Bug 908907 - Fill in gaps in seccomp-bpf whitelist for b2g. r=kang, r=bsmith

Relatively harmless syscalls:
* dup, used by mozilla::ipc::Shmem
* getuid, for android::IPCThreadState, used in audio decode
* nanosleep, used by android::AudioTrack

Of potential concern:
* sched_setscheduler, used by audio threads in e.g. CubeVid
  This might be restrictable somewhat by inspecting its arguments.

Of serious concern:
* unlink, as a workaround for bug 906996 (q.v.).

Note that we already allow open(), including for writing (temporary
files, /dev/genlock on qcom devices, probably more), so allowing unlink
won't make the situation much worse.

security/sandbox/seccomp_filter.h

@@ -69,18 +69,24 @@
   ALLOW_SYSCALL(munmap), \
   ALLOW_SYSCALL(mmap2), \
   ALLOW_SYSCALL(mprotect), \
+  ALLOW_SYSCALL(dup), \
+  ALLOW_SYSCALL(getuid32), \
+  ALLOW_SYSCALL(nanosleep), \
   /* Must remove all of the following in the future, when no longer used */ \
   /* open() is for some legacy APIs such as font loading. */ \
+  /* See bug 906996 for removing unlink(). */ \
   ALLOW_SYSCALL(open), \
   ALLOW_SYSCALL(fstat64), \
   ALLOW_SYSCALL(stat64), \
   ALLOW_SYSCALL(prctl), \
   ALLOW_SYSCALL(access), \
   ALLOW_SYSCALL(getdents64), \
+  ALLOW_SYSCALL(unlink), \
   /* Should remove all of the following in the future, if possible */ \
   ALLOW_SYSCALL(getpriority), \
   ALLOW_SYSCALL(setpriority), \
   ALLOW_SYSCALL(sigprocmask), \
+  ALLOW_SYSCALL(sched_setscheduler), \
   /* Always last and always OK calls */ \
   SECCOMP_WHITELIST_ADD \
   /* restart_syscall is called internally, generally when debugging */ \