[PATCH] microblaze: restore r4 in sys_rt_sigreturn to fix corruption after signal

[Richard Theil]

From: Richard Theil <linux@xxxxxxxxxx>

As already identified in http://marc.info/?l=microblaze-uclinux&m=126203029729306 ,
the Microblaze signal handling code will corrupt r4 on a signal return. This was never
fixed upstream. ret_from_trap in entry.S expects both r3 and r4, but sys_rt_sigreturn
will only provide r3 by means of a 32-bit return value. The patch explicitly loads r4 instead
of returning a 64 bit compound to avoid surprises from the compiler. Applicable at least
from 3.6 on to today's git master.

Signed-off-by: Richard Theil <linux@xxxxxxxxxx>
---
--- linux-3.13.5/arch/microblaze/kernel/signal.c.orig	2014-02-22 22:35:30.000000000 +0100
+++ linux-3.13.5/arch/microblaze/kernel/signal.c	2014-04-07 19:20:54.282023372 +0200
@@ -105,6 +105,10 @@ asmlinkage long sys_rt_sigreturn(struct
 	if (restore_altstack(&frame->uc.uc_stack))
 		goto badframe;
 
+	/* after signal, ret_from_trap needs r3 _and_ r4 from context */
+	__asm__ __volatile__("lwi r4,%0,%1"::
+		"r"(regs),"i"(offsetof(struct pt_regs,r4)));
+
 	return rval;
 
 badframe:

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