[sanitizer] Fix __sanitizer_syscall_post_epoll_wait

Syscall return number of initialized events which
needs to be used for unposoning.

Differential Revision: https://reviews.llvm.org/D107207

Author: vitalybuka

compiler-rt/lib/sanitizer_common/sanitizer_common_syscalls.inc

@@ -2107,7 +2107,7 @@ POST_SYSCALL(epoll_wait)
 (long res, long epfd, void *events, long maxevents, long timeout) {
   if (res >= 0) {
     if (events)
-      POST_WRITE(events, struct_epoll_event_sz);
+      POST_WRITE(events, res * struct_epoll_event_sz);
   }
 }
 
@@ -2123,7 +2123,7 @@ POST_SYSCALL(epoll_pwait)
  const void *sigmask, long sigsetsize) {
   if (res >= 0) {
     if (events)
-      POST_WRITE(events, struct_epoll_event_sz);
+      POST_WRITE(events, res * struct_epoll_event_sz);
   }
 }
 

compiler-rt/test/msan/Linux/syscalls.cpp

@@ -9,6 +9,7 @@
 
 #include <linux/aio_abi.h>
 #include <signal.h>
+#include <sys/epoll.h>
 #include <sys/ptrace.h>
 #include <sys/stat.h>
 #include <sys/uio.h>
@@ -128,5 +129,17 @@ int main(int argc, char *argv[]) {
   __sanitizer_syscall_post_sigaltstack(0, nullptr, (stack_t *)buf);
   assert(__msan_test_shadow(buf, sizeof(buf)) == sizeof(stack_t));
 
+  __msan_poison(buf, sizeof(buf));
+  long max_events = sizeof(buf) / sizeof(epoll_event);
+  __sanitizer_syscall_pre_epoll_wait(0, buf, max_events, 0);
+  __sanitizer_syscall_post_epoll_wait(max_events, 0, buf, max_events, 0);
+  assert(__msan_test_shadow(buf, sizeof(buf)) == max_events * sizeof(epoll_event));
+
+  __msan_poison(buf, sizeof(buf));
+  sigset_t sigset = {};
+  __sanitizer_syscall_pre_epoll_pwait(0, buf, max_events, 0, &sigset, sizeof(sigset));
+  __sanitizer_syscall_post_epoll_pwait(max_events, 0, buf, max_events, 0, &sigset, sizeof(sigset));
+  assert(__msan_test_shadow(buf, sizeof(buf)) == max_events * sizeof(epoll_event));
+
   return 0;
 }