Index: compiler-rt/lib/msan/msan_interceptors.cpp
===================================================================
--- compiler-rt/lib/msan/msan_interceptors.cpp
+++ compiler-rt/lib/msan/msan_interceptors.cpp
@@ -21,6 +21,7 @@
 #include "msan_report.h"
 #include "msan_thread.h"
 #include "msan_poisoning.h"
+#include "sanitizer_common/sanitizer_errno_codes.h"
 #include "sanitizer_common/sanitizer_platform_limits_posix.h"
 #include "sanitizer_common/sanitizer_platform_limits_netbsd.h"
 #include "sanitizer_common/sanitizer_allocator.h"
@@ -1373,11 +1374,14 @@
 static int sigaction_impl(int signo, const __sanitizer_sigaction *act,
                           __sanitizer_sigaction *oldact) {
   ENSURE_MSAN_INITED();
+  if (signo <= 0 || signo >= kMaxSignals) {
+    errno = errno_EINVAL;
+    return -1;
+  }
   if (act) read_sigaction(act);
   int res;
   if (flags()->wrap_signals) {
     SpinMutexLock lock(&sigactions_mu);
-    CHECK_LT(signo, kMaxSignals);
     uptr old_cb = atomic_load(&sigactions[signo], memory_order_relaxed);
     __sanitizer_sigaction new_act;
     __sanitizer_sigaction *pnew_act = act ? &new_act : nullptr;
@@ -1411,8 +1415,11 @@
 
 static uptr signal_impl(int signo, uptr cb) {
   ENSURE_MSAN_INITED();
+  if (signo <= 0 || signo >= kMaxSignals) {
+    errno = errno_EINVAL;
+    return -1;
+  }
   if (flags()->wrap_signals) {
-    CHECK_LT(signo, kMaxSignals);
     SpinMutexLock lock(&sigactions_mu);
     if (cb != __sanitizer::sig_ign && cb != __sanitizer::sig_dfl) {
       atomic_store(&sigactions[signo], cb, memory_order_relaxed);
Index: compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp
===================================================================
--- compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp
+++ compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp
@@ -2422,6 +2422,10 @@
   // the signal handler through rtl_sigaction, very bad things will happen.
   // The handler will run synchronously and corrupt tsan per-thread state.
   SCOPED_INTERCEPTOR_RAW(sigaction, sig, act, old);
+  if (sig <= 0 || sig >= kSigCount) {
+    errno = errno_EINVAL;
+    return -1;
+  }
   __sanitizer_sigaction *sigactions = interceptor_ctx()->sigactions;
   __sanitizer_sigaction old_stored;
   if (old) internal_memcpy(&old_stored, &sigactions[sig], sizeof(old_stored));
Index: compiler-rt/test/sanitizer_common/TestCases/Posix/signal.cpp
===================================================================
--- /dev/null
+++ compiler-rt/test/sanitizer_common/TestCases/Posix/signal.cpp
@@ -0,0 +1,233 @@
+// RUN: %clangxx -O0 -g %s -o %t && %run %t 2>&1 | FileCheck %s
+
+#include <assert.h>
+#include <climits>
+#include <errno.h>
+#include <stdio.h>
+#include <signal.h>
+
+#include <initializer_list>
+
+constexpr int std_signals[] = {
+  SIGHUP,
+  SIGINT,
+  SIGQUIT,
+  SIGILL,
+  SIGTRAP,
+  SIGABRT,
+  SIGIOT,
+  SIGBUS,
+  SIGFPE,
+  SIGUSR1,
+  SIGSEGV,
+  SIGUSR2,
+  SIGPIPE,
+  SIGALRM,
+  SIGTERM,
+  SIGCHLD,
+  SIGCONT,
+  SIGTSTP,
+  SIGTTIN,
+  SIGTTOU,
+  SIGURG,
+  SIGXCPU,
+  SIGXFSZ,
+  SIGVTALRM,
+  SIGPROF,
+  SIGWINCH,
+  SIGIO,
+  SIGSYS,
+};
+
+constexpr int no_change_act_signals[] = {
+  SIGKILL,
+  SIGSTOP,
+};
+
+void signal_handler(int) {}
+void signal_action_handler(int, siginfo_t*, void*) {}
+
+void test_signal_custom() {
+  for (int signum : std_signals) {
+    sighandler_t ret = signal(signum, &signal_handler);
+    assert(ret != SIG_ERR);
+  }
+  for (int signum = SIGRTMIN; signum <= SIGRTMAX; ++signum) {
+    sighandler_t ret = signal(signum, &signal_handler);
+    assert(ret != SIG_ERR);
+  }
+  for (int signum : no_change_act_signals) {
+    sighandler_t ret = signal(signum, &signal_handler);
+    int err = errno;
+    assert(ret == SIG_ERR);
+    assert(err == EINVAL);
+  }
+  for (int signum : { 0, SIGRTMAX + 1, INT_MAX}) {
+    sighandler_t ret = signal(signum, &signal_handler);
+    int err = errno;
+    assert(ret == SIG_ERR);
+    assert(err == EINVAL);
+  }
+}
+
+void test_signal_ignore() {
+  for (int signum : std_signals) {
+    sighandler_t ret = signal(signum, SIG_IGN);
