Only copy mcontext data from sigcontext.

The ucontext_t data structure could be bigger than the kernel data
structure. Since the unwinder only cares about the mcontext data, only
copy that out of the structure. The mcontext data is the same size in
the kernel and in the ucontext_t structure.

Bug: 15189014
Change-Id: I5978169c4425b8212e11db85a57eb319cd0e264b

2 files changed, 9 insertions, 5 deletions

diff --git a/libbacktrace/BacktraceThread.cpp b/libbacktrace/BacktraceThread.cpp
index 018d51f04..b47cd2ad2 100644
--- a/libbacktrace/BacktraceThread.cpp
+++ b/libbacktrace/BacktraceThread.cpp
@@ -117,6 +117,12 @@ void ThreadEntry::Wake() {
   futex(&futex_, FUTEX_WAKE, INT_MAX, NULL, NULL, 0);
 }
 
+void ThreadEntry::CopyUcontextFromSigcontext(void* sigcontext) {
+  ucontext_t* ucontext = reinterpret_cast<ucontext_t*>(sigcontext);
+  // The only thing the unwinder cares about is the mcontext data.
+  memcpy(&ucontext_.uc_mcontext, &ucontext->uc_mcontext, sizeof(ucontext->uc_mcontext));
+}
+
 //-------------------------------------------------------------------------
 // BacktraceThread functions.
 //-------------------------------------------------------------------------
@@ -129,7 +135,7 @@ static void SignalHandler(int, siginfo_t*, void* sigcontext) {
     return;
   }
 
-  entry->CopyUcontext(reinterpret_cast<ucontext_t*>(sigcontext));
+  entry->CopyUcontextFromSigcontext(sigcontext);
 
   // Indicate the ucontext is now valid.
   entry->Wake();
diff --git a/libbacktrace/BacktraceThread.h b/libbacktrace/BacktraceThread.h
index a75a807be..ff3e9f3f4 100644
--- a/libbacktrace/BacktraceThread.h
+++ b/libbacktrace/BacktraceThread.h
@@ -40,14 +40,12 @@ public:
 
   static void Remove(ThreadEntry* entry);
 
-  inline void CopyUcontext(ucontext_t* ucontext) {
-    memcpy(&ucontext_, ucontext, sizeof(ucontext_));
-  }
-
   void Wake();
 
   void Wait(int);
 
+  void CopyUcontextFromSigcontext(void*);
+
   inline void Lock() {
     pthread_mutex_lock(&mutex_);
     // Reset the futex value in case of multiple unwinds of the same thread.