Point the child stack at the top-most address rather than the bottom-most add...

breakpad/linux/exception_handler.cc

 // Copyright (c) 2009, Google Inc.
 // All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
@@ skipping 61 matching lines @@
 #include <signal.h>
 #include <stdio.h>
 #include <sys/mman.h>
 #include <sys/signal.h>
 #include <sys/syscall.h>
 #include <sys/ucontext.h>
 #include <sys/user.h>
 #include <sys/wait.h>
 #include <unistd.h>
 
+#include "breakpad/linux/linux_libc_support.h"
 #include "breakpad/linux/linux_syscall_support.h"
 #include "breakpad/linux/memory.h"
 #include "breakpad/linux/minidump_writer.h"
 #include "common/linux/guid_creator.h"
 
 // A wrapper for the tgkill syscall: send a signal to a specific thread.
 static int tgkill(pid_t tgid, pid_t tid, int sig) {
   syscall(__NR_tgkill, tgid, tid, sig);
 }
 
@@ skipping 167 matching lines @@
 
   CrashContext context;
   memcpy(&context.siginfo, info, sizeof(siginfo_t));
   memcpy(&context.context, uc, sizeof(struct ucontext));
   context.tid = sys_gettid();
 
   if (crash_handler_ && crash_handler_(&context, sizeof(context),
                                        callback_context_))
     return true;
 
+  static const unsigned kChildStackSize = 8000;
   PageAllocator allocator;
-  void* const stack = allocator.Alloc(8000);
+  uint8_t* stack = (uint8_t*) allocator.Alloc(kChildStackSize);
+  if (!stack)
+    return false;
+  // clone() needs the top-most address. (scrub just to be safe)
+  stack += kChildStackSize;
+  my_memset(stack - 16, 0, 16);
 
   ThreadArgument thread_arg;
   thread_arg.handler = this;
   thread_arg.pid = getpid();
   thread_arg.context = &context;
   thread_arg.context_size = sizeof(context);
 
   const pid_t child = sys_clone(
       ThreadEntry, stack, CLONE_FILES | CLONE_FS | CLONE_UNTRACED,
       &thread_arg, NULL, NULL, NULL);
@@ skipping 20 matching lines @@
 
 // This function runs in a compromised context: see the top of the file.
 // Runs on the cloned process.
 bool ExceptionHandler::DoDump(pid_t crashing_process, const void* context,
                               size_t context_size) {
   return google_breakpad::WriteMinidump(
       next_minidump_path_c_, crashing_process, context, context_size);
 }
 
 }  // namespace google_breakpad