powerpc/signal64: Rewrite rt_sigreturn() to minimise uaccess switches

Add uaccess blocks and use the 'unsafe' versions of functions doing user
access where possible to reduce the number of times uaccess has to be
opened/closed.

Co-developed-by: Christopher M. Riedl <cmr@codefail.de>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Christopher M. Riedl <cmr@codefail.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210227011259.11992-10-cmr@codefail.de

arch/powerpc/kernel/signal_64.c

@@ -822,11 +822,11 @@ SYSCALL_DEFINE0(rt_sigreturn)
 		 */
 		current->thread.regs->msr &= ~MSR_TS_MASK;
 		if (!user_read_access_begin(&uc->uc_mcontext, sizeof(uc->uc_mcontext)))
-			return -EFAULT;
-		if (__unsafe_restore_sigcontext(current, NULL, 1, &uc->uc_mcontext)) {
-			user_read_access_end();
 			goto badframe;
-		}
+
+		unsafe_restore_sigcontext(current, NULL, 1, &uc->uc_mcontext,
+					  badframe_block);
+
 		user_read_access_end();
 	}
 
@@ -836,6 +836,8 @@ SYSCALL_DEFINE0(rt_sigreturn)
 	set_thread_flag(TIF_RESTOREALL);
 	return 0;
 
+badframe_block:
+	user_read_access_end();
 badframe:
 	signal_fault(current, regs, "rt_sigreturn", uc);