[PATCH] [sanitizer] Fix log_path behavior with StopTheWorld.

[Sergey Matveev]

Hi samsonov,

Fix race on report_fd/report_fd_pid between the parent process and the
tracer task.

http://llvm-reviews.chandlerc.com/D2306

Files:
  lib/sanitizer_common/sanitizer_common.cc
  lib/sanitizer_common/sanitizer_common.h
  lib/sanitizer_common/sanitizer_posix.cc
  lib/sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cc

Index: lib/sanitizer_common/sanitizer_common.cc
===================================================================
--- lib/sanitizer_common/sanitizer_common.cc
+++ lib/sanitizer_common/sanitizer_common.cc
@@ -42,6 +42,12 @@
 // child thread will be different from |report_fd_pid|.
 uptr report_fd_pid = 0;
 
+// Alternative report FD and PID for use by the tracer task in StopTheWorld. It
+// shares the address space with the main process, but has a different PID and
+// thus requires special handling.
+fd_t tracer_report_fd = kStderrFd;
+uptr tracer_report_fd_pid = 0;
+
 static DieCallbackType DieCallback;
 void SetDieCallback(DieCallbackType callback) {
   DieCallback = callback;
Index: lib/sanitizer_common/sanitizer_common.h
===================================================================
--- lib/sanitizer_common/sanitizer_common.h
+++ lib/sanitizer_common/sanitizer_common.h
@@ -136,6 +136,8 @@
 extern bool log_to_file;
 extern char report_path_prefix[4096];
 extern uptr report_fd_pid;
+extern fd_t tracer_report_fd;
+extern uptr tracer_report_fd_pid;
 
 uptr OpenFile(const char *filename, bool write);
 // Opens the file 'file_name" and reads up to 'max_len' bytes.
Index: lib/sanitizer_common/sanitizer_posix.cc
===================================================================
--- lib/sanitizer_common/sanitizer_posix.cc
+++ lib/sanitizer_common/sanitizer_posix.cc
@@ -197,33 +197,53 @@
   return 0;
 }
 
-void MaybeOpenReportFile() {
-  if (!log_to_file || (report_fd_pid == internal_getpid())) return;
+static uptr DoOpenReportFile(uptr pid) {
   InternalScopedBuffer<char> report_path_full(4096);
-  internal_snprintf(report_path_full.data(), report_path_full.size(),
-                    "%s.%d", report_path_prefix, internal_getpid());
+  internal_snprintf(report_path_full.data(), report_path_full.size(), "%s.%d",
+                    report_path_prefix, pid);
   uptr openrv = OpenFile(report_path_full.data(), true);
   if (internal_iserror(openrv)) {
-    report_fd = kStderrFd;
+    if (pid == tracer_report_fd_pid)
+      tracer_report_fd = kStderrFd;
+    else
+      report_fd = kStderrFd;
     log_to_file = false;
     Report("ERROR: Can't open file: %s\n", report_path_full.data());
     Die();
   }
-  if (report_fd != kInvalidFd) {
-    // We're in the child. Close the parent's log.
-    internal_close(report_fd);
-  }
-  report_fd = openrv;
-  report_fd_pid = internal_getpid();
+  return openrv;
 }
 
+void MaybeOpenReportFile() {
+  if (!log_to_file) return;
+  uptr pid = internal_getpid();
+  bool in_tracer = (pid == tracer_report_fd_pid);
+  if (in_tracer) {
+    if (tracer_report_fd != kInvalidFd)
+      return;
+    uptr openrv = DoOpenReportFile(pid);
+    tracer_report_fd = openrv;
+  } else {
+    if (pid == report_fd_pid)
+      return;
+    uptr openrv = DoOpenReportFile(pid);
+    if (report_fd != kInvalidFd) {
+      // We're in the child. Close the parent's log.
+      internal_close(report_fd);
+    }
+    report_fd = openrv;
+    report_fd_pid = pid;
+  }
+}
 void RawWrite(const char *buffer) {
   static const char *kRawWriteError =
       "RawWrite can't output requested buffer!\n";
   uptr length = (uptr)internal_strlen(buffer);
   MaybeOpenReportFile();
-  if (length != internal_write(report_fd, buffer, length)) {
-    internal_write(report_fd, kRawWriteError, internal_strlen(kRawWriteError));
+  uptr fd = (internal_getpid() == tracer_report_fd_pid) ? tracer_report_fd
+                                                        : report_fd;
+  if (length != internal_write(fd, buffer, length)) {
+    internal_write(fd, kRawWriteError, internal_strlen(kRawWriteError));
     Die();
   }
 }
Index: lib/sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cc
===================================================================
--- lib/sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cc
+++ lib/sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cc
@@ -356,6 +356,26 @@
   int process_was_dumpable_;
 };
 
+// When sanitizer output is being redirected to file (i.e. by using log_path) we
+// must prevent the tracer task from conflicting with the parent over file
+// descriptors. This scoper manipulates some sanitizer_common globals to achieve
+// that.
+struct ScopedHandleReportFD {
+  explicit ScopedHandleReportFD(uptr tracer_pid) {
+    tracer_report_fd = kInvalidFd;
+    tracer_report_fd_pid = tracer_pid;
+    if (report_fd == kStderrFd || report_fd == kStdoutFd)
+      tracer_report_fd = report_fd;
+  }
+  ~ScopedHandleReportFD() {
+    if (tracer_report_fd != kStderrFd && tracer_report_fd != kStdoutFd &&
+        tracer_report_fd != kInvalidFd)
+      internal_close(tracer_report_fd);
+    tracer_report_fd = kInvalidFd;
+    tracer_report_fd_pid = 0;
+  }
+};
+
 void StopTheWorld(StopTheWorldCallback callback, void *argument) {
   StopTheWorldScope in_stoptheworld;
   // Prepare the arguments for TracerThread.
@@ -379,6 +399,7 @@
       Report("Failed spawning a tracer thread (errno %d).\n", local_errno);
     tracer_thread_argument.mutex.Unlock();
   } else {
+    ScopedHandleReportFD scoped_handle_report_fd(tracer_pid);
     // On some systems we have to explicitly declare that we want to be traced
     // by the tracer thread.
 #ifdef PR_SET_PTRACER
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