powerpc/signal64: Use __get_user() to copy sigset_t

Usually sigset_t is exactly 8B which is a "trivial" size and does not
warrant using __copy_from_user(). Use __get_user() directly in
anticipation of future work to remove the trivial size optimizations
from __copy_from_user(). Calling __get_user() also results in a small
boost to signal handling throughput here.

Signed-off-by: Christopher M. Riedl <cmr@codefail.de>

arch/powerpc/kernel/signal_64.c

@@ -97,6 +97,14 @@ static void prepare_setup_sigcontext(struct task_struct *tsk, int ctx_has_vsx_re
 #endif /* CONFIG_VSX */
 }
 
+static inline int get_user_sigset(sigset_t *dst, const sigset_t *src)
+{
+	if (sizeof(sigset_t) <= 8)
+		return __get_user(dst->sig[0], &src->sig[0]);
+	else
+		return __copy_from_user(dst, src, sizeof(sigset_t));
+}
+
 /*
  * Set up the sigcontext for the signal frame.
  */
@@ -701,8 +709,9 @@ SYSCALL_DEFINE3(swapcontext, struct ucontext __user *, old_ctx,
 	 * We kill the task with a SIGSEGV in this situation.
 	 */
 
-	if (__copy_from_user(&set, &new_ctx->uc_sigmask, sizeof(set)))
+	if (get_user_sigset(&set, &new_ctx->uc_sigmask))
 		do_exit(SIGSEGV);
+
 	set_current_blocked(&set);
 
 	if (!user_read_access_begin(new_ctx, ctx_size))
@@ -740,8 +749,9 @@ SYSCALL_DEFINE0(rt_sigreturn)
 	if (!access_ok(uc, sizeof(*uc)))
 		goto badframe;
 
-	if (__copy_from_user(&set, &uc->uc_sigmask, sizeof(set)))
+	if (get_user_sigset(&set, &uc->uc_sigmask))
 		goto badframe;
+
 	set_current_blocked(&set);
 
 #ifdef CONFIG_PPC_TRANSACTIONAL_MEM