[MIPS] Add seccomp bpf support

sandbox/linux/seccomp-bpf-helpers/sigsys_handlers.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Note: any code in this file MUST be async-signal safe.
 
 #include "sandbox/linux/seccomp-bpf-helpers/sigsys_handlers.h"
 
 #include <unistd.h>
 
@@ skipping 26 matching lines @@
     // TODO(jln): query the current policy to check if send() is available and
     // use it to perform a non-blocking write.
     const int ret = HANDLE_EINTR(write(STDERR_FILENO, error_message, size));
     // We can't handle any type of error here.
     if (ret <= 0 || static_cast<size_t>(ret) > size) break;
     size -= ret;
     error_message += ret;
   }
 }
 
+// Invalid syscall values are truncated to zero.
+// On architectures where base value is zero (Intel and Arm),
+// syscall number is the same as offset from base.
+// This function returns values between 0 and 1023 on all architectures.
+// On architectures where base value is different than zero (currently only
+// Mips), we are truncating valid syscall values to offset from base.
+uint32_t SyscallNumberToOffsetFromBase(uint32_t sysno) {
+#if defined(__mips__)
+  // On MIPS syscall numbers are in different range than on x86 and ARM.
+  // Valid MIPS O32 ABI syscall __NR_syscall will be truncated to zero for
+  // simplicity.
+    sysno = sysno - __NR_Linux;
+#endif
+
+  if (sysno >= 1024)
+    sysno = 0;
+
+  return sysno;
+}
+
 // Print a seccomp-bpf failure to handle |sysno| to stderr in an
 // async-signal safe way.
 void PrintSyscallError(uint32_t sysno) {
   if (sysno >= 1024)
     sysno = 0;
   // TODO(markus): replace with async-signal safe snprintf when available.
   const size_t kNumDigits = 4;
   char sysno_base10[kNumDigits];
   uint32_t rem = sysno;
   uint32_t mod = 0;
   for (int i = kNumDigits - 1; i >= 0; i--) {
     mod = rem % 10;
     rem /= 10;
     sysno_base10[i] = '0' + mod;
   }
+#if defined(__mips__) && (_MIPS_SIM == _MIPS_SIM_ABI32)
+  static const char kSeccompErrorPrefix[] =
+      __FILE__":**CRASHING**:" SECCOMP_MESSAGE_COMMON_CONTENT
+                               " in syscall 4000 + ";
+#else
   static const char kSeccompErrorPrefix[] =
       __FILE__":**CRASHING**:" SECCOMP_MESSAGE_COMMON_CONTENT " in syscall ";
+#endif
   static const char kSeccompErrorPostfix[] = "\n";
   WriteToStdErr(kSeccompErrorPrefix, sizeof(kSeccompErrorPrefix) - 1);
   WriteToStdErr(sysno_base10, sizeof(sysno_base10));
   WriteToStdErr(kSeccompErrorPostfix, sizeof(kSeccompErrorPostfix) - 1);
 }
 
 }  // namespace.
 
 namespace sandbox {
 
 intptr_t CrashSIGSYS_Handler(const struct arch_seccomp_data& args, void* aux) {
-  uint32_t syscall = args.nr;
-  if (syscall >= 1024)
-    syscall = 0;
+  uint32_t syscall = SyscallNumberToOffsetFromBase(args.nr);
+
   PrintSyscallError(syscall);
 
   // Encode 8-bits of the 1st two arguments too, so we can discern which socket
   // type, which fcntl, ... etc., without being likely to hit a mapped
   // address.
   // Do not encode more bits here without thinking about increasing the
   // likelihood of collision with mapped pages.
   syscall |= ((args.args[0] & 0xffUL) << 12);
   syscall |= ((args.args[1] & 0xffUL) << 20);
   // Purposefully dereference the syscall as an address so it'll show up very
@@ skipping 90 matching lines @@
 
 const char* GetIoctlErrorMessageContentForTests() {
   return SECCOMP_MESSAGE_IOCTL_CONTENT;
 }
 
 const char* GetKillErrorMessageContentForTests() {
   return SECCOMP_MESSAGE_KILL_CONTENT;
 }
 
 }  // namespace sandbox.