MIPS: seccomp: Handle indirect system calls (o32)

When userland uses syscall() to perform an indirect system call
the actually system call that needs to be checked by the filter
is on the first argument. The kernel code needs to handle this case
by looking at the original syscall number in v0 and if it's
NR_syscall, then it needs to examine the first argument to
identify the real system call that will be executed.
Similarly, we need to 'virtually' shift the syscall() arguments
so the syscall_get_arguments() function can fetch the correct
arguments for the indirect system call.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6404/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/scall64-n32.S

@@ -72,6 +72,7 @@ n32_syscall_trace_entry:
 	SAVE_STATIC
 	move	s0, t2
 	move	a0, sp
+	daddiu	a1, v0, __NR_N32_Linux
 	jal	syscall_trace_enter
 
 	bltz	v0, 2f			# seccomp failed? Skip syscall