[esan] Ensure internal_sigaction() bypasses interceptors

[bruening]

Summary

Implements real_sigaction() which it turns out is required for
internal_sigaction() to bypass the libc interceptors.

[aizatsky]

Is there (or will there by) a test that exercises this?

[bruening]

In D20832#447421, @aizatsky wrote:

Is there (or will there by) a test that exercises this?

Without this CL, our internal_sigaction calls that we make every run are only during init, where the EsanDuringInit check in COMMON_INTERCEPTOR_ENTER does end up calling the original libc sigaction and we do avoid running our interception action on our own calls that way (but it does not feel right for an "internal_" call to go through the interceptor). The real problem is when we call internal_sigaction after we're initialized, which only happens on an unhandled SIGSEGV for which the app has no handler: then we'll spin in an infinite loop as our attempts to remove our own handler repeatedly fail. It's not easy to add such a test. I'll add some of this explanation to the CL.

compiler-rt/trunk/lib/esan/esan_interceptors.cpp

 }
 #define ESAN_MAYBE_INTERCEPT_SIGNAL INTERCEPT_FUNCTION(signal)
 #else
 #error Platform not supported
 #define ESAN_MAYBE_INTERCEPT_SIGNAL
 #endif
 
 #if SANITIZER_LINUX
+DECLARE_REAL(int, sigaction, int signum, const struct sigaction *act,
+             struct sigaction *oldact)
 INTERCEPTOR(int, sigaction, int signum, const struct sigaction *act,
             struct sigaction *oldact) {
   void *ctx;
   COMMON_INTERCEPTOR_ENTER(ctx, sigaction, signum, act, oldact);
   if (!processSigaction(signum, act, oldact))
     return 0;
   else
     return REAL(sigaction)(signum, act, oldact);
 }
+
+// This is required to properly use internal_sigaction.
+namespace __sanitizer {
+int real_sigaction(int signum, const void *act, void *oldact) {
+  return REAL(sigaction)(signum, (const struct sigaction *)act,
+                         (struct sigaction *)oldact);
+}
+} // namespace __sanitizer
+
 #define ESAN_MAYBE_INTERCEPT_SIGACTION INTERCEPT_FUNCTION(sigaction)
 #else
 #error Platform not supported
 #define ESAN_MAYBE_INTERCEPT_SIGACTION
 #endif
 
 namespace __esan {
 

compiler-rt/trunk/lib/sanitizer_common/sanitizer_posix.h

#define DEFINE_REAL_PTHREAD_FUNCTIONS                                          \
   }                                                                            \
   int real_pthread_join(void *th, void **ret) {                                \
     return REAL(pthread_join(th, ret));                                        \
   }                                                                            \
   }  // namespace __sanitizer
 
 int my_pthread_attr_getstack(void *attr, void **addr, uptr *size);
 
+// A routine named real_sigaction() must be implemented by each sanitizer in
+// order for internal_sigaction() to bypass interceptors.
 int internal_sigaction(int signum, const void *act, void *oldact);
 void internal_sigfillset(__sanitizer_sigset_t *set);
 
 uptr internal_execve(const char *filename, char *const argv[],
                      char *const envp[]);
 }  // namespace __sanitizer
 
 #endif  // SANITIZER_POSIX_H