[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 c4a6f92c01d423521aee83ffcaf2e4cc35f4e82a..7e4aac29f5e3c190229a93dee294f0dd592917ec 100644
--- a/sandbox/linux/seccomp-bpf-helpers/sigsys_handlers.cc
+++ b/sandbox/linux/seccomp-bpf-helpers/sigsys_handlers.cc
@@ -44,11 +44,29 @@ void WriteToStdErr(const char* error_message, size_t size) {
   }
 }
 
+uint32_t TruncSysnoToValue(uint32_t sysno) {
+#if defined(__mips__)
+  // On MIPS syscall numbers are in different range than on x86 and ARM
+  if (sysno < __NR_Linux || sysno > __NR_Linux + __NR_Linux_syscalls)
+    // 4000 (truncated to zero) is valid syscall number on mips O32 ABI.
+    // Invalid syscall will be printed with a value that is out of range
+    // on apropriate ABI.
+    sysno = sysno - __NR_Linux + 1000;

[jln (very slow on Chromium), 2014/05/16 19:30:17]

I don't understand what this does.

Please document the function above in a comment.

Invalid syscall values should be transformed to zero. Valid syscall values
should be transformed to sysno - __NR_linux.

Please choose another name as well. The name should be something along the lines
of: "SyscallNumberToOffsetFromBase()"

[nedeljko, 2014/05/19 17:37:23]

On MIPS O32 ABI value of __NR_Linux is 4000.

The value of the first syscall (__NR_syscall) is also 4000.

With sysno - __NR_Linux this valid syscall value would be transformed to zero.
That is the reason why I transformed invalid values to 1000 + offset.

On the other hand, I see that this is complicating things to much just for one
syscall in one of the ABIs, so I'll do as you said in your comment and transform
invalid values to zero.

[nedeljko, 2014/05/22 17:38:55]

Done.

+  else
+    sysno = sysno - __NR_Linux;
+#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;
+
+  sysno = TruncSysnoToValue(sysno);
+
   // TODO(markus): replace with async-signal safe snprintf when available.
   const size_t kNumDigits = 4;
   char sysno_base10[kNumDigits];
@@ -59,8 +77,14 @@ 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));
@@ -73,8 +97,7 @@ namespace sandbox {
 
 intptr_t CrashSIGSYS_Handler(const struct arch_seccomp_data& args, void* aux) {
   uint32_t syscall = args.nr;
-  if (syscall >= 1024)
-    syscall = 0;
+
   PrintSyscallError(syscall);
 
   // Encode 8-bits of the 1st two arguments too, so we can discern which socket