Fix a couple of bugs where we generate incorrect minidump files on Linux.o

Patch by Markus Gutschke <markus@chromium.org>.  R=thestig

Review URL: http://breakpad.appspot.com/150001
Review URL: http://breakpad.appspot.com/155001

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@649 4c0a9323-5329-0410-9bdc-e9ce6186880e

src/client/linux/minidump_writer/linux_dumper.cc

@@ -75,6 +75,26 @@ static bool SuspendThread(pid_t pid) {
       return false;
     }
   }
+#if defined(__i386) || defined(__x86_64)
+  // On x86, the stack pointer is NULL or -1, when executing trusted code in
+  // the seccomp sandbox. Not only does this cause difficulties down the line
+  // when trying to dump the thread's stack, it also results in the minidumps
+  // containing information about the trusted threads. This information is
+  // generally completely meaningless and just pollutes the minidumps.
+  // We thus test the stack pointer and exclude any threads that are part of
+  // the seccomp sandbox's trusted code.
+  user_regs_struct regs;
+  if (sys_ptrace(PTRACE_GETREGS, pid, NULL, &regs) == -1 ||
+#if defined(__i386)
+      !regs.esp
+#elif defined(__x86_64)
+      !regs.rsp
+#endif
+      ) {
+    sys_ptrace(PTRACE_DETACH, pid, NULL, NULL);
+    return false;
+  }
+#endif
   return true;
 }
 
@@ -111,11 +131,19 @@ bool LinuxDumper::Init() {
 bool LinuxDumper::ThreadsSuspend() {
   if (threads_suspended_)
     return true;
-  bool good = true;
-  for (size_t i = 0; i < threads_.size(); ++i)
-    good &= SuspendThread(threads_[i]);
+  for (size_t i = 0; i < threads_.size(); ++i) {
+    if (!SuspendThread(threads_[i])) {
+      // If the thread either disappeared before we could attach to it, or if
+      // it was part of the seccomp sandbox's trusted code, it is OK to
+      // silently drop it from the minidump.
+      memmove(&threads_[i], &threads_[i+1],
+              (threads_.size() - i - 1) * sizeof(threads_[i]));
+      threads_.resize(threads_.size() - 1);
+      --i;
+    }
+  }
   threads_suspended_ = true;
-  return good;
+  return threads_.size() > 0;
 }
 
 bool LinuxDumper::ThreadsResume() {