Linux sandbox: add behind-flag USR2 handler for crash debugging.

When --allow-sandbox-debugging is used, we set-up a new signal
handler in the Zygote (inherited by all renderers) that performs a
chroot().

This allows testing of sandbox violation crash report with official
binaries shipping to users.

BUG=415842
R=mdempsky@chromium.org

Review URL: https://codereview.chromium.org/585123003

Cr-Commit-Position: refs/heads/master@{#295826}

content/zygote/zygote_main_linux.cc

@@ -7,6 +7,7 @@
 #include <dlfcn.h>
 #include <fcntl.h>
 #include <pthread.h>
+#include <signal.h>
 #include <string.h>
 #include <sys/socket.h>
 #include <sys/types.h>
@@ -22,6 +23,7 @@
 #include "base/posix/eintr_wrapper.h"
 #include "base/posix/unix_domain_socket_linux.h"
 #include "base/rand_util.h"
+#include "base/strings/safe_sprintf.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/sys_info.h"
 #include "build/build_config.h"
@@ -64,6 +66,43 @@
 
 namespace content {
 
+namespace {
+
+void DoChrootSignalHandler(int) {
+  const int old_errno = errno;
+  const char kFirstMessage[] = "Chroot signal handler called.\n";
+  ignore_result(write(STDERR_FILENO, kFirstMessage, sizeof(kFirstMessage) - 1));
+
+  const int chroot_ret = chroot("/");
+
+  char kSecondMessage[100];
+  const ssize_t printed =
+      base::strings::SafeSPrintf(kSecondMessage,
+                                 "chroot() returned %d. Errno is %d.\n",
+                                 chroot_ret,
+                                 errno);
+  if (printed > 0 && printed < static_cast<ssize_t>(sizeof(kSecondMessage))) {
+    ignore_result(write(STDERR_FILENO, kSecondMessage, printed));
+  }
+  errno = old_errno;
+}
+
+// This is a quick hack to allow testing sandbox crash reports in production
+// binaries.
+// This installs a signal handler for SIGUSR2 that performs a chroot().
+// In most of our BPF policies, it is a "watched" system call which will
+// trigger a SIGSYS signal whose handler will crash.
+// This has been added during the investigation of https://crbug.com/415842.
+void InstallSandboxCrashTestHandler() {
+  struct sigaction act = {};
+  act.sa_handler = DoChrootSignalHandler;
+  CHECK_EQ(0, sigemptyset(&act.sa_mask));
+  act.sa_flags = 0;
+
+  PCHECK(0 == sigaction(SIGUSR2, &act, NULL));
+}
+}  // namespace
+
 // See http://code.google.com/p/chromium/wiki/LinuxZygote
 
 static void ProxyLocaltimeCallToBrowser(time_t input, struct tm* output,
@@ -410,7 +449,12 @@ static bool EnterSuidSandbox(sandbox::SetuidSandboxClient* setuid_sandbox,
       LOG(ERROR) << "Failed to set non-dumpable flag";
       return false;
     }
+  } else {
+    // If sandbox debugging is allowed, install a handler for sandbox-related
+    // crash testing.
+    InstallSandboxCrashTestHandler();
   }
+
 #endif
 
   return true;