Merge trunk r1454 to the chrome_43 branch.

> Fix signal propagation logic for Linux/Android exception handler.
>
> The current code is relying on info->si_pid to figure out whether
> the exception handler was triggered by a signal coming from the kernel
> (that will re-trigger until the cause that triggered the signal has
> been cleared) or from user-space e.g., kill -SIGNAL pid, which will NOT
> automatically re-trigger in the next signal handler in the chain.
> While the intentions are good (manually re-triggering user-space
> signals), the current implementation mistakenly looks at the si_pid
> field in siginfo_t, assuming that it is coming from the kernel if
> si_pid == 0.
> This is wrong. siginfo_t, in fact, is a union and si_pid is meaningful
> only for userspace signals. For signals originated by the kernel,
> instead, si_pid overlaps with si_addr (the faulting address).
> As a matter of facts, the current implementation is mistakenly
> re-triggering the signal using tgkill for most of the kernel-space
> signals (unless the fault address is exactly 0x0).
> This is not completelly correct for the case of SIGSEGV/SIGBUS. The
> next handler in the chain will stil see the signal, but the |siginfo|
> and the |context| arguments of the handler will be meaningless
> (retriggering a signal with tgkill doesn't preserve them).
> Therefore, if the next handler in the chain expects those arguments
> to be set, it will fail.
> Concretelly, this is causing problems to WebView. In some rare
> circumstances, the next handler in the chain is a user-space runtime
> which does SIGSEGV handling to implement speculative null pointer
> managed exceptions (see as an example
> http://www.mono-project.com/docs/advanced/runtime/docs/exception-handling/)
>
> The fix herein proposed consists in using the si_code (see SI_FROMUSER
> macros) to determine whether a signal is coming form the kernel
> (and therefore just re-establish the next signal handler) or from
> userspace (and use the tgkill logic).
>
> Repro case:
> This issue is visible in Chrome for Android with this simple repro case:
> - Add a non-null pointer dereference in the codebase:
>   *((volatile int*)0xbeef) = 42
> Without this change: the next handler (the libc trap) prints:
>   F/libc  (  595): Fatal signal 11 (SIGSEGV), code 1, fault addr 0x487
>   where 0x487 is actually the PID of the process (which is wrong).
> With this change: the next handler prints:
>   F/libc  (  595): Fatal signal 11 (SIGSEGV), code 1, fault addr 0xbeef
>   which is the correct answer.
>
> BUG=chromium:481937
> R=mark@chromium.org
>
> Review URL: https://breakpad.appspot.com/6844002

BUG=chromium:481937

git-svn-id: http://google-breakpad.googlecode.com/svn/branches/chrome_43@1457 4c0a9323-5329-0410-9bdc-e9ce6186880e
diff --git a/src/client/linux/handler/exception_handler.cc b/src/client/linux/handler/exception_handler.cc
index 6a8849d..e515a73 100644
--- a/src/client/linux/handler/exception_handler.cc
+++ b/src/client/linux/handler/exception_handler.cc
@@ -365,7 +365,8 @@
 
   pthread_mutex_unlock(&g_handler_stack_mutex_);
 
-  if (info->si_pid || sig == SIGABRT) {
+  // info->si_code <= 0 iff SI_FROMUSER (SI_FROMKERNEL otherwise).
+  if (info->si_code <= 0 || sig == SIGABRT) {
     // This signal was triggered by somebody sending us the signal with kill().
     // In order to retrigger it, we have to queue a new signal by calling
     // kill() ourselves.  The special case (si_pid == 0 && sig == SIGABRT) is