Use real time signal for threads instead of SIGURG.

This guarantees that any application is not also using this signal
for some other purpose.

Change-Id: I7c9bbb0ec8bb4e13322ecda951bcd43c6bf6ee1a

3 files changed, 23 insertions, 5 deletions

diff --git a/libbacktrace/BacktraceThread.cpp b/libbacktrace/BacktraceThread.cpp
index 4cda19e..e0bab24 100644
--- a/libbacktrace/BacktraceThread.cpp
+++ b/libbacktrace/BacktraceThread.cpp
@@ -136,7 +136,7 @@ void BacktraceThread::FinishUnwind() {
 bool BacktraceThread::TriggerUnwindOnThread(ThreadEntry* entry) {
   entry->state = STATE_WAITING;
 
-  if (tgkill(Pid(), Tid(), SIGURG) != 0) {
+  if (tgkill(Pid(), Tid(), THREAD_SIGNAL) != 0) {
     BACK_LOGW("tgkill failed %s", strerror(errno));
     return false;
   }
@@ -196,9 +196,9 @@ bool BacktraceThread::Unwind(size_t num_ignore_frames) {
     act.sa_sigaction = SignalHandler;
     act.sa_flags = SA_RESTART | SA_SIGINFO | SA_ONSTACK;
     sigemptyset(&act.sa_mask);
-    if (sigaction(SIGURG, &act, &oldact) == 0) {
+    if (sigaction(THREAD_SIGNAL, &act, &oldact) == 0) {
       retval = TriggerUnwindOnThread(entry);
-      sigaction(SIGURG, &oldact, NULL);
+      sigaction(THREAD_SIGNAL, &oldact, NULL);
     } else {
       BACK_LOGW("sigaction failed %s", strerror(errno));
     }
diff --git a/libbacktrace/BacktraceThread.h b/libbacktrace/BacktraceThread.h
index 3412d58..9310a44 100644
--- a/libbacktrace/BacktraceThread.h
+++ b/libbacktrace/BacktraceThread.h
@@ -18,6 +18,7 @@
 #define _LIBBACKTRACE_BACKTRACE_THREAD_H
 
 #include <inttypes.h>
+#include <signal.h>
 #include <sys/types.h>
 
 #include "BacktraceImpl.h"
@@ -29,6 +30,14 @@ enum state_e {
   STATE_CANCEL,
 };
 
+// The signal used to cause a thread to dump the stack.
+#if defined(__GLIBC__)
+// GLIBC reserves __SIGRTMIN signals, so use SIGRTMIN to avoid errors.
+#define THREAD_SIGNAL SIGRTMIN
+#else
+#define THREAD_SIGNAL (__SIGRTMIN+1)
+#endif
+
 class BacktraceThreadInterface;
 
 struct ThreadEntry {
diff --git a/libbacktrace/backtrace_test.cpp b/libbacktrace/backtrace_test.cpp
index a5e141b..9744922 100644
--- a/libbacktrace/backtrace_test.cpp
+++ b/libbacktrace/backtrace_test.cpp
@@ -33,6 +33,9 @@
 #include <backtrace/BacktraceMap.h>
 #include <UniquePtr.h>
 
+// For the THREAD_SIGNAL definition.
+#include "BacktraceThread.h"
+
 #include <cutils/atomic.h>
 #include <gtest/gtest.h>
 
@@ -460,9 +463,15 @@ TEST(libbacktrace, thread_level_trace) {
   // Wait up to 2 seconds for the tid to be set.
   ASSERT_TRUE(WaitForNonZero(&thread_data.state, 2));
 
+  // Make sure that the thread signal used is not visible when compiled for
+  // the target.
+#if !defined(__GLIBC__)
+  ASSERT_LT(THREAD_SIGNAL, SIGRTMIN);
+#endif
+
   // Save the current signal action and make sure it is restored afterwards.
   struct sigaction cur_action;
-  ASSERT_TRUE(sigaction(SIGURG, NULL, &cur_action) == 0);
+  ASSERT_TRUE(sigaction(THREAD_SIGNAL, NULL, &cur_action) == 0);
 
   UniquePtr<Backtrace> backtrace(Backtrace::Create(getpid(), thread_data.tid));
   ASSERT_TRUE(backtrace.get() != NULL);
@@ -475,7 +484,7 @@ TEST(libbacktrace, thread_level_trace) {
 
   // Verify that the old action was restored.
   struct sigaction new_action;
-  ASSERT_TRUE(sigaction(SIGURG, NULL, &new_action) == 0);
+  ASSERT_TRUE(sigaction(THREAD_SIGNAL, NULL, &new_action) == 0);
   EXPECT_EQ(cur_action.sa_sigaction, new_action.sa_sigaction);
   EXPECT_EQ(cur_action.sa_flags, new_action.sa_flags);
 }