[tsan] Fix sigaction implementation when it's called only to get handler

[vitalybuka]

format

[vitalybuka]

remove debug printf

compiler-rt/lib/tsan/rtl/tsan_interceptors.cc

 int sigaction_impl(int sig, const __sanitizer_sigaction *act,
                    __sanitizer_sigaction *old) {
   // Note: if we call REAL(sigaction) directly for any reason without proxying
   // the signal handler through rtl_sigaction, very bad things will happen.
   // The handler will run synchronously and corrupt tsan per-thread state.
   SCOPED_INTERCEPTOR_RAW(sigaction, sig, act, old);
   __sanitizer_sigaction *sigactions = interceptor_ctx()->sigactions;
   __sanitizer_sigaction old_stored;
-  internal_memcpy(&old_stored, &sigactions[sig], sizeof(old_stored));
-  if (act == 0) return 0;
+  if (old) internal_memcpy(&old_stored, &sigactions[sig], sizeof(old_stored));
+  __sanitizer_sigaction newact;
+  if (act) {
     // Copy act into sigactions[sig].
     // Can't use struct copy, because compiler can emit call to memcpy.
     // Can't use internal_memcpy, because it copies byte-by-byte,
     // and signal handler reads the handler concurrently. It it can read
     // some bytes from old value and some bytes from new value.
     // Use volatile to prevent insertion of memcpy.
     sigactions[sig].handler =
         *(volatile __sanitizer_sighandler_ptr const *)&act->handler;
     sigactions[sig].sa_flags = *(volatile int const *)&act->sa_flags;
     internal_memcpy(&sigactions[sig].sa_mask, &act->sa_mask,
                     sizeof(sigactions[sig].sa_mask));
 #if !SANITIZER_FREEBSD && !SANITIZER_MAC && !SANITIZER_NETBSD
     sigactions[sig].sa_restorer = act->sa_restorer;
 #endif
-  __sanitizer_sigaction newact;
     internal_memcpy(&newact, act, sizeof(newact));
     internal_sigfillset(&newact.sa_mask);
     if ((uptr)act->handler != sig_ign && (uptr)act->handler != sig_dfl) {
       if (newact.sa_flags & SA_SIGINFO)
         newact.sigaction = rtl_sigaction;
       else
         newact.handler = rtl_sighandler;
     }
     ReleaseStore(thr, pc, (uptr)&sigactions[sig]);
-  int res = REAL(sigaction)(sig, &newact, old);
+    act = &newact;
+  }
+  int res = REAL(sigaction)(sig, act, old);
   if (res == 0 && old) {
     uptr cb = (uptr)old->sigaction;
     if (cb == (uptr)rtl_sigaction || cb == (uptr)rtl_sighandler) {
       internal_memcpy(old, &old_stored, sizeof(*old));
     }
   }
   return res;
 }

compiler-rt/test/sanitizer_common/TestCases/Linux/signal_segv_handler.cc

-// RUN: %clangxx -O1 %s -o %t && TSAN_OPTIONS="flush_memory_ms=1 memory_limit_mb=1" ASAN_OPTIONS="handle_segv=0" %run %t 2>&1 | FileCheck %s
+// RUN: %clangxx -O1 %s -o %t && TSAN_OPTIONS="flush_memory_ms=1
+// memory_limit_mb=1" %run %t 2>&1 | FileCheck %s
 
 // JVM uses SEGV to preempt threads. All threads do a load from a known address
 // periodically. When runtime needs to preempt threads, it unmaps the page.
 // Threads start triggering SEGV one by one. The signal handler blocks
 // threads while runtime does its thing. Then runtime maps the page again
 // and resumes the threads.
 // Previously this pattern conflicted with stop-the-world machinery,
 // because it briefly reset SEGV handler to SIG_DFL.
 // As the consequence JVM just silently died.
 
 // This test sets memory flushing rate to maximum, then does series of
 // "benign" SEGVs that are handled by signal handler, and ensures that
 // the process survive.
 
+#include <assert.h>
+#include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include <signal.h>
-#include <sys/mman.h>
 #include <string.h>
+#include <sys/mman.h>
 #include <unistd.h>
 
 unsigned long page_size;
 void *guard;
 
 void handler(int signo, siginfo_t *info, void *uctx) {
   mprotect(guard, page_size, PROT_READ | PROT_WRITE);
 }
 
 int main() {
   page_size = sysconf(_SC_PAGESIZE);
   struct sigaction a, old;
   memset(&a, 0, sizeof(a));
   memset(&old, 0, sizeof(old));
   a.sa_sigaction = handler;
   a.sa_flags = SA_SIGINFO;
   sigaction(SIGSEGV, &a, &old);
+
+  memset(&a, 0, sizeof(a));
+  sigaction(SIGSEGV, 0, &a);
+  assert(a.sa_sigaction == handler);
+  fprintf(stderr, "%d %d\n", (int)a.sa_flags, (int)SA_SIGINFO);
+  assert(a.sa_flags & SA_SIGINFO);
+
   guard = mmap(0, 3 * page_size, PROT_NONE, MAP_ANON | MAP_PRIVATE, -1, 0);
   guard = (char*)guard + page_size;  // work around a kernel bug 
   for (int i = 0; i < 1000000; i++) {
     mprotect(guard, page_size, PROT_NONE);
     *(int*)guard = 1;
   }
   sigaction(SIGSEGV, &old, 0);
   fprintf(stderr, "DONE\n");
 }
 
 // CHECK: DONE