Tests: Add tests for inheriting signal handlers

Tests/Kernel/CMakeLists.txt

@@ -43,6 +43,7 @@ set(LIBTEST_BASED_SOURCES
     TestProcFS.cpp
     TestProcFSWrite.cpp
     TestSigAltStack.cpp
+    TestSigHandler.cpp
     TestSigWait.cpp
 )
 

Tests/Kernel/TestSigHandler.cpp

@@ -0,0 +1,78 @@
+/*
+ * Copyright (c) 2022, Tim Schumacher <timschumi@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibTest/TestCase.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+static void signal_handler(int)
+{
+    VERIFY_NOT_REACHED();
+}
+
+TEST_CASE(default_handlers)
+{
+    struct sigaction current_action { };
+
+    int rc = sigaction(SIGUSR2, nullptr, &current_action);
+
+    EXPECT_EQ(rc, 0);
+    EXPECT_EQ(current_action.sa_handler, SIG_DFL);
+}
+
+TEST_CASE(handlers_after_fork)
+{
+    struct sigaction new_action {
+        { signal_handler }, 0, 0
+    };
+    int rc = sigaction(SIGUSR2, &new_action, nullptr);
+    EXPECT_EQ(rc, 0);
+
+    pid_t pid = fork();
+
+    if (pid == 0) {
+        struct sigaction current_action { };
+        rc = sigaction(SIGUSR2, nullptr, &current_action);
+        EXPECT_EQ(rc, 0);
+        EXPECT_EQ(current_action.sa_handler, signal_handler);
+        exit(rc == 0 && current_action.sa_handler == signal_handler ? EXIT_SUCCESS : EXIT_FAILURE);
+    } else {
+        int exit_status = 0;
+        rc = waitpid(pid, &exit_status, 0);
+        EXPECT_EQ(rc, pid);
+        EXPECT(WIFEXITED(exit_status));
+        EXPECT_EQ(WEXITSTATUS(exit_status), 0);
+    }
+}
+
+TEST_CASE(handlers_after_exec)
+{
+    struct sigaction new_action {
+        { signal_handler }, 0, 0
+    };
+    int rc = sigaction(SIGUSR2, &new_action, nullptr);
+    EXPECT_EQ(rc, 0);
+
+    pid_t pid = fork();
+
+    if (pid == 0) {
+        // Hide the confusing "Running 1 cases out of 3" output.
+        freopen("/dev/null", "w", stdout);
+
+        // This runs the 'default_handlers' test in this binary again, but after exec.
+        execl("/proc/self/exe", "TestSigHandler", "default_handlers", nullptr);
+        FAIL("Failed to exec.");
+    } else {
+        int exit_status = 0;
+        rc = waitpid(pid, &exit_status, 0);
+        EXPECT_EQ(rc, pid);
+        EXPECT(WIFEXITED(exit_status));
+        EXPECT_EQ(WEXITSTATUS(exit_status), 0);
+    }
+}