[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..8a23af8df354f8f01853503572d2f202c0a3ef33 100644
--- a/sandbox/linux/seccomp-bpf-helpers/sigsys_handlers.cc
+++ b/sandbox/linux/seccomp-bpf-helpers/sigsys_handlers.cc
@@ -47,8 +47,14 @@ void WriteToStdErr(const char* error_message, size_t size) {
 // Print a seccomp-bpf failure to handle |sysno| to stderr in an
 // async-signal safe way.
 void PrintSyscallError(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))

[jln (very slow on Chromium), 2014/05/02 20:42:04]

What is the value of __NR_linux?

We need to make sure that sysno can be printed by the code below (with 4
digits).

We might want to do
if (sysno >= __NR_linux) sysno = sysno - __NR_linux; else sysno = 0;

(and keep the following if (sysno >= 1024) sysno = 0)

[nedeljko, 2014/05/07 15:40:05]

Value of __NR_Linux for is either 4000, 5000 or 6000 depending on ABI and max
value for syscall is __NR_Linux + __NR_Linux_syscalls.
Nevertheless, I decided to truncate value as you suggested because of the
function CrashSIGSYS_Handler that uses only 12 bits for syscall number.

As for keeping if (sysno >= 1024) sysno = 0), on mips max offset from min
syscall number to max number is 350 (__NR_Linux_syscalls) so I didnt keep that
if statement.

One more thing: on O32 ABI the start of syscall range (4000) is also valid
syscall number, so I decided to use the value that is out of range for
appropriate ABI (__NR_Linux + 1000)

+    sysno = 0;
+#else
   if (sysno >= 1024)
     sysno = 0;
+#endif
   // TODO(markus): replace with async-signal safe snprintf when available.
   const size_t kNumDigits = 4;
   char sysno_base10[kNumDigits];
@@ -73,8 +79,14 @@ namespace sandbox {
 
 intptr_t CrashSIGSYS_Handler(const struct arch_seccomp_data& args, void* aux) {
   uint32_t syscall = args.nr;
+#if defined(__mips__)
+  // On MIPS syscall numbers are in different range than on x86 and ARM

[jln (very slow on Chromium), 2014/05/02 20:42:04]

Let's make a function TruncSysnoToValu(int sysno, int max_value); instead of
duplicating.

[nedeljko, 2014/05/07 15:40:05]

Done.

+  if ((syscall < __NR_Linux) || (syscall >= __NR_Linux + __NR_Linux_syscalls))
+    syscall = 0;
+#else
   if (syscall >= 1024)
     syscall = 0;
+#endif
   PrintSyscallError(syscall);
 
   // Encode 8-bits of the 1st two arguments too, so we can discern which socket