[sanitizer] Add cloak_sanitizer_signal_handlers runtime option

compiler-rt/lib/sanitizer_common/sanitizer_common.h

@@ -390,6 +390,9 @@ void ReportDeadlySignal(const SignalContext &sig, u32 tid,
 void SetAlternateSignalStack();
 void UnsetAlternateSignalStack();
 
+bool IsSignalHandlerFromSanitizer(int signum);
+bool SetSignalHandlerFromSanitizer(int signum, bool new_state);
+
 // Construct a one-line string:
 //   SUMMARY: SanitizerToolName: error_message
 // and pass it to __sanitizer_report_error_summary.

compiler-rt/lib/sanitizer_common/sanitizer_flags.inc

@@ -113,6 +113,11 @@ COMMON_FLAG(HandleSignalMode, handle_sigfpe, kHandleSignalYes,
 COMMON_FLAG(bool, allow_user_segv_handler, true,
             "Deprecated. True has no effect, use handle_sigbus=1. If false, "
             "handle_*=1 will be upgraded to handle_*=2.")
+COMMON_FLAG(bool, cloak_sanitizer_signal_handlers, false,
+            "If set, signal/sigaction will pretend that sanitizers did not "
+            "preinstall any signal handlers. If the user subsequently installs "
+            "a signal handler, this will disable cloaking for the respective "
+            "signal.")
 COMMON_FLAG(bool, use_sigaltstack, true,
             "If set, uses alternate stack for signal handling.")
 COMMON_FLAG(bool, detect_deadlocks, true,

compiler-rt/lib/sanitizer_common/sanitizer_posix_libcdep.cpp

@@ -47,6 +47,8 @@ typedef void (*sa_sigaction_t)(int, siginfo_t *, void *);
 
 namespace __sanitizer {
 
+static atomic_uint8_t signal_handler_is_from_sanitizer[64];
+
 u32 GetUid() {
   return getuid();
 }
@@ -210,6 +212,21 @@ void UnsetAlternateSignalStack() {
   UnmapOrDie(oldstack.ss_sp, oldstack.ss_size);
 }
 
+bool IsSignalHandlerFromSanitizer(int signum) {
+  return atomic_load(&signal_handler_is_from_sanitizer[signum],
+                     memory_order_relaxed);
+}
+
+bool SetSignalHandlerFromSanitizer(int signum, bool new_state) {
+  bool old_state = false;
+  if (signum >= 0 && static_cast<unsigned>(signum) <
+                         ARRAY_SIZE(signal_handler_is_from_sanitizer))
+    old_state = atomic_exchange(&signal_handler_is_from_sanitizer[signum],
+                                new_state, memory_order_relaxed);
+
+  return old_state;
+}
+
 static void MaybeInstallSigaction(int signum,
                                   SignalHandlerType handler) {
   if (GetHandleSignalMode(signum) == kHandleSignalNo) return;
@@ -223,6 +240,9 @@ static void MaybeInstallSigaction(int signum,
   if (common_flags()->use_sigaltstack) sigact.sa_flags |= SA_ONSTACK;
   CHECK_EQ(0, internal_sigaction(signum, &sigact, nullptr));
   VReport(1, "Installed the sigaction for signal %d\n", signum);
+
+  if (common_flags()->cloak_sanitizer_signal_handlers)
+    SetSignalHandlerFromSanitizer(signum, true);
 }
 
