[2/2] selftests: Add a test mangling with uc_sigmask

diff --git a/tools/testing/selftests/signal/.gitignore b/tools/testing/selftests/signal/.gitignore
index 98a7bbc4f325..ccba56247942 100644
--- a/tools/testing/selftests/signal/.gitignore
+++ b/tools/testing/selftests/signal/.gitignore
@@ -1,2 +1,3 @@ 
 # SPDX-License-Identifier: GPL-2.0-only
 sigaltstack
+mangle_uc_sigmask
diff --git a/tools/testing/selftests/signal/Makefile b/tools/testing/selftests/signal/Makefile
index dd6be992fd81..4ebf6ac2e303 100644
--- a/tools/testing/selftests/signal/Makefile
+++ b/tools/testing/selftests/signal/Makefile
@@ -1,6 +1,7 @@ 
 # SPDX-License-Identifier: GPL-2.0-only
 CFLAGS = -Wall
 TEST_GEN_PROGS = sigaltstack
+TEST_GEN_PROGS += mangle_uc_sigmask
 
 include ../lib.mk
 
diff --git a/tools/testing/selftests/signal/mangle_uc_sigmask.c b/tools/testing/selftests/signal/mangle_uc_sigmask.c
new file mode 100644
index 000000000000..0803aeb248a0
--- /dev/null
+++ b/tools/testing/selftests/signal/mangle_uc_sigmask.c
@@ -0,0 +1,141 @@ 
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2024 ARM Ltd.
+ *
+ * Author: Dev Jain <dev.jain@arm.com>
+ *
+ * Test describing a clear distinction between signal states - delivered and
+ * blocked, and their relation with ucontext.
+ */
+
+#include <signal.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <ucontext.h>
+#include <assert.h>
+
+#include "../kselftest.h"
+
+void handler_verify_ucontext(int signo, siginfo_t *info, void *uc)
+{
+	int ret;
+
+	/* Kernel dumps ucontext with USR2 blocked */
+	ret = sigismember(&(((ucontext_t *)uc)->uc_sigmask), SIGUSR2);
+	ksft_test_result(ret == 1, "USR2 in ucontext\n");
+
+	raise(SIGUSR2);
+}
+
+void handler_segv(int signo, siginfo_t *info, void *uc)
+{
+	/*
+	 * Three cases possible:
+	 * 1. Program already terminated due to segmentation fault.
+	 * 2. SEGV was blocked even after returning from handler_usr.
+	 * 3. SEGV was delivered on returning from handler_usr.
+	 * The last option must happen.
+	 */
+	ksft_test_result_pass("SEGV delivered\n");
+}
+
+static int cnt;
+
+void handler_usr(int signo, siginfo_t *info, void *uc)
+{
+	int ret;
+
+	/*
+	 * Break out of infinite recursion caused by raise(SIGUSR1) invoked
+	 * from inside the handler
+	 */
+	++cnt;
+	if (cnt > 1)
+		return;
+
+	ksft_print_msg("In handler_usr\n");
+
+	/* SEGV blocked during handler execution, delivered on return */
+	raise(SIGPIPE);
+	ksft_print_msg("SEGV bypassed successfully\n");
+
+	/*
+	 * Signal responsible for handler invocation is blocked by default;
+	 * delivered on return, leading to an infinite recursion
+	 */
+	raise(SIGUSR1);
+	ksft_test_result(cnt == 1,
+			 "USR1 is blocked, cannot invoke handler again\n");
+
+	/* SIGPIPE has been blocked in sa_mask, but ucontext is invariant */
+	ret = sigismember(&(((ucontext_t *)uc)->uc_sigmask), SIGPIPE);
+	ksft_test_result(ret == 0, "USR1 not in ucontext\n");
+
+	/* SIGUSR1 has been blocked, but ucontext is invariant */
+	ret = sigismember(&(((ucontext_t *)uc)->uc_sigmask), SIGUSR1);
+	ksft_test_result(ret == 0, "SEGV not in ucontext\n");
+
+	/*
+	 * Mangle ucontext; this will be copied back into &current->blocked
+	 * on return from the handler.
+	 */
+	if (sigaddset(&((ucontext_t *)uc)->uc_sigmask, SIGUSR2))
+		ksft_exit_fail_perror("Cannot add into uc_sigmask");
+}
+
+int main(int argc, char *argv[])
+{
+	struct sigaction act, act2;
+	sigset_t *set, *oldset;
+
+	ksft_print_header();
+	ksft_set_plan(6);
+
+	act.sa_flags = SA_SIGINFO;
+	act.sa_sigaction = &handler_usr;
+
+	/* add SEGV to blocked mask */
+	if (sigemptyset(&act.sa_mask) || sigaddset(&act.sa_mask, SIGPIPE)
+	    || (sigismember(&act.sa_mask, SIGPIPE) != 1))
+		ksft_exit_fail_msg("Cannot add SEGV to blocked mask\n");
+
+	if (sigaction(SIGUSR1, &act, NULL))
+		ksft_exit_fail_perror("Cannot install handler");
+
+	act2.sa_flags = SA_SIGINFO;
+	act2.sa_sigaction = &handler_segv;
+
+	if (sigaction(SIGPIPE, &act2, NULL))
+		ksft_exit_fail_perror("Cannot install handler");
+
+	/* invoke handler */
+	raise(SIGUSR1);
+
+	/* Mangled ucontext implies USR2 is blocked for current thread */
+	raise(SIGUSR2);
+	ksft_print_msg("USR2 bypassed successfully\n");
+
+	act.sa_sigaction = &handler_verify_ucontext;
+	if (sigaction(SIGUSR1, &act, NULL))
+		ksft_exit_fail_perror("Cannot install handler");
+
+	raise(SIGUSR1);
+
+	ksft_print_msg("USR2 still blocked on return from handler\n");
+
+	/* Confirm USR2 blockage by sigprocmask() too */
+	set = malloc(sizeof(sigset_t *));
+	oldset = malloc(sizeof(sigset_t *));
+
+	if (sigemptyset(set))
+		ksft_exit_fail_perror("Cannot empty set");
+
+	if (sigprocmask(SIG_BLOCK, set, oldset))
+		ksft_exit_fail_perror("sigprocmask()");
+
+	ksft_test_result(sigismember(oldset, SIGUSR2) == 1,
+			 "USR2 present in &current->blocked\n");
+
+	ksft_finished();
+}