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

content/zygote/zygote_main_linux.cc

Index: content/zygote/zygote_main_linux.cc
diff --git a/content/zygote/zygote_main_linux.cc b/content/zygote/zygote_main_linux.cc
index b77471a88d956a5fae66617ba90718fa4e187460..7cc9cb27776a2fd3efd1e6b045bc5b419641a2c0 100644
--- a/content/zygote/zygote_main_linux.cc
+++ b/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 +66,40 @@
 
 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));
+
+  ignore_result(chroot("/"));
+
+  char kSecondMessage[100];
+  const ssize_t printed = base::strings::SafeSPrintf(
+      kSecondMessage, "chroot() returned. Errno is %d.\n", errno);

[mdempsky, 2014/09/19 23:35:40]

Maybe go ahead and include chroot()'s return value in the message string too?

Also, no SafeSNPrintf in bafe? :(

[jln (very slow on Chromium), 2014/09/19 23:42:30]

Done
There is, but SafeSprintf is safe as well (it uses templates and knows the size
of the buffer).

+  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));
+}
+}

[mdempsky, 2014/09/19 23:35:40]

(Should have a "// namespace" comment, I think?)

[jln (very slow on Chromium), 2014/09/19 23:42:30]

Done.

+
 // See http://code.google.com/p/chromium/wiki/LinuxZygote
 
 static void ProxyLocaltimeCallToBrowser(time_t input, struct tm* output,
@@ -411,6 +447,8 @@ static bool EnterSuidSandbox(sandbox::SetuidSandboxClient* setuid_sandbox,
       return false;
     }
   }
+
+  InstallSandboxCrashTestHandler();
 #endif
 
   return true;