[MIPS] Add seccomp bpf support

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

Index: sandbox/linux/seccomp-bpf-helpers/sigsys_handlers.cc
diff --git a/sandbox/linux/seccomp-bpf-helpers/sigsys_handlers.cc b/sandbox/linux/seccomp-bpf-helpers/sigsys_handlers.cc
index 6b959ac928c0091904c4445dc01685e5ff679925..e82eb0dd1d5633895334dbf8e0df25110d231c2c 100644
--- a/sandbox/linux/seccomp-bpf-helpers/sigsys_handlers.cc
+++ b/sandbox/linux/seccomp-bpf-helpers/sigsys_handlers.cc
@@ -44,11 +44,35 @@ void WriteToStdErr(const char* error_message, size_t size) {
   }
 }
 
+// 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 an 1023 on archs other than Mips.

[jln (very slow on Chromium), 2014/06/03 01:00:33]

s/an/and/

Also, this statement should be "on all architectures". On MIPS the range is
narrower, but this statement is still true.

[nedeljko, 2014/06/03 15:32:18]

Done.

+// On architectures where base value is different than zero (currently only
+// Mips), we are truncating valid syscall value to offset from base.
+// This function returns values between 0 and 350 on Mips (O32 ABI).
+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
+  // simlicity.
+  if (sysno > __NR_Linux && sysno <= __NR_Linux + __NR_Linux_syscalls)
+    sysno = sysno - __NR_Linux;
+  else
+    sysno = 0;
+#else
+  if (sysno >= 1024)
+    sysno = 0;
+#endif
+  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];
@@ -59,8 +83,14 @@ void PrintSyscallError(uint32_t sysno) {
     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));
@@ -72,9 +102,8 @@ void PrintSyscallError(uint32_t sysno) {
 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