[RFC PATCH v1 33/37] KVM: selftests: Make TEST_EXPECT_SIGBUS thread-safe

[Ackerley Tng <ackerleytng@google.com>]

The TEST_EXPECT_SIGBUS macro is not thread-safe as it uses a global
sigjmp_buf and installs a global SIGBUS signal handler. If multiple threads
execute the macro concurrently, they will race on installing the signal
handler and stomp on other threads' jump buffers, leading to incorrect test
behavior.

Make TEST_EXPECT_SIGBUS thread-safe with the following changes:

Share the KVM tests' global signal handler. sigaction() applies to all
threads; without sharing a global signal handler, one thread may have
removed the signal handler that another thread added, hence leading to
unexpected signals.

The alternative of layering signal handlers was considered, but calling
sigaction() within TEST_EXPECT_SIGBUS() necessarily creates a race. To
avoid adding new setup and teardown routines to do sigaction() and keep
usage of TEST_EXPECT_SIGBUS() simple, share the KVM tests' global signal
handler.

Opportunistically rename report_unexpected_signal to
catchall_signal_handler.

To continue to only expect SIGBUS within specific regions of code, use a
thread-specific variable, expecting_sigbus, to replace installing and
removing signal handlers.

Make the execution environment for the thread, sigjmp_buf, a
thread-specific variable.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 .../testing/selftests/kvm/include/test_util.h | 27 +++++++++----------
 tools/testing/selftests/kvm/lib/kvm_util.c    | 18 +++++++++----
 tools/testing/selftests/kvm/lib/test_util.c   |  7 -----
 3 files changed, 25 insertions(+), 27 deletions(-)

diff --git a/tools/testing/selftests/kvm/include/test_util.h b/tools/testing/selftests/kvm/include/test_util.h
index 2871a42928471..0e4e6f7dab8fb 100644
--- a/tools/testing/selftests/kvm/include/test_util.h
+++ b/tools/testing/selftests/kvm/include/test_util.h
@@ -80,22 +80,19 @@ do {									\
 	__builtin_unreachable(); \
 } while (0)
 
-extern sigjmp_buf expect_sigbus_jmpbuf;
-void expect_sigbus_handler(int signum);
+extern __thread sigjmp_buf expect_sigbus_jmpbuf;
+extern __thread bool expecting_sigbus;
 
-#define TEST_EXPECT_SIGBUS(action)						\
-do {										\
-	struct sigaction sa_old, sa_new = {					\
-		.sa_handler = expect_sigbus_handler,				\
-	};									\
-										\
-	sigaction(SIGBUS, &sa_new, &sa_old);					\
-	if (sigsetjmp(expect_sigbus_jmpbuf, 1) == 0) {				\
-		action;								\
-		TEST_FAIL("'%s' should have triggered SIGBUS", #action);	\
-	}									\
-	sigaction(SIGBUS, &sa_old, NULL);					\
-} while (0)
+#define TEST_EXPECT_SIGBUS(action)                                     \
+	do {                                                           \
+		expecting_sigbus = true;			       \
+		if (sigsetjmp(expect_sigbus_jmpbuf, 1) == 0) {         \
+			action;                                        \
+			TEST_FAIL("'%s' should have triggered SIGBUS", \
+				  #action);                            \
+		}                                                      \
+		expecting_sigbus = false;			       \
+	} while (0)
 
 size_t parse_size(const char *size);
 
diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
index 8603bd5c705ed..41169e8cbf8af 100644
--- a/tools/testing/selftests/kvm/lib/kvm_util.c
+++ b/tools/testing/selftests/kvm/lib/kvm_util.c
@@ -2302,13 +2302,20 @@ __weak void kvm_selftest_arch_init(void)
 {
 }
 
-static void report_unexpected_signal(int signum)
+__thread sigjmp_buf expect_sigbus_jmpbuf;
+__thread bool expecting_sigbus;
+
+static void catchall_signal_handler(int signum)
 {
+	switch (signum) {
+	case SIGBUS: {
+		if (expecting_sigbus)
+			siglongjmp(expect_sigbus_jmpbuf, 1);
+
+		TEST_FAIL("Unexpected SIGBUS (%d)\n", signum);
+	}
 #define KVM_CASE_SIGNUM(sig)					\
 	case sig: TEST_FAIL("Unexpected " #sig " (%d)\n", signum)
-
-	switch (signum) {
-	KVM_CASE_SIGNUM(SIGBUS);
 	KVM_CASE_SIGNUM(SIGSEGV);
 	KVM_CASE_SIGNUM(SIGILL);
 	KVM_CASE_SIGNUM(SIGFPE);
@@ -2320,12 +2327,13 @@ static void report_unexpected_signal(int signum)
 void __attribute((constructor)) kvm_selftest_init(void)
 {
 	struct sigaction sig_sa = {
-		.sa_handler = report_unexpected_signal,
+		.sa_handler = catchall_signal_handler,
 	};
 
 	/* Tell stdout not to buffer its content. */
 	setbuf(stdout, NULL);
 
+	expecting_sigbus = false;
 	sigaction(SIGBUS, &sig_sa, NULL);
 	sigaction(SIGSEGV, &sig_sa, NULL);
 	sigaction(SIGILL, &sig_sa, NULL);
diff --git a/tools/testing/selftests/kvm/lib/test_util.c b/tools/testing/selftests/kvm/lib/test_util.c
index 8a1848586a857..03eb99af9b8de 100644
--- a/tools/testing/selftests/kvm/lib/test_util.c
+++ b/tools/testing/selftests/kvm/lib/test_util.c
@@ -18,13 +18,6 @@
 
 #include "test_util.h"
 
-sigjmp_buf expect_sigbus_jmpbuf;
-
-void __attribute__((used)) expect_sigbus_handler(int signum)
-{
-	siglongjmp(expect_sigbus_jmpbuf, 1);
-}
-
 /*
  * Random number generator that is usable from guest code. This is the
  * Park-Miller LCG using standard constants.
-- 
2.51.0.858.gf9c4a03a3a-goog