+    assert(ret != SIG_ERR);
+  }
+  for (int signum = SIGRTMIN; signum <= SIGRTMAX; ++signum) {
+    sighandler_t ret = signal(signum, SIG_IGN);
+    assert(ret != SIG_ERR);
+  }
+  for (int signum : no_change_act_signals) {
+    sighandler_t ret = signal(signum, SIG_IGN);
+    int err = errno;
+    assert(ret == SIG_ERR);
+    assert(err == EINVAL);
+  }
+  for (int signum : { 0, SIGRTMAX + 1, INT_MAX}) {
+    sighandler_t ret = signal(signum, SIG_IGN);
+    int err = errno;
+    assert(ret == SIG_ERR);
+    assert(err == EINVAL);
+  }
+}
+
+void test_signal_default() {
+  for (int signum : std_signals) {
+    sighandler_t ret = signal(signum, SIG_DFL);
+    assert(ret != SIG_ERR);
+  }
+  for (int signum = SIGRTMIN; signum <= SIGRTMAX; ++signum) {
+    sighandler_t ret = signal(signum, SIG_DFL);
+    assert(ret != SIG_ERR);
+  }
+  for (int signum : { 0, SIGRTMAX + 1, INT_MAX}) {
+    sighandler_t ret = signal(signum, SIG_DFL);
+    int err = errno;
+    assert(ret == SIG_ERR);
+    assert(err == EINVAL);
+  }
+}
+
+void test_sigaction_custom() {
+  struct sigaction act = {}, oldact;
+
+  act.sa_handler = &signal_handler;
+  sigemptyset(&act.sa_mask);
+  act.sa_flags = 0;
+
+  for (int signum : std_signals) {
+    int ret = sigaction(signum, &act, &oldact);
+    assert(ret == 0);
+  }
+  for (int signum = SIGRTMIN; signum <= SIGRTMAX; ++signum) {
+    int ret = sigaction(signum, &act, &oldact);
+    assert(ret == 0);
+  }
+  for (int signum : no_change_act_signals) {
+    int ret = sigaction(signum, &act, &oldact);
+    int err = errno;
+    assert(ret == -1);
+    assert(err == EINVAL);
+  }
+  for (int signum : { 0, SIGRTMAX + 1, INT_MAX}) {
+    int ret = sigaction(signum, &act, &oldact);
+    int err = errno;
+    assert(ret == -1);
+    assert(err == EINVAL);
+  }
+
+  act.sa_handler = nullptr;
+  act.sa_sigaction = &signal_action_handler;
+  act.sa_flags = SA_SIGINFO;
+
+  for (int signum : std_signals) {
+    int ret = sigaction(signum, &act, &oldact);
+    assert(ret == 0);
+  }
+  for (int signum = SIGRTMIN; signum <= SIGRTMAX; ++signum) {
+    int ret = sigaction(signum, &act, &oldact);
+    assert(ret == 0);
+  }
+  for (int signum : no_change_act_signals) {
+    int ret = sigaction(signum, &act, &oldact);
+    int err = errno;
+    assert(ret == -1);
+    assert(err == EINVAL);
+  }
+  for (int signum : { 0, SIGRTMAX + 1, INT_MAX}) {
+    int ret = sigaction(signum, &act, &oldact);
+    int err = errno;
+    assert(ret == -1);
+    assert(err == EINVAL);
+  }
+}
+
+void test_sigaction_ignore() {
+  struct sigaction act = {}, oldact;
+
+  act.sa_handler = SIG_IGN;
+  sigemptyset(&act.sa_mask);
+  act.sa_flags = 0;
+
+  for (int signum : std_signals) {
+    int ret = sigaction(signum, &act, &oldact);
+    assert(ret == 0);
+  }
+  for (int signum = SIGRTMIN; signum <= SIGRTMAX; ++signum) {
+    int ret = sigaction(signum, &act, &oldact);
+    assert(ret == 0);
+  }
+  for (int signum : no_change_act_signals) {
+    int ret = sigaction(signum, &act, &oldact);
+    int err = errno;
+    assert(ret == -1);
+    assert(err == EINVAL);
+  }
+  for (int signum : { 0, SIGRTMAX + 1, INT_MAX}) {
+    int ret = sigaction(signum, &act, &oldact);
+    int err = errno;
+    assert(ret == -1);
+    assert(err == EINVAL);
+  }
+}
+
+void test_sigaction_default() {
+  struct sigaction act = {}, oldact;
+
+  act.sa_handler = SIG_DFL;
+  sigemptyset(&act.sa_mask);
+  act.sa_flags = 0;
+
+  for (int signum : std_signals) {
+    int ret = sigaction(signum, &act, &oldact);
+    assert(ret == 0);
+  }
+  for (int signum = SIGRTMIN; signum <= SIGRTMAX; ++signum) {
+    int ret = sigaction(signum, &act, &oldact);
+    assert(ret == 0);
+  }
+  for (int signum : { 0, SIGRTMAX + 1, INT_MAX}) {
+    int ret = sigaction(signum, &act, &oldact);
+    int err = errno;
+    assert(ret == -1);
+    assert(err == EINVAL);
+  }
+}
+
+int main(void) {
+  printf("sigaction\n");
+
+  test_signal_custom();
+  test_signal_ignore();
+  test_signal_default();
+
+  test_sigaction_custom();
+  test_sigaction_ignore();
+  test_sigaction_default();
+
+  // CHECK: sigaction
+
+  return 0;
+}