Subject: sigaltstack bad behavior on powerpc64

I may have found a bug in powerpc64 arch specific code.

*) When setting a sighandler using sigaction() call, if the flag
SA_ONSTACK is set and no alternate stack is provided via sigaltstack(),
the kernel still try to install the alternate stack. This behavior is
the opposite of the one which is documented in Single Unix
Specifications V3.

*) Also when setting an alternate stack using sigaltstack() with the
flag SS_DISABLE, 
the kernel try to install the alternate stack on signal delivery.

These two use cases makes the process crash at signal delivery.

I wrote a small patch to add a condition in get_sigframe().

Hope that is relevant and helpfull.

Regards,
Laurent MEYER.

Signed-off-by: Laurent Meyer <meyerlau@fr.ibm.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>

Index: linus-2.6/arch/powerpc/kernel/signal_64.c
===================================================================
--- linus-2.6.orig/arch/powerpc/kernel/signal_64.c
+++ linus-2.6/arch/powerpc/kernel/signal_64.c
@@ -214,7 +214,7 @@ static inline void __user * get_sigframe
         /* Default to using normal stack */
         newsp = regs->gpr[1];
 
-	if (ka->sa.sa_flags & SA_ONSTACK) {
+	if ((ka->sa.sa_flags & SA_ONSTACK) && current->sas_ss_size) {
 		if (! on_sig_stack(regs->gpr[1]))
 			newsp = (current->sas_ss_sp + current->sas_ss_size);
 	}