Linux Sandbox: check no threads before fork().

sandbox/linux/seccomp-bpf/sandbox_bpf.cc

Index: sandbox/linux/seccomp-bpf/sandbox_bpf.cc
diff --git a/sandbox/linux/seccomp-bpf/sandbox_bpf.cc b/sandbox/linux/seccomp-bpf/sandbox_bpf.cc
index b598d769ba15b6f251a3f724f83c2033ab91d0b8..9fe503134015df4d3bd3ed1325b91b05172ee773 100644
--- a/sandbox/linux/seccomp-bpf/sandbox_bpf.cc
+++ b/sandbox/linux/seccomp-bpf/sandbox_bpf.cc
@@ -268,6 +268,10 @@ bool Sandbox::RunFunctionInPolicy(void (*code_in_sandbox)(),
     SANDBOX_DIE("Process started without standard file descriptors");
   }
 
+  // This code is using fork() and should only ever run single threaded.

[Jorge Lucangeli Obes, 2013/12/10 21:14:08]

nit: single-threaded.

[jln (very slow on Chromium), 2013/12/10 21:42:00]

Done.

+  // Most of the code below is "async-signal-safe" and only minor changes
+  // would be needed to support threads.
+  DCHECK(IsSingleThreaded(proc_fd_));
   pid_t pid = fork();
   if (pid < 0) {
     // Die if we cannot fork(). We would probably fail a little later