[PATCH] ARM: unwind: Fix handling of "Pop r4-r[4+nnn],r14" opcode

[Nikolay Borisov]

The arm EABI states that opcode 10100nnn means pop register r4-4[4+nnn],
aditionally there is a similar opcode: 10101nnn which means the same thing plus
popping r14. Those two cases are handled by the unwind_exec_pop_r4_to_rN
function which checks whether the 4th bit is set and does r14 popping.

However, up until now it has been checking whether the 8th was set (mask & 0x80)
instead of the 4th (mask & 0x8), a simple to make typo but this meant that we
were always popping r14 even if we had the former opcode.

This patch changes the mask so that the 2 opcodes are being handled correctly.

Signed-off-by: Nikolay Borisov <Nikolay.Borisov at arm.com>
---

I tested this patch using magic sysrq to generate backtrace of every process and
it was working. However, inspecting the unwind table for vmlinuz it seems that
the majority of the instructions do in fact require popping r14 so that's why
this has gone unnoticed for a long period, however I'm open to suggestions how
to more thoroughly test this. 

The first thing that comes to mind is manually fiddle with the unwind table of a
well-known function so that it no longer requires r14 being popped. 

 arch/arm/kernel/unwind.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm/kernel/unwind.c b/arch/arm/kernel/unwind.c
index 3c21769..cb791ac 100644
--- a/arch/arm/kernel/unwind.c
+++ b/arch/arm/kernel/unwind.c
@@ -285,7 +285,7 @@ static int unwind_exec_pop_r4_to_rN(struct unwind_ctrl_block *ctrl,
 		if (unwind_pop_register(ctrl, &vsp, reg))
 				return -URC_FAILURE;
 
-	if (insn & 0x80)
+	if (insn & 0x8)
 		if (unwind_pop_register(ctrl, &vsp, 14))
 				return -URC_FAILURE;
 
-- 
1.8.1.5