[sanitizer] Add cloak_sanitizer_signal_handlers runtime option (llvm#162746)

If set, signal/sigaction will pretend that the sanitizers did not preinstall any signal handlers. If a user successfully installs a signal handler, it will not be cloaked.
    
The flag is currently off by default, which means this patch should not affect the behavior of any sanitizers.
    
This can be useful in an ecosystem where:
1) there exists a library that will install a signal handler iff it does not detect a preinstalled signal handler (a heuristic to prevent overriding user-installed exception handlers etc.)
2) the aforementioned library is linked in to some, but not all, apps
3) user-installed signal handlers are intended to have the highest priority, followed by the library-installed signal handler, and then the sanitizer's signal handler
    
The flag is in sanitizer_common, though it is currently only supported in ASan, LSan, MSan, TSan and UBSan.

Author: thurstond

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 +212,20 @@ 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) {
+  if (signum < 0 || static_cast<unsigned>(signum) >=
+                        ARRAY_SIZE(signal_handler_is_from_sanitizer))
+    return false;
+
+  return atomic_exchange(&signal_handler_is_from_sanitizer[signum], new_state,
+                         memory_order_relaxed);
+}
+
 static void MaybeInstallSigaction(int signum,
                                   SignalHandlerType handler) {
   if (GetHandleSignalMode(signum) == kHandleSignalNo) return;
@@ -223,6 +239,9 @@ 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;
+}