seccomp: ignore secure_computing return values

This change is inspired by
https://lkml.org/lkml/2012/4/16/14
which fixes the build warnings for arches that don't support
CONFIG_HAVE_ARCH_SECCOMP_FILTER.

In particular, there is no requirement for the return value of
secure_computing() to be checked unless the architecture supports
seccomp filter. Instead of silencing the warnings with (void)
a new static inline is added to encode the expected behavior
in a compiler and human friendly way.

v2: - cleans things up with a static inline
- removes sfr's signed-off-by since it is a different approach
v1: - matches sfr's original change

Reported-by: Stephen Rothwell <[e-mail]>
Signed-off-by: Will Drewry <[e-mail]>
Acked-by: Kees Cook <[e-mail]>
Signed-off-by: James Morris <[e-mail]>

arch/microblaze/kernel/ptrace.c

···
 {
 	long ret = 0;
 
-	secure_computing(regs->r12);
+	secure_computing_strict(regs->r12);
 
 	if (test_thread_flag(TIF_SYSCALL_TRACE) &&
 	    tracehook_report_syscall_entry(regs))

arch/mips/kernel/ptrace.c

···
 asmlinkage void syscall_trace_enter(struct pt_regs *regs)
 {
 	/* do the secure computing check first */
-	secure_computing(regs->regs[2]);
+	secure_computing_strict(regs->regs[2]);
 
 	if (!(current->ptrace & PT_PTRACED))
 		goto out;

arch/powerpc/kernel/ptrace.c

···
 {
 	long ret = 0;
 
-	secure_computing(regs->gpr[0]);
+	secure_computing_strict(regs->gpr[0]);
 
 	if (test_thread_flag(TIF_SYSCALL_TRACE) &&
 	    tracehook_report_syscall_entry(regs))

arch/s390/kernel/ptrace.c

···
 	long ret = 0;
 
 	/* Do the secure computing check first. */
-	secure_computing(regs->gprs[2]);
+	secure_computing_strict(regs->gprs[2]);
 
 	/*
 	 * The sysc_tracesys code in entry.S stored the system

arch/sh/kernel/ptrace_32.c

···
 {
 	long ret = 0;
 
-	secure_computing(regs->regs[0]);
+	secure_computing_strict(regs->regs[0]);
 
 	if (test_thread_flag(TIF_SYSCALL_TRACE) &&
 	    tracehook_report_syscall_entry(regs))

arch/sh/kernel/ptrace_64.c

···
 {
 	long long ret = 0;
 
-	secure_computing(regs->regs[9]);
+	secure_computing_strict(regs->regs[9]);
 
 	if (test_thread_flag(TIF_SYSCALL_TRACE) &&
 	    tracehook_report_syscall_entry(regs))

arch/sparc/kernel/ptrace_64.c

···
 	int ret = 0;
 
 	/* do the secure computing check first */
-	secure_computing(regs->u_regs[UREG_G1]);
+	secure_computing_strict(regs->u_regs[UREG_G1]);
 
 	if (test_thread_flag(TIF_SYSCALL_TRACE))
 		ret = tracehook_report_syscall_entry(regs);

include/linux/seccomp.h

···
 	return 0;
 }
 
+/* A wrapper for architectures supporting only SECCOMP_MODE_STRICT. */
+static inline void secure_computing_strict(int this_syscall)
+{
+	BUG_ON(secure_computing(this_syscall) != 0);
+}
+
 extern long prctl_get_seccomp(void);
 extern long prctl_set_seccomp(unsigned long, char __user *);
 
···
 struct seccomp_filter { };
 
 static inline int secure_computing(int this_syscall) { return 0; }
+static inline void secure_computing_strict(int this_syscall) { return; }
 
 static inline long prctl_get_seccomp(void)
 {