.c

@@ -504,6 +504,25 @@ int cmd__path_utils(int argc, const char **argv)
 		return !!res;
 	}
 
+	if (argc > 1 && !strcmp(argv[1], "is_path_owned_by_current_user")) {
+		int res = 0;
+
+		for (int i = 2; i < argc; i++) {
+			struct strbuf buf = STRBUF_INIT;
+
+			if (is_path_owned_by_current_user(argv[i], &buf))
+				printf("'%s' is owned by current SID\n", argv[i]);
+			else {
+				printf("'%s' is not owned by current SID: %s\n", argv[i], buf.buf);
+				res = 1;
+			}
+
+			strbuf_release(&buf);
+		}
+
+		return res;
+	}
+
 	fprintf(stderr, "%s: unknown function name: %s\n", argv[0],
 		argv[1] ? argv[1] : "(there was none)");
 	return 1;