[MIPS] Add seccomp bpf support

sandbox/linux/seccomp-bpf/syscall_unittest.cc

Index: sandbox/linux/seccomp-bpf/syscall_unittest.cc
diff --git a/sandbox/linux/seccomp-bpf/syscall_unittest.cc b/sandbox/linux/seccomp-bpf/syscall_unittest.cc
index 60db69bcd6bb5388bb8929740fdef48156cba5ed..0cdd767252c467dee50bbcebb06fb5a7a1f23ecc 100644
--- a/sandbox/linux/seccomp-bpf/syscall_unittest.cc
+++ b/sandbox/linux/seccomp-bpf/syscall_unittest.cc
@@ -50,6 +50,9 @@ TEST(Syscall, WellKnownEntryPoint) {
 #else
   EXPECT_EQ(0xEF000000u, ((uint32_t*)SandboxSyscall(-1))[-1]);  // SVC 0
 #endif
+#elif defined(__mips__)
+  // Opcode for MIPS sycall is in the lower 16-bits
+  EXPECT_EQ(0x0cu, (((uint32_t *)SandboxSyscall(-1))[-1])&0x0000FFFF);
 #else
 #warning Incomplete test case; need port for target platform
 #endif
@@ -75,7 +78,12 @@ intptr_t CopySyscallArgsToAux(const struct arch_seccomp_data& args, void* aux) {
       static_cast<std::vector<uint64_t>*>(aux);
   BPF_ASSERT(arraysize(args.args) == 6);
   seen_syscall_args->assign(args.args, args.args + arraysize(args.args));
+#if defined(__mips__)
+  // On MIPS, kernel returns errno and not -errno
+  return ENOMEM;
+#else
   return -ENOMEM;
+#endif
 }
 
 ErrorCode CopyAllArgsOnUnamePolicy(SandboxBPF* sandbox, int sysno, void* aux) {
@@ -107,6 +115,15 @@ BPF_TEST(Syscall,
 
   // We could use pretty much any system call we don't need here. uname() is
   // nice because it doesn't have any dangerous side effects.
+#if defined(__mips__)
+  BPF_ASSERT(SandboxSyscall(__NR_uname,
+                            syscall_args[0],
+                            syscall_args[1],
+                            syscall_args[2],
+                            syscall_args[3],
+                            syscall_args[4],
+                            syscall_args[5]) == ENOMEM);
+#else
   BPF_ASSERT(SandboxSyscall(__NR_uname,
                             syscall_args[0],
                             syscall_args[1],
@@ -114,6 +131,7 @@ BPF_TEST(Syscall,
                             syscall_args[3],
                             syscall_args[4],
                             syscall_args[5]) == -ENOMEM);
+#endif
 
   // We expect the trap handler to have copied the 6 arguments.
   BPF_ASSERT(BPF_AUX.size() == 6);