 void InstallDeadlySignalHandlers(SignalHandlerType handler) {

compiler-rt/lib/sanitizer_common/sanitizer_signal_interceptors.inc

@@ -45,6 +45,8 @@ using namespace __sanitizer;
 INTERCEPTOR(uptr, bsd_signal, int signum, uptr handler) {
   SIGNAL_INTERCEPTOR_ENTER();
   if (GetHandleSignalMode(signum) == kHandleSignalExclusive) return 0;
+
+  // TODO: support cloak_sanitizer_signal_handlers
   SIGNAL_INTERCEPTOR_SIGNAL_IMPL(bsd_signal, signum, handler);
 }
 #define INIT_BSD_SIGNAL COMMON_INTERCEPT_FUNCTION(bsd_signal)
@@ -56,19 +58,55 @@ INTERCEPTOR(uptr, bsd_signal, int signum, uptr handler) {
 INTERCEPTOR(uptr, signal, int signum, uptr handler) {
   SIGNAL_INTERCEPTOR_ENTER();
   if (GetHandleSignalMode(signum) == kHandleSignalExclusive)
+    // The user can neither view nor change the signal handler, regardless of
+    // the cloak_sanitizer_signal_handlers setting. This differs from
+    // sigaction().
     return (uptr) nullptr;
-  SIGNAL_INTERCEPTOR_SIGNAL_IMPL(signal, signum, handler);
+
+  uptr ret = +[](auto signal, int signum, uptr handler) {
+    SIGNAL_INTERCEPTOR_SIGNAL_IMPL(signal, signum, handler);
+  }(signal, signum, handler);
+
+  if (ret != sig_err && SetSignalHandlerFromSanitizer(signum, false))
+    // If the user sets a signal handler, it becomes uncloaked, even if they
+    // reuse a sanitizer's signal handler.
+    ret = sig_dfl;
+
+  return ret;
 }
 #define INIT_SIGNAL COMMON_INTERCEPT_FUNCTION(signal)
 
 INTERCEPTOR(int, sigaction_symname, int signum,
             const __sanitizer_sigaction *act, __sanitizer_sigaction *oldact) {
   SIGNAL_INTERCEPTOR_ENTER();
+
   if (GetHandleSignalMode(signum) == kHandleSignalExclusive) {
     if (!oldact) return 0;
     act = nullptr;
+    // If cloak_sanitizer_signal_handlers=true, the user can neither view nor
+    // change the signal handle.
+    // If false, the user can view but not change the signal handler. This
+    // differs from signal().
   }
-  SIGNAL_INTERCEPTOR_SIGACTION_IMPL(signum, act, oldact);
+
+  int ret = +[](int signum, const __sanitizer_sigaction* act,
+                __sanitizer_sigaction* oldact) {
+    SIGNAL_INTERCEPTOR_SIGACTION_IMPL(signum, act, oldact);
+  }(signum, act, oldact);
+
+  if (act) {
+    if (ret == 0 && SetSignalHandlerFromSanitizer(signum, false)) {
+      // If the user sets a signal handler, it becomes uncloaked, even if they
+      // reuse a sanitizer's signal handler.
+
+      if (oldact)
+        oldact->handler = reinterpret_cast<__sanitizer_sighandler_ptr>(sig_dfl);
+    }
+  } else if (ret == 0 && oldact && IsSignalHandlerFromSanitizer(signum)) {
+    oldact->handler = reinterpret_cast<__sanitizer_sighandler_ptr>(sig_dfl);
+  }
+
+  return ret;
 }
 #define INIT_SIGACTION COMMON_INTERCEPT_FUNCTION(sigaction_symname)
 

compiler-rt/test/sanitizer_common/TestCases/Linux/allow_user_segv.cpp

@@ -23,6 +23,10 @@
 // Flaky errors in debuggerd with "waitpid returned unexpected pid (0)" in logcat.
 // UNSUPPORTED: android && i386-target-arch
 
+// Note: this test case is unusual because it retrieves the original
+// (ASan-installed) signal handler; thus, it is incompatible with the
+// cloak_sanitizer_signal_handlers runtime option.
+
 #include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>

compiler-rt/test/sanitizer_common/TestCases/Linux/cloak_sigaction.cpp

@@ -0,0 +1,51 @@
+// UNSUPPORTED: android
+// UNSUPPORTED: hwasan
+
+// RUN: %clangxx -O0 %s -o %t
+
+// Sanitizer signal handler not installed; custom signal handler installed
+// RUN: %env_tool_opts=handle_segv=0:cloak_sanitizer_signal_handlers=false not %run %t 2>&1 | FileCheck %s --check-prefixes=DEFAULT,CUSTOM
+// RUN: %env_tool_opts=handle_segv=0:cloak_sanitizer_signal_handlers=true not %run %t 2>&1 | FileCheck %s --check-prefixes=DEFAULT,CUSTOM
+
+// Sanitizer signal handler installed but overriden by custom signal handler
+// RUN: %env_tool_opts=handle_segv=1:cloak_sanitizer_signal_handlers=false not %run %t 2>&1 | FileCheck %s --check-prefixes=NONDEFAULT,CUSTOM
+// RUN: %env_tool_opts=handle_segv=1:cloak_sanitizer_signal_handlers=true not %run %t 2>&1 | FileCheck %s --check-prefixes=DEFAULT,CUSTOM
+
+// Sanitizer signal handler installed immutably
+// N.B. for handle_segv=2 with cloaking off, there is a pre-existing difference
+//      in signal vs. sigaction: signal effectively cloaks the handler.
+// RUN: %env_tool_opts=handle_segv=2:cloak_sanitizer_signal_handlers=false not %run %t 2>&1 | FileCheck %s --check-prefixes=NONDEFAULT,SANITIZER
+// RUN: %env_tool_opts=handle_segv=2:cloak_sanitizer_signal_handlers=true not %run %t 2>&1 | FileCheck %s --check-prefixes=DEFAULT,SANITIZER
+
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+void handler(int signum, siginfo_t *info, void *context) {
+  printf("Custom signal handler\n");
+  exit(1);
+}
+
+int main(int argc, char *argv[]) {
+  struct sigaction sa = {0};
+  struct sigaction old = {0};
+  sa.sa_flags = SA_SIGINFO;
+  sa.sa_sigaction = &handler;
+  sigaction(SIGSEGV, &sa, &old);
+
+  if (reinterpret_cast<void *>(old.sa_sigaction) == SIG_DFL)
+    printf("Old handler: default\n");
+  // DEFAULT: Old handler: default
+  else
+    printf("Old handler: non-default\n");
+  // NONDEFAULT: Old handler: non-default
+
+  fflush(stdout);
+
+  char *c = (char *)0x123;
+  printf("%d\n", *c);
+  // CUSTOM: Custom signal handler
+  // SANITIZER: Sanitizer:DEADLYSIGNAL
+
+  return 0;
+}

compiler-rt/test/sanitizer_common/TestCases/Linux/cloak_signal.cpp

@@ -0,0 +1,46 @@
+// UNSUPPORTED: android
+// UNSUPPORTED: hwasan
+
+// RUN: %clangxx -O0 %s -o %t
+
+// Sanitizer signal handler not installed; custom signal handler installed
+// RUN: %env_tool_opts=handle_segv=0:cloak_sanitizer_signal_handlers=false not %run %t 2>&1 | FileCheck %s --check-prefixes=DEFAULT,CUSTOM
+// RUN: %env_tool_opts=handle_segv=0:cloak_sanitizer_signal_handlers=true not %run %t 2>&1 | FileCheck %s --check-prefixes=DEFAULT,CUSTOM
+
+// Sanitizer signal handler installed but overriden by custom signal handler
+// RUN: %env_tool_opts=handle_segv=1:cloak_sanitizer_signal_handlers=false not %run %t 2>&1 | FileCheck %s --check-prefixes=NONDEFAULT,CUSTOM
+// RUN: %env_tool_opts=handle_segv=1:cloak_sanitizer_signal_handlers=true not %run %t 2>&1 | FileCheck %s --check-prefixes=DEFAULT,CUSTOM
+
+// Sanitizer signal handler installed immutably
+// N.B. for handle_segv=2 with cloaking off, there is a pre-existing difference
+//      in signal vs. sigaction: signal effectively cloaks the handler.
+// RUN: %env_tool_opts=handle_segv=2:cloak_sanitizer_signal_handlers=false not %run %t 2>&1 | FileCheck %s --check-prefixes=DEFAULT,SANITIZER
+// RUN: %env_tool_opts=handle_segv=2:cloak_sanitizer_signal_handlers=true not %run %t 2>&1 | FileCheck %s --check-prefixes=DEFAULT,SANITIZER
+
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+void my_signal_sighandler(int signum) {
+  printf("Custom signal handler\n");
+  exit(1);
+}
+
+int main(int argc, char *argv[]) {
+  __sighandler_t old = signal(SIGSEGV, &my_signal_sighandler);
+  if (old == SIG_DFL)
+    printf("Old handler: default\n");
+  // DEFAULT: Old handler: default
+  else
+    printf("Old handler: non-default\n");
+  // NONDEFAULT: Old handler: non-default
+
+  fflush(stdout);
+
+  char *c = (char *)0x123;
+  printf("%d\n", *c);
+  // CUSTOM: Custom signal handler
+  // SANITIZER: Sanitizer:DEADLYSIGNAL
+
+  return 0;
+}