x86: signal: check sas_ss_size instead of sas_ss_flags()

Impact: fix redundant and incorrect check

Oleg Nesterov noticed wrt commit:

  14fc9fb: x86: signal: check signal stack overflow properly

>> No need to check SA_ONSTACK if we're already using alternate signal stack.
>
> Yes, but this also mean that we don't need sas_ss_flags() under
> "if (!onsigstack)",

Checking on_sig_stack() in sas_ss_flags() at get_sigframe() is redundant
and not correct on 64 bit. To check sas_ss_size is enough.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Cc: roland@redhat.com
LKML-Reference: <49CBB54C.5080201@ct.jp.nec.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/signal.c

@@ -221,7 +221,7 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
 	if (!onsigstack) {
 		/* This is the X/Open sanctioned signal stack switching.  */
 		if (ka->sa.sa_flags & SA_ONSTACK) {
-			if (sas_ss_flags(sp) == 0)
+			if (current->sas_ss_size)
 				sp = current->sas_ss_sp + current->sas_ss_size;
 		} else {
 #ifdef CONFIG_X86